Basic Principles and Introduction to Chest Radiographs

Basic Radiology Module
 revised 2018 D. Magid MD M.Ed

1st version Kopal Kulkarni MS IV and Donna Magid MD, M.Ed 2011

Requesting an imaging study

While giving a brilliant patient presentation on rounds, you mention that your patient would benefit from getting diagnostic imaging. The resident says, “Great, go for it.” Now what?

To start with, decide on an appropriate imaging technique. Will a contrast CT be the best study? Or do you need a “dry” (non-contrast) CT? Or is it MRI, ultrasound, fluoroscopy or radiographythat will be the best and most cost-efficient test?  Radiology need to know what your clinical question is ('R/O subdural"; "R/O aspiration"; "Hx breast Ca, new left lateral rib pain"); so as to then choose the details-- contrast, no contrast, MR sequences, views, etc-- best tailored to provide the answer.  We do the protocolling, you provide the necessary and adequate information, PLEASE!   And please remember to explain the planned study to the opatient-- what they will feel, hear, whether it hurts, what are the risks-- students are often better at this than the busier more senior team members. is a great guide to explaining it to a patient-- you are unlikely to know exactly how an enteroclysis or bone scintigraphy  or other mysteriousprocess actually works, feels, sounds. Frequent explorations of the American College of radiology Appropriateness Criteria (ACR AC) will also build your comprehension  of how/when each these imaging modalities may be used best. The following reading material will give you basic pointers on the strengths, weaknesses and contraindications of a few common imaging techniques. But first, two simple rules to remember when requesting imaging.

Rule #1: Consult the radiologist!

First, research your patient's specific clinical question ("R/O P.E."; "?Aspirations"; "Low Back Pain for 1 yr, no trauma") at the ACR Appropriateness Criteria, If you have any questions about which is the best study-- remember, the real experts in imaging are the radiologists. Even medicine and surgery docs can become confused with the many evolving and ever-changing options of imaging (Contrast? Non-contrast? FatSat? Proton density? Arterial or venous phase?).Call your friendly neighborhood radiologist. lists the phones of the various CT and MR scanners, US locales, specifies the relevant technologists' numbers, and delivers clear color-coded groups of need-to-find numbers to help guide you through the confusion to knowledgeable imaging people.  They can tell you the best study for your particular patient, making you look like a shining star in front of your team, saving your patient from extra radiation, and avoiding wasted money on unnecessary studies for the healthcare system. Use this site!

Rule #2: When requesting imaging studies, give a brief patient history and a clinical question to be answered by the study.

Once you’ve decided on the best diagnostic and most cost-efficient study, your clinical service can show you how to request the study-- this too is constantly evolving and may have some differences at differnt sites or services.
Remember that requesting an imaging study is more like calling a consult from a specialist than ordering a prescription. Just as you would send a short but informative page to a consulting physician, you must write a brief but relevant history of the patient and reason for imaging on the imaging request forms. This will not only help your radiologist decide whether you have requested the correct test, but also will help them focus on particular findings and generate a more accurate and relevant report for your specific clinical question. [For example: if requesting an abdominal CT, tell us you are worried about a patient with 'newly-diagnosed colon cancer, CT to stage and assess primary and mets' ; don't just write 'CT abdomen with and without contrast, thin slices'.  The technical way we will execute your desired search is our call).

Introduction to Imaging Modalities

X-ray: the development of x-rays was the first step in creating the field of medical imaging as we know it today. The first paper describing possible medical uses was written  in 1895 by a German physicist named Wilhelm Röentgen (or Rontgen), who eventually won the first Nobel Prize in Physics (1901) for his discovery.

 First Radiograph

The first radiograph, performed in 1885, shows Mrs. Roentgen's hand -- allegedly causing her to gasp and exclaim "I have seen my death!"

As you may vaguely remember from physics, x-rays are a type of electromagnetic radiation with a frequency somewhere between gamma rays on the high end and ultraviolet and visible light on the low end. Due to the frequency of the waves, x-rays are considered a form of ionizing radiation, meaning that they have enough energy to knock electrons off of atoms. This property also confers the mutagenic risk of radiation, which caused many of the early pioneers in x-ray technology to succumb to cancer.

When undergoing a radiograph , a patient is instructed to stand between an x-ray source and a detector. The x-rays pass through the patient and basically cast a 'shadow' of the body on the detector, conveying information about the varying radiodensities of the traversed structures. In a very radiodense tissue such as bone, fewer beams will pass through the tissue; these areas will appear white (radiodense, high attenuation) on radiographs. When a tissue such a lung is less dense (either in terms of thickness and/or atomic number), more beams pass through and the area will appear dark (radiolucent, lower attenuation) on the radiograph. When describing a finding on a radiograph, learn to call the whiter areas (like bone) radiopaque or radiodense and the blacker areas (like lung) radiolucent.

Foot Example


 Radiographs are informative and quick and if necessary, can be performed at the bedside of very ill patients or in the operating room (although the quality of these images may be diminished). For this reason, they are often used as first-line imaging. Although getting a radiograph will expose a patient to radiation, the radiation dose is actually quite little as compared to that of CT (see ‘radiation safety’ section for more information). At the same time, when imaging a radiation sensitive area like reproductive organs, or young patients, a non-radiation-emitting imaging modality like ultrasound should be considered first. Likewise, a small dose is still a significant dose, so request radiographs prudently; every patient who steps in the hospital does NOT need a chest image (check ACR AC guidelines). Situations in which radiographs are helpful and commonly requested: initial assessment/triage of the chest, abdomen, bones and joints, airway, foreign matter localization. 
Radiography is NOT necessarily the initial/best choice for vessels, some soft tissues, brain or neural tissue, or radiolucent structures such as ligaments, tendons and cartilage.

  • Lung processes- Pneumonia, lung nodules, pneumothorax, COPD (chest image)
  • Chest pain- causes such as aortic dissection, cardiomegaly and other findings may be seen on chest radiograph
  • Abdominal processes- pneumoperitoneum (free air in the abdomen), obstruction, calcifications/calculi, bowel caliber or bowel wall thickening, bowel displaced by mass; other causes of acute abdomen. You may need to use oral contrast when imaging the abdomen.
  • C-spine- initial screen for fracture/injury that may threaten the spinal cord
  • Bone and joint problems- fractures, deformities, infections, neoplasms, cartilage loss, derangement.  Subtle soft-tissue  issues will not adequately be seen although radiographs remain the entrance-point for virtually all bone and joint assessments.  

X-rays are (invisible) electromagnetic waves; plain films or radiographs are the images that they create. Avoid colloquialisms by saying 'image', 'study',  'view', or 'radiograph' and not “x-ray”, nor 'plain film" (digital means no film involved) when referring to a visible image.


  • Do not image  the trunk or most proximal extremities of a  pregnant woman due to radiation risk- always ask/test a woman if she might be pregnant, and discuss with/forewarn Radiology. (see Kiddy Physics section)

Computed Tomography (CT)

CT scanning was invented in 1972, evolved rapidly and is now a fast, readily available, highly detailed and likely highly over-used imaging modality.

In the CT scanner, the x-ray source and detector are located within the donut-shaped ring, called the gantry. A series of x-ray images are taken at various angles along 360 degrees around the ring. The computer-constructed composite of this data yields a cross-sectional slice of the body. Recent advances in obtaining more and more slices simultaneously (multislice, which initialy meant 4, then 16, then 64, and now hundreds slices at once) continue to explode the speed, resolution, and cephalocaudal body volume, that we can rapidly obtain without patient motion. This is of use for 2D reconstruction (putting the transaxial slices back together and digitally ‘reslicing’ the patient in other planes, usually coronal or sagittal) or simulated 3D images.  However (see Kiddy Physics), this has also created an enormous increase in the dose-per-patient and per tissue volume, creating enormous concern and heightening both medical and layperson awareness of the need for careful technical modifications, cost/benefit analyses, and thoughtful use..

Since CT scanners use the same x-rays that radiographs use, there are tissue/structure similarities in terms of radiodensity. Bones are more attenuating of the initial beam, and will thus show up as white, whereas lungs are mostly air and will be low attenuation just as on radiographs. Just as with radiographs, the black/white is determined by the relative attenuating properties (radiodensity or radiolucency, determined by atomic number) of tissues and the path length through these tissues (thickness). The preferred CT descriptors you may hear when on the wards or consulting with Radiology are “high attenuation” (radiodense or whiter) and “low attenuation” (radiolucent or blacker), rather than 'radiodense/lucent'. “Attenuation” refers to the extent to which interposed tissue attenuates, or stops, the x-ray beams; ie, is related to the decrease in intensity of x-rays as they pass through different media. As you can imagine, bone or lead block x-rays much more so than air or fat do. Thus, x-rays have a higher attenuation (are stopped or absorbed) as they attempt to pass through dense (high atomic number) objects.

CT Scan Example

In CT scans, the image is presented such that the right side of the patient is on the left side of your screen (ie, as if you were standing at the feet looking cephalad with patient supine). Quiz yourself: Identify the tissue types and identify them from highest to lowest

Hounsfield units (HU, named after one of the primary inventors of the CT scanner (hence The Hounsfield Units, the Radiology Dept’s previous superb intramural basketball team) quantify the density of a certain area on a CT scan. On this scale water is defined as having a value of zero HU . Anything more dense is positive on the scale, while anything less dense is negative. You don’t have to memorize these values right now, but just to give you an idea of the scale:

Table 1

*From highest to lowest attenuation: bone (black arrow); contrast (green); muscle (red); fat (blue); air in bowel (purple).

Quiz Yourself:


Looking at an abdominal CT of one of your patients, you notice an incidental mass in the adrenal gland (see arrow). When describing the mass to your residents, you would say that the mass is _______ than the liver.

 Higher or lower attenuation?  

Lower or higher Hounsfield units?


Answer: more radiolucent, lower attenuation, lower Hounsfield units

High attenuation= radiodense= high density (bone, lead, contrast)= high Housfield units
Low attenuation=radiolucent= low density (fat, air, vacuum)= low Hounsfield units

In this example, finding the Hounsfield units would be particularly useful because adrenal incidentalomas (referring to an unexpected finding when scanning the abdomen for other reasons) are common. Those dispaying a high fat content (therefore very low HU) are very often benign adenomas rather than pheochromocytomas, carcinomas or metastases. Combined with the fact that the finding is very small, it is unlikely that your patient has a malignant adrenal lesion. No further work-up is needed other than serial imaging.

CT uses:  Throughout your time on the wards, you will learn the nuances of when to request a CT and how it can be useful in the diagnosis or staging of particular disease processes. In your initial consideration, you should be aware that a CT scan is very fast and detailed. A full body scan can  take less than 30 seconds to complete which is why CT is often a good, quick option for trauma in the Emergency Department. CT scanning is also fast enough that a patient usually can successfully hold his or her breath during the entire actual data acquisition (ie, the scan), allowing for low motion artifact when imaging the chest.

At the same time, it obviously cannot be done at the bedside, and is thus not for patients who have unstable vital signs, are in traction or in spinal immobilization frames, or who cannot leave the floor for other reasons. (Patients in the ICUs often can be transported via ‘the Purple People” teams, who can provide ICU-style nursing care and monitoring of life-support equipment through transport, study, and return to the ICU; your team will assess whether, in any case, a patient can in fact go downstairs for CT).  CT is expensive, relatively high-dose, and there are additional risks from, for instance, intravenous contrast (particularly if renal function marginal or there is question of allergies), so do not order CT scans lightly.Again, check the ACR AC for general guidelines on clinical applications in given scenarios.

There are four major substances that typically are quite high attenuation (white) on CT: bone, blood, metal and contrast (ie the 4 B’s: ‘Blood, Bone, Bullets and…Barium’?). Know this! I guarantee that at some point you will be quizzed on this point. 

  1. Bone: As bone (and anything well-calcified, such as some lung nodules) shows up very distinctly on CT, you can imagine that CT would be useful for looking at fractures or other local changes in bone such as neoplasm or infection. For long bone fractures though, start with radiography (coned and centered to area of interest, please!) and consult with Radiology, don’t jump to CT- a radiograph is usually diagnostic and sufficient. If scanned, using CT 'bone windows' allows better analysis of fine trabecular, medullary, articular, and cortical detail.
  2. Blood: CT may be useful in trauma situations when acute hemorrhage is suspected. Head injury or possible hemorrhagic stroke patients, for example, can get rapid assessment for intracranial bleeding with the so-called ‘dry’ (non-contrast) CT.  (IV contrast may increase the attenuation of non-acute structures, and is not used if ruling out intracranial bleeding).  
  3. Metal object: Surgical material, swallowed or imbedded metal objects and bullets will be obvious on CT.
  4. Contrast: More on this subject later. (See TTW CONTRAST section).



  • Chest and Abdomen: useful for visualizing internal organs. You may need contrast depending on the clinical scenario.
  • Pelvis: Usually not used first in children/young adults due to risk of radiation to reproductive organs. Consider ultrasound or MRI as initial imaging alternatives with no radiation risk. Pelvic fractures/trauma (which are often acute emergencies), however, usually best seen on CT-which will depict not just bone but all the tissues, structures, and organ systems (GI, GU, neurovascular..) involved.
  • Extremities and Spine: Good for imaging of bone, although a radiograph may be sufficient. Not good for soft tissues in the extremities or back (ligaments tears, spinal disc herniation, nerve root or cord questions). As a rule, for Orthopaedic questions, CT is best when the primary question involves bone, MR when the question involves complex soft tissue, ligaments or tendons, cord, or nerve root questions.

When looking at particular structures on CT, you can adjust the “window” setting. A Bone window will give more differentiation in the high HU range (ie, spreads out the black-grey-white spectrum centered over the highest-attenuation tissues), while the lung window is better in the very low HU range (spreads out the black-grey-white scale over the most radiolucent tissues, allowing fine parenchymal detail or visualization of tiny nodules). There are also various windows such as mediastinal, liver, and kidney windows for everything in between.  Any one CT study is going to involve multiple windows (for example in chest, we will review the lung parenchyma, the bones, and the mediastinum/great vessels—so every chest image/data set will be reviewed at least 3 ways/times).


What is Contrast?:
Notice that in the mediastinum above, it’s easy to tell apart the vessels from the adjacent solid structures such as lymph nodes because of accentuated attenuation (radiodensity) differences between adjacent structures. Intravenous contrast agents such as iodine are very useful to look at the blood flow and, being high attenuation (high atomic number), also differentiate between different types of tissues. Contrast can be administered in different ways such as intravenous to delineate blood vessels, oral and rectal to highlight the lumen of gastrointestinal organs, and even rarely, inhalational contrast for lung imaging. With CT acquisition now so fast, the timing of a scan can be calculated to capture the arterial and/or venous phases of IV enhancement, further characterizing or targetting a lesion or tissue.  Administering contrast, however, can have risks (minor and major allergic reactions, nephrotoxicity, etc) and only should be done when needed. Consulting a radiologist is a good way to decide whether or not contrast would be helpful in a particular study. Explain, in any requisition, what information you wish to obtain, rather than just telling us ‘contrast’ or ‘non-contrast’—better communication leads to better imaging.

Relative contraindications to CT:

  • Documented reaction to contrast: Contrast may also cause an anaphylactic reaction, so always ask your patients if they have had a bad reaction to contrast in the past. (They may not know the word ‘contrast’; ask if they were injected before previous studies, and check the patient records if possible.)
  • Pregnancy due to radiation risks- always ask/test a woman if she might be pregnant (remember, we have had both pregnant 9 year olds and 57 year olds! The latter with triplets--her own grandchildren.).
  • Obesity > 300 lbs (One table in the Emergency Dept. may take closer to 650 lb but warn us, we need to modify the table) but most of the CT scanners have lower table limits.If you mis-represent the weight to get the scan done, it is likely the table will break down. compromising your scan and the 120 patients scheduled to follow on that machine over the next two days as we undergo repairs.  And no, despite persistent urban legend, we do not send obese patients to the Zoo to be scanned.  There is NO CT scanner at the Maryland Zoo, DO NOT call Dr. Magid!)
  • Unstable vital signs, certain life support equipment.
  • Contrast risks can include nephrotoxicity. For this reason, patients at high-risk (those with poor renal function or blood flow to the kidneys) should only receive contrast if absolutely necessary. Patients on dialysis may get CT immediately before dialysis, but discuss with Radiology and more senior clinicians on team.

Remember that CT scanning involves a relatively large amount of radiation compared to other imaging modalities. This is most necessary to consider when imaging pregnant women and children, but being cognizant of radiation exposure and using CT imaging prudently for any of your patients is important (see ‘Kiddy Physics’ section, the American College of Radiology Appropriateness Criteria, and/or Inappropriate imaging choices compromise patient care, put patients at unnecessary risk, may limit 3rd party reimbursement, and may 'ding' the requesting physician in impending national evaluations of quality of care.

Magnetic Resonance Imaging (MRI)


MRI evolved in the  early 1970’s but was not widely used in clinical settings begiuntil the mid/late1980’s. MRI is very useful for visualizing detailed internal soft tissue structures.

Unlike CT, MRI machines do not use radiation. Instead, a powerful permanent magnetic field aligns (polar) atoms in the body. When the field is turned off, the polar nuclei ‘relax’ and emit photons, which produce an electromagnetic signal that is detected by the scanner (and if you ever wondered why physics was a pre-med requirement...). Everything within a demarcated MR field must be non-ferromagnetic, requiring significant changes in furnishings, medical equipment, and affiliated personnel's clothing including jewelry, wallets and ID badges. Everything going into the room-- the patient, the clothing, the transport and support equipment, anything intentionally or accidently implanted in the patient-- must also be screened for compliance.

MR images reflect technical and directional variations on the applied magnetic field. These variations, or sequences, reflect measurements of different parameters.For example, T1 is the longitudinal relaxation time whereas T2 reflects the transverse relaxation time. The physics of these scans are beyond this text—and are not necessary to learn the intelligent clinical applications of MR.  It is just important to know that the different sequences highlight different types of soft tissue, creating visual or ‘signal’ differences based on physiology and molecular composition rather than radiodensity. When describing these scans, use ‘high signal’ to describe bright (white) areas and ‘low signal’ or 'absent signal' for dark (grey, black) areas. There are dauntingly long lists of constantly-changing terms associated with the various possible sequences (and with various brands of MR scanners) such as dual echo, turbo spin, half Fourier, inversion recovery, FLASH, GRASS, spoiled gradient, and many more. Do not try to keep these straight (even most Radiologists struggle to keep up) and do not request specific sequences—again, communicate with us in detail laying out the clinical concerns and known factors, and let us protocol (design) the best possible study for the clinical query.

  • T1- fat has a high signal (bright) and water/CSF or more fluid containing tissues have low signal (dark).
  • T2- fat has low signal (dark) and water or more fluid containing tissues have high signal (bright).
  • FLAIR- Like a T2 except free water is dark, so only edematous tissues light up, making it easier to spot pathology.


MRI Uses:

MRI is a great tool for examining soft tissue pathology. Since it relies on a magnetic field and not radiation, there is no risk of radiation-induced cancers or mutation, making this a safe option for pregnant women or children if necessary (while MR of the fetus increasingly is more common, it should not be undertaken lightly). At the same time, MRI scans take much longer than CT scans- about 20-30 minutes for a head MRI, up to over an hour for complex body studies; while a CT head scan make take less than 10 seconds. During this time, patients must be able to lie very still within the machine and tolerate a loud banging noise (there are MR-compliant headphones that let the patient listen to music). For most body parts, an MR means lying supine inside a long narrow tube-like enclosure which some patients describe as ‘coffin-like’.  As you can imagine, the prolonged immobility under unusual circumstances may be difficult for claustrophobic patients or those with pain, limited intellectual or comprehensive capacity, altered mental status, or uncontrollable involuntary motions.l. Also, as a responsible future physician, be aware that MRIs are very costly. As with any imaging modality, order MRIs judiciously, and consult the American College of Radiology Appropriateness Criteria often to enhance your understanding of what to order when. Clinical scenarios in which MRI scans may be useful include:

  • Brain tumors or ischemic stokes
  • Any soft tissue pathology- ligament or meniscal tears in the knee, spinal disc herniation, nerve root impingement, bone marrow changes
  • Soft tissue pelvic imaging (both because of the lack of radiation exposure to reproductive organs and because of good soft tissue discrimination)- uterine fibroids
  • MRI angiograms can be useful in the study of coronary or cerebral vasculature
  • MRI’s are NOT useful in visualizing bone or calcified structures, and are impeded by metal.  The non-polar molecules do not react to the field, therefore send off no signal, and appear as absent data or uninformative black regions (absent signal, signal void) in the MR image. These information gaps markedly limit MR utility for any bony structure (calcified bone appears as black absent signal) and can, particularly around metal, create artifact and limit the imaging of adjacent structures (eg, the bone around an arthroplasty stem; tissue detail near surgical clips) . Recent advances at Hopkins in metal artifact reduction techniques (MARS) has allowed great improvement in imaging around orthopaedic and other metal implants. 


Gadolinium, the contrast agent used for MRI, has very rare associated complication of nephrogenic systemic fibrosis in patients with renal failure. This condition ivolves diffuse fibrosis of the eyes, skin and other organs. It is thus recommended that patients with renal failure do not get gadolinium unless absolutely required and if so, receive dialysis shortly after.


  • Claustrophobia: MRI scanning is a claustrophobic and very loud experience- one that is often anxiety provoking. Prepare your patients and assess their ability to go through the procedure. Anti-anxiolytics may be helpful, as may the special headphones. .
  • Unstable vital signs
  • Obesity. (Again, no- there is no MRI at the Zoo).  While table limits have improved with newer MR models,a weight limit reflects both the mechanical limits of the table, and the very rigid inner diameter of the tube/gantry, meaning some oddly shaped patients may not fit the cross-sectional diameter even if technically within weight limits.
  • Pregnant women should not receive gadolinium, particularly in the first trimester
  • Metal objects: An MRI machine is basically a very large, very strong magnet. It is therefore absolutely contraindicated to allow patients with ferromagnetic metal foreign objects, intentional (staples, aneurysm clips, cochlear implants, certain plates, pacemaker wires or defibrillators, infusion ports, certain cardiac valves, nerve stimulators, stents, etc) or unintentional (certain shrapnel, bullets, metal shards or filings secondary to machine work, needles, etc- even some old tattoos with iron in certain colors!) to be scanned. There is a questionnaire which will screen for this information, and for some studies, such as head MR, it is common to get a screening radiograph to rule out such metal in the field of the proposed study. Other metals, such as most orthopaedic and dental implants, may be technically safe but will generate image artifacts which may compromise the study adjacent to these implants.

Quick comparison: When to get MR vs. CT?    

Table 2


When most people think about ultrasound, the only thing that pops into mind is the pregnancy exam. In fact ultrasound is a very versatile tool and is commonly used in many different types of clinical settings. 


Ultrasound is an imaging modality that emits very high frequency sound waves from the transducer and analyzes the reflected energy bouncing back to the transducer, to create a real-time image. Based on their properties, including the mass and molecular spacing and the compressibility of the materials, different materials will reflect the sound differently; in this way, ultrasound is particularly effective at detecting areas in which there is a change in density between tissues, ie, the boundaries between tissues.  Bats use the same type of technique to find bugs at night. 


In the beginning, ultrasound may be difficult for you to interpret, but use the correct terminology to describe structures: hypoechoic, echopenic, or sonolucent for dark areas (cysts, vessel lumens), and hyperechoic, echogenic, or sonogenic for bright areas (bone, solid organs).


Sonography can also employ the Doppler effect (we told you - you need that pre-med physics!) to measure whether flow is moving away from or towards the probe, and assess the velocity of the blood. This can distinguish a blood vessel from surrounding tissue, an artery from an adjacent vein, and may detect asymmetries of flow, and is particularly helpful in situations such as line placement.


Ultrasound uses:

One of the major benefits of ultrasound is that it is quickly and easily performed at bedside. While you can request a formal ultrasound study from the radiology department, you will also probably perform an ultrasound examination yourself sometime, even as a medical student. Since ultrasound is performed directly by the operator (as opposed to CT or MR, where the patient and physician are physically separated), it can be used in an interactive manner- for example not only looking for stones and signs of inflammation in a suspected case of cholecystitis, but also using the transducer to elicit Murphy’s sign (pain with compression over the gall bladder) while directly visualizing the gall bladder. However, this also means marked variability in image quality, depending both on equipment (like radiography, the smaller most portable hand-held units may not produce the best quality images; and huge variability in both image quality and in clinician interpretation are seen with less experienced operators). 

Ultrasound is safe for almost all patients as it operates on sound waves and thus emits no radiation. It does not require a patient to be “healthy” or stable enough to hold absolutely still nor to be sent down to a scanner (although there are some advantages to going down to the US Section, and like the AP chest, the portable US  should only be requested for sound medical reasons, not patient convenience/preference).  There is no preparation or contrast needed (except the hydration and full bladder preferred for transabdominal/pelvic ultrasound) and the machine itself is easy to use. It is also one of the less expensive imaging modalities, both in terms of basic equipment and patient charges. Ultrasound rooms can be small and require no special shielding, and only minimal extra supplies.  All of these qualities can make ultrasound a great first-line option, whether to expedite emergency care, expand medical services in an underserved area, or to obviate the need for a more expensive imaging test. Some clinical scenarios in which ultrasound is commonly used include:

  • Gynecologic exams- the lack of radiation makes ultrasound one of the most frequently used imaging tests in both pregnancy and other gynecologic exams. If done transabdominally (ie transducer applied to skin, lower abdomen), a full bladder helps expand the acoustic window into the female pelvis.Transvaginal scanning bypasses some of the difficulties of scanning the female patient with bladder issues, low anterior wall surgery or wounds, and to some extent morbid obesity since fat blocks US transmission. 
  • Echocardiography- this very common test used to detect wall motion abnormalities in the heart is actually a form of ultrasound. Done externally it is limited to the acoustic windows between ribs, and by patient subcutaneous fat; the transesophageal approach overcomes these issues but is more invasive, potentially risky, and expensive.
  • Abdominal exams- useful to examine for cysts, bleeding, or inflammation of organs. Ideal for the RUQ, where the liver’s physical properties produce an acoustic window allowing excellent visualization of the liver, gall bladder and right kidney. Imaging modality of choice in the RLQ for the thin child with suspected appendicitis, although far less useful in an obese adult or an adult with diffuse air in bowel (both air and fat block sonographic transmission).
  • Procedures- live-time imaging makes ultrasound a perfect on-site tool to guide procedures like biopsies, line placements, and thoracentesis/paracentesis (removing excess fluid from the thorax or abdomen without injuring other structures).
  • Checking for deep venous thromboses in the extremities, especially proximally; Doppler effects can be used to assess volume, speed, and direction of flow
  • Examining breast masses to further distinguish cyst/fluid from focal fat or tissueUsing Doppler ultrasound to examine the severity of carotid artery blockage

As a medical student, it will often be your job to clarify issues and reassure your patient (what will it feel like to get an MRI? What will the cystourethrogram tell me?). Refer to, which gives simple overviews of many different procedures, including an explanation of the procedure and its results, what the patient can expect to experience (see, hear, taste, feel) and benefits vs. risks:




Chest Film Anatomy

2018 D.Magid MD, M.Ed; first version 2011 K.Kulkarni MS IV and D.Magid MD, M.Ed

The final section in this module will focus on the basics of reading a chest study. But before we get to the new stuff, take some time to re-learn the anatomy by quizzing yourself. Answers are at the bottom of the page.

PA View:

Answers: 1. Spinous process of T2    2. Trachea    3. Clavicle    4. Spine of scapula    5. Humerus    6. Fifth rib    7. Manubrium    8. Aortic Arch    9. Right main bronchus    10. Pulmonary artery (R)    11. Main pulmonary artery    12. Pulmonary artery (L)    13. Lateral margin of the descending aorta    14. Lateral surface of right atrium    15. Lateral surface of left ventricle    16. Eleventh rib- posterior    17. Left hemidiaphragm    18. Acromium of scapula    19. Right hemidiaphragm

Lateral View: 

Answers: 1. Arch of aorta  2. Right hemidiaphragm  3. Left hemidiaphragm   4. Trachea  5. Scapula  6. Vertebral body  7. Posterior border of the heart   8. Sternum    9. Costophrenic angle    10. Pulmonary artery    11. Heart    12. Anterior border of the heart

How to Approach a Chest Radiograph

Learning how to read a chest study is a basic skill that you will find useful on the wards. This is the one study most likely to be tested on the wards or on national exams. The key to this process is being systematic and consistent. Consider this video:

I’m sure many of you have seen this, but the point is that it is surprisingly easy to miss a gorilla. When focusing on one main abnormality, it is easy to miss other more subtle but equally important findings. But, by being consistently systematic, you can become a much more thorough and observant reader.

1. Identify: First, make sure that the patient name and medical record number is as expected: 

2. The second step is to assess the technical adequacy of the film.  Keep this assessment in mind when reading the rest of the film, as it may limit your ability to comment on certain findings. A mnemonic which may help here is RIPE (is the film RIPE to be read?):


a. Rotation: In a good frontal chest image, the patient should be perfectly perpendicular to the beams. Any axial (side-to-side) rotation from this position may alter the appearance of normal structures or hide abnormalities. Make sure the patient is not rotated by checking the medial margin of the clavicles. The midline spinous process (posterior midline structure) should be exactly centered between the medial margins of the clavicles (anterior midline structures); and in turn the radiolucent trachea should be centered over the spinous process (see below).


b. Inpiratory effort: When a chest radiograph is taken, the patient is instructed to take in a deep breath. This expands the lungs and allows us to examine the lung fields. In a patient who has for some reason not taken a deep breath (ie, hypoinflated), the film may falsely look as though there are more prominent lung markings than normal or that the heart is enlarged. You can check for good inspiratory effort by counting 9-10 posterior ribs in the lung field. Over-inflation can also be abnormal and is often present in emphysema (and occasionally in very healthy young adults with terrific inspiratory effort).

c. Position: First check if left and right are correct. Remember, the film should look as though you are facing the patient, with the patient’s right on the left side of the screen and vice versa. Usually there is a handy marker placed by the x-ray technician: [see red circle on figure A above.] If you’ve been paying attention, you may have noticed that figure B is backwards (as is the chest image on the opening credits of the TV show Scrubs.  Radiologists are deeply offended by many medical shows, since a Radiologist is rarely shown yet  some general internist or surgeon appears to perform and interpret everything, from MR to angiography to fetal US…’not’.).

You also need to check whether the film is AP or PA. When an x-ray is taken, the patient is placed between the x-ray source and x-ray detector (image cassette), with the patient as close to the film cassette as possible. Why? Think about making shadow puppets in a dark room. When you place your hand closer to the flashlight and further from the wall, the image on the walls is large, fuzzy and distorted. When you move your hand closer to the wall, the shadow is much more sharp and true-to life in size and shape. Likewise, the closer the patient gets to the film cassette, the better the image.

When the patient stands against the plate, facing the image cassette, this is called a PA view (posterior-anterior). The heart is closer to the anterior chest wall and thus the detector plate, so in this type of view, the image of the heart is more geometrically accurate and sharp. This is the preferred frontal chest image.

When a patient is too sick to leave the bed, a portable chest x-ray can be taken. In this situation, the cassette is placed posterior to the patient in bed so the patient rests the back against the plate and elevated mattress. This is called an AP image (anterior-posterior). The heart is farther from the plate and may be magnified, thus you CANNOT assess for cardiomegaly (enlarged heart size) on an AP study. Portable studies also tend to be poorer quality because patients often cannot be properly positioned, may not be able to sit completely upright, and often cannot take or hold a deep breath.

You can identify whether a film is PA or AP by reading the margins: 

d.Exposure: Like photographic film, radiographs can be over- or under-exposed (therefore technically limited) depending on the amount of energy used and the duration of exposure. An underexposed study is too light (white) whereas an overexposed study is too dark (black). If this seems counterintuitive to you, the reason is that the radiographs we see are actually negatives of the original image by convention. Dr. Howell has popularized a mnemonic to remember this classification: a radiograph is like a piece of toast: when it is underdone (under exposed) it is too white and when it is overdone (over exposed) it is too dark. Adequate penetration is assessed by looking at the thoracic spine. You should be able to identify the distal thoracic vertebral body end plates through the cardiac silhouette just proximal to the midline convergence of the hemidiaphragms.

4. It is now time to evaluate the film itself, which means it is also time for another mnemonic. While there are many systematic ways to go through a chest film, one simple starting point is  ABCDEF(f)G:

a. Airway: Check to see if the airway is midline and patent, following its path all the way to the carina and main bronchii. The often-obscure carina position can be approximated by finding proximal left main bronchus and working retrograde towards the midline and is an important landmark for intubation assessment. In a tension pneumothorax, the collapse of one lung can cause the trachea to shift. This is an emergency- you must recognize it as soon as possible BUT caveat—did you check for film rotation first? ( Rotation is FAR more common than tension pneumothorax or other causes of true tracheal shift! Which, P.S., is far LESS common than those medical TV shows would lead you to believe). In children, a foreign body or acute laryngotracheobronchitis (aka viral croup) can also cause upper airway obstruction.

b. Bones: Next, check the ribs, spine, clavicles and arms for any signs of pathology. Look for fractures, excess curvature of the spine (scoliosis), radiodense or radiolucent or destructive bone changes which may indicate a neoplastic or metastatic process, missing or deformed ribs, and dislocation or arthritis in the shoulder. On the lateral, assess vertebral bodies for isolated or multi-level compression, anterior vertebral body wedging, bone destruction, end plate irregularity, or  diskitis (infection destroying apposed end plates)

c. Cardiovascular system: An easy way to assess for cardiomegaly is to compare the diameter of the base of the heart to the diameter of the thoracic cavity just proximal to the hemidiaphragms (cardiothoracic ratio: diameter of the heart over the widest diameter of the lower chest, on a PA image). The heart should be less than half the width of the basal thoracic cavity. Remember, because of the distorted projection on an AP image, the heart often appears 'larger' than on the last PA image-- can't compare apples to oranges.

Below is a PA radiograph of someone with CHF. As you can see, the widest point of the heart is greater than half the width of the thoracic cavity. CHF can cause cardiomegaly as well as pulmonary congestion and pleural effusions. In this film, effusions are not present but we can also see diffuse opacity which on a properly exposed image is a sign of pulmonary congestion. There is also an implanted cardiac assistance device-- nice generic term to use if you are unclear what something is, or have not yet learned to distinguish the implanted defibrillator (like this) from a simpler pacer implant.

d. Diaphragm: The diaphragms should be superiorlyconvex, symmetric and sharply outlined. Bilaterally flat diaphragms can be a sign of over inflation (emphysema) while an asymmetric/unilateral diaphragm change may be an indication of pneumothorax, lung collapse, phrenic nerve injury or other processes. Apparent hemidiaphragm elevation may actually represent fluid collecting at the lung base; being a similar radiodensity, it spuriously ‘elevates’ the hemidiaphragm silhouette, as with the right hemidiaphragm below. Also remember to look for air under the diaphragm, which is a sign of pneumoperitoneum.

e. Effusions:  Pleural effusions are collections of fluid in the pleural space. A small amount of fluid (2-10 cc) is normal and required for lubrication. Pleural effusions may visible on chest images, allegedly when the volume of accumulated fluid is at least 200-300 cc.but this is markedly modified by position, habitus, one vs two views, etc.  The posterior costophrenic sulcus is much deeper than the lateral costophrenic angles (CPAs) in most patients,and on frontal projects inferior to the hemidiaphragms, meaning a great deal of fluid may be present but not diagnosed when there is a frontal image only.  Abnormal collections of fluid in the pleural space come in two flavors: transudative and exudative. Transudative fluid accumulates due to increased hydrostatic pressure (eg CHF) or decreased oncotic pressure (eg hypoalbuminemia). Exudative effusions include blood, pus or lymphatic fluid.

You can spot pleural effusions by checking the angle between the diaphragm and the chest wall, known as the costophrenic angle. The angle should normally be sharp and pointing caudally. When fluid accumulates in the pleural cavity, it collects at the lowest point and forms a meniscus, which is why pleural effusions appear as curved lines blunting or obliterating the angles of the diaphragm. In the case above (PA) and below (lateral), the effusion is extensive:


f. Fields: the lung parenchyma itself, including the first few generations of bronchial brnaches, the alveolar spaces and the fine tracery of more radiodense vessels extending from the hila towards the periphery.  
f. Foreign bodies: Another "F".  Some of the more interesting radiographs you will see will include foreign bodies. This being Baltimore, you’ll find bullets in almost everyone (almost). You may also find yourself asking, what did that kid eat? And how did that collection of needle fragments get there? More commonly, chest studies are used to evaluate intentional therapeutic iatrogenic foreign bodies, such as chest tubes, lines, and endotracheal tubes. Some simple rules of thumb to assess for correct placement:

i. Chest tube: both tip and side hole medial to chest wall, not kinked.
ii. Line: apparently directed caudal in superior vena cava or cavo-atrial junction (can’t confirm unless there is a lateral as well)
iii. ET tube: in ‘safe zone’, ie, distal to T1-2 and at least 2 cm proximal to the carina. iv. NG tube-- both tip and side hole must be distal to the lower esophageal sphincter, approximately positioned where the medial left hemidiaphragm margin dips caudally towards the midline. This area is cut off or poorly seen on many patients, requiring an image centered over the hemidiaphragms (ok, you will hear this from someone: a view some have dubbed 'Chabdomen").

The patient below has two foreign bodies: a superior vena cava stent and a central line:

      Many people put the lung fields last on the list on purpose, since this area is the “gorilla”-- usually the first (and only) feature that gets any attention on a chest film. When examining the lung fields, symmetry is your friend, so remember to compare right and left, keeping in mind lung volume and markings.

A common pathology that you may be asked to identify is that of pneumonia. Bacterial pneumonia can appear as fluffy, opaque patches on chest films. The combination of these opacities and so-called “air bronchograms” are indicative of a consolidative pattern. Consolidation is characterized by large areas of fluid filled or increasingly radiodense alveoli and can be caused by multiple processes not limited to pnemonia, including hemorrhage, ARDS and malignancy. The confluent radiodensities may heighten visualization of the radiolucent air-filled bronchial branches.

Infectious, inflammatory, or edematous changes may also present with an interstitial pattern, fine linear branching and tapering reflecting enhanced visualization of engorged or thickened underlying parenchyma. (In reality, most processes, even if seemingly predominantly interstitial or alveolar, prove to be mixed at higher resolution of CT). 

Nodules have a wide differential and may represent benign or malignant tumors, a granuloma of TB or sarcoid, or even a septic emboli. Keep your patient’s history and immune status in mind when creating a differential. In assessing a potential new nodule, it is imperitve to confirm if there are old films, and if they show the same finding—or if perhaps they show it has grown or changed in the interval. The lung fields may also show areas of opacity.

Also note the contours of the mediastinum. A widened mediastinum has a broad differential and will be discussed in further detail below.


G. Gastric bubble - under the left hemidiaphragm in upright patient; helps assess position of patient (especially APs which may be modified by semi-upright position). Also helps correlate R/L labelling, and may help decide if an 'elevated' left hemidiaphragm is truly elevated, or is there is a large fluid collection at that base.  
4. The lateral view is an important tool when examining a chest study because it can help you determine the depth and anterior-posterior extent of a lesion. Consider for example a chest image which shows a round lesion in the left lung field. While this may initially seem like a cause for concern, the lateral image may demonstrate that the “lesion” is very superficial and is, in fact, a nipple or related to clthing artifact or chest wall finding external to the bony thorax. On the other hand, a similar appearing lesion, when evaluated with a lateral film, may actually be within the lung parenchyma and therefore a clearly more significant finding.

Any two-dimensional image of a bulky 3 dimensional structure can only show two of the 3 (X,Y,Z) axes at a time. One important function of the lateral image (and why One View Is No View) is to distinguish the anterior-posterior Z-axis  which allows deternimation of position and therefore differential diagnosis of a mediastinal lesion, or may show fluid deep in the posterio costophrenic angle as discussed under 'E". :

c. Anterior Mediastinum: located between the ventral cardiac surface and the sternum. If you see a mass, think of the “4 T’s”:

i. Teratoma
ii. Thyroid
iii. Thoracic Aorta
iv. ‘Terrible Lymphoma’ (Thomas Hodgkins, making this a Terrible mnemonic)

d. Middle Mediastinum: located between the anterior surface of the spine and the ventral cardiac surface. Lesions here should make you think of enlarged lymph nodes, as are found in lymphoma and other cancers, esophageal anomalies, or hiatal hernias.

e. Posterior mediastinum: located behind the anterior surface of the spine. Masses here most often originate from the nervous system and include schwanomas and meningocoeles but can also be metastatic spinal disease.

i. Spine sign- As one scans the posterior chest downward along the T spine, the spine should become less dense on the lateral chest film. If the vertebrae are more radiodense inferiorly, this is most often abnormal and is referred to as the “spine sign.” Think of pneumonia, a mass, atelectasis or pleural disease in the lower lung.

And remember—while it is advisable to acquire a certain familiarity with chest radiographic analysis, it is imperative that you pursue and document the final official interpretation of the study. For the rest of your careers, it will be absolutely medically and legally mandated that you are aware of and consider the official final interpretation of any requested test—imaging, ECG, lab work, histopathology or cytology, microbiology, specialty physician consultation—in a timely manner.  You will get better at picking up acute and chronic abnormalities, but the Radiologist is trained to do so faster and (we certainly hope) more precisely.  Radiology also often picks up unexpected 2nd or 3rd findings (the metastasis, bone lesion, old granulomatous disease, IVDA needle fragments...) unlikely to be seen by the house officers or non-radiologists. Remember also that the monitors in Radiology are far higher resolution than the vast majority of computer monitors in clinical areas, and allow image manipulation, magnification, and other technical tricks to enhance appreciation of subtle findings.

    Chest imaging is a skill acquired gradually, in layers-- be patient, keep practicing, ask questions, and compare the radiograph to the subsequent or prior radiographs or CTs. GO FOR IT!