Basic Principles and Introduction to Chest Radiographs

Basic Radiology Module
Transition to the Wards 2011, revised 2016 D. Magid MD M.Ed

Kopal Kulkarni MS IV and Donna Magid MD, M.Ed

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 plain film that will be the best and most cost-efficient test?  We need to know what your clinical question is ('R/O subdural"; "R/O aspiration"; "Hx breast Ca, new left lateral rib pain"); we 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!   "You should also have an idea of how each of these ....used best." 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.  Refer to or Google a test before attempting to explain it to a patient-- you are unlikely to know exactly how an enteroclysis or bone scintigraphy  or other mysterious new detail can best be explained otherwise. You should also have an idea of how 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, you can always ask your medical team. But remember, the real experts in imaging are the radiologists. Even medicine and surgery docs can become confused with the many options of imaging (Contrast? Non-contrast? Which protocol to use?), so if there is ever any question, 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, ordering it is relatively simple. Requesting imaging is done on the computer through either POE at Hopkins or Meditech at Bayview.
Remember that requesting an imaging study is more like calling a consult for a procedure 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 ordered the correct test, but also will help them focus on particular findings and give you 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. Though x-rays were discovered in the 1870’s, the first paper describing possible medical uses was written much later 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 

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. In a very dense tissue such as bone, fewer beams will pass through the tissue; these areas will appear bright 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 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 order plain films prudently; every patient who steps in the hospital does NOT need a chest film. Situations in which plain films are helpful and commonly ordered:

In general, this is a good modality for detecting regions or tissues well-visualized on plain film,such as most bones, the lung fields, the mediastinal outline, the distribution of air in bowel, alignment of spine, etc. Radiography (plain film) is NOT the best choice for most soft tissue or vessels, or brain or neural tissue, or radiolucent structures such as ligaments, tendons and cartilage.

  • Lung processes- Pneumonia, lung nodules, pneumothorax, COPD (chest film)
  • Chest pain- aortic dissection, cardiomegaly and other findings may be seen on plain film (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', 'plain film '(an anachronism in the digital age--no more film!) or 'radiograph' and not “x-ray” when referring to a visible image.


  • Do not image a pregnant women in the area of the fetus due to radiation risk- always ask/test a woman if she might be pregnant, and discuss with (and forewarn!) Radiology. (see Kiddy Physics section)


Computed Tomography (CT)

CT scanning was invented in 1972. Since then, it has developed greatly and is now a fast, readily available, and highly detailed 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 just a few years ago meant first 4, then 16, then 64, and now 256 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, creating enormous concern and heightening both medical and layperson awareness of the need for careful technical modifications and cost/benefit analyses.

Since CT scanners use the same x-rays that plain films use, there are tissue/structure similarities in terms of radiodensity. Bones are more radiodense, and will thus show up as white, whereas lungs are mostly air and will be radiolucent just as on plain films. Just as with plain films, the black/white is determined by the relative radiodensity or radiolucency of tissues and the path length (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). “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) through dense 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 foot 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 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 Hounsfield units. 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.

More radiodense or more radiolucent?

Lower or higher 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 (yes, they are actually called this!) which have 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, you can be reassured that your patient is unlikely to have a malignant adrenal cancer. 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 order 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. Some general guidelines on clinical application:

There are four major substances that typically are quite radiodense (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 plain film may be perfectly sufficient. Using '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 equipment, 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 plain film 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 on campus may take closer to 400 lb but as of now, patients exceeding that cannot be scanned.  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 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


Magnetic Resonance Imaging (MRI)


MRI is a relatively new imaging modality; it was developed in the early 1970’s but 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 magnetic field aligns (polar) hydrogen atoms in the body. When the field is turned off, the hydrogen 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, here you go).

When looking at an MRI, you will notice that there are a variety of different types of sequences that are produced, most importantly T1 and T2. 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 where a CT scan make take far less than a minute. During this time, patients must be able to lie very still within the machine. For most body parts, this means laying inside a long narrow enclosure which some patients describe as ‘coffin-like’.  As you can imagine, this may be difficult for claustrophobic patients or those with pain or mental status changes who cannot stay still. Also, as a responsible doctor-to-be, 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
  • 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) .


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.
  • Unstable vital signs
  • Obesity> 300lbs (Again, no- there is no MRI at the Zoo).  This reflects both the mechanical limits of the table, and the very rigid inner diameter of the tube, 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 a BAD IDEA 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 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 impress your residents by using the correct terminology to describe structures: hypoechoic or sonolucent for dark areas (cysts, vessel lumens), and hyperechoic 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, and an artery from an adjacent vein, 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.

Ultrasound is safe for almost all patients (how often can you say this?!?) as it operates on sound waves and thus emits no radiation. It does not require a patient to be “healthy” or stable enough 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), or to be able to hold absolutely still.  There is no preparation or contrast needed (except the hydration and full bladder preferred for abdominal/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 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.
  • 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 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 this website, 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

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 film. 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. 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 (fairly lame) mnemonic which may help here is RIPE (is the film RIPE to be read?):


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


b. Inpiratory effort: When a chest x-ray 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 is more pulmonary congestion than normal or that the heart is enlarged. You can check if there is 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 film on the opening credits of the TV show Scrubs.  Radiologists are deeply offended by these shows, since the Radiologist is rarely shown yet the average 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 (film cassette), with the patient as close to the film cassette as possible. Why? Think about making shadow puppets. 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 film 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 film, 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 film cassette is placed posterior to the patient in bed so the patient  has their back to the plate. 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 cannot be properly positioned, may not be 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. A brilliant mnemonic to remember this classification, brought to you by Dr. Howell, is that a plain film 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, but not below the diaphragm.

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 of the most popular methods among the house staff is ABCDEF plus the lungs:

a. Airway: Check to see if the airway is midline and patent, following its path all the way to the carina and main bronchii. 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? FAR more common than tension pneumothorax! 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 size of the heart to the size of the thoracic cavity (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 film, you should not make any conclusions about cardiomegaly on an AP film.

Below is a PA radiograph of someone with CHF. As you can see, 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 is a sign of pulmonary congestion. There is also evidence of an AICD (implantable cardioversion defibrillator)- a device given to patients with severe heart failure.


d. Diaphragm: The diaphragms should be convex, symmetric and sharply outlined. A symmetrically flat diaphragm can be a sign of over inflation (emphysema) while an asymmetric  diaphragm may be an indication of pneumothorax, lung collapse or other processes which could cause a differential in lung volumes. 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 are visible on chest films when the volume of accumulated fluid is at least 200-300 cc. Abnormal collections of fluid in the pleural space come in two flavors: transudative and exudative. Transudative fluid accumulates due to increased hydrostatic pressure (ex/CHF) or decreased oncotic pressure (ex/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 to form 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. Foreign bodies: 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.

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

g. Lungs and Mediastinum- 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.


4. The lateral view is an important tool when examining a chest film because it can help you determine the depth and anterior-posterior extent of a lesion. Consider for example a chest film 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. 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.

One important function of the lateral film is to distinguish the z-axis position and therefore differential diagnosis of a mediastinal lesion:

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’ (Hodgkins)

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 the vertebrae progress downward, the spine should become less dense on the lateral chest film. If the vertebrae are more radiodense, this is most often abnormal and is referred to as the “spine sign.” Think of pneumonia, a mass, atelectasis or pleural disease.

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 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.