Kiddy Physics

2018 - Donna Magid, M.D., M.Ed.

Opinions are mine alone, and do not reflect the Dept., Institution, or American Board of Radiology.  Potential errors also mine alone, accuracy not guaranteed. For orientation/introductory purposes only, do not disseminate or quote.

Brief History

  • Nov. 1895: Roentgen publishes description of ‘x-rays’, capable of penetrating tissue and creating an image on photographic plates.
  • 1896: 1st 1000 articles (some of dubious medical value) follow (Lancet 1897 described locating a baker’s ring in a freshly baked cake).  (Over the next few years, an x-ray crazy public would start exchanging hand images instead of photographs—although the first report of radiation burn to the hand also appeared this year, in “Deutsche medicinische Wochenschrift”-- or other tokens, and would be offered irrelevant and dangerous products and services such as a 15 minute x-ray exposure ‘guaranteed to cure headaches”, similar ‘x-ray headache tablets,' x-ray-associated shoe polishes,radium-laced  toothpastes, cocktails, stove cleaners,and  golf balls [more bounce?], and - perhaps most horrifying—radioactive “disease-preventing prophylactics,” aka condoms).  
  • 1896: was also the year a child with a gunshot injury (GSW) to the head was brought to Vanderbilt.  Before attempting a head study, a physicist asked the Dean of Medicine to sit for a trial exposure.  One hour with the tube close to the head produced an image, but also led to toal hair loss and skin irritation within 3 weeks.  Later that year, a man with an old intracranial GSW asked Minnesota physicians to find his bullet.  Allegedly that exposure was fourteen hours -- producing the desired localizing images but also leading to ferocious blistering and marked swelling within 24 hours, with hair loss, massive skin breakdown and ulcerations within the field.                                                                                                                         
  • 1897: Osler proposes acquiring a ‘Roentgen Ray Apparatus” (RRA) which does not arrive until 1901, with Drs.Baetjer and Cushing.
  • 1901: Roentgen awarded first Physics Nobel prize.  Exposure times at this point ranged from the shortest - 5 minutes - up to 20 minutes.
  • 1902: 1st skin neoplasm reported
  • 1910: 1st published report of Roentgen ray ‘poisoning’ (see 1896-- slow learners?)
  • 1928: 1st Committee on Protection and Mortality.  Established measurement units, little else.
  • 1929: Committee advised ‘Skiagraphers’ to wear ‘kid gloves”. Great Depression Oct. 1929.
  • 1936: “X-Ray Martyrs” published; a monument to ‘radium martyrs’ inscribed with the names of those dying of radiation-associated disease was also erected in Hamburg, Germany. Dr. Baetjer had died, age 59, in 1933 of radiation-related disease. Extensive over-exposure of his hands in particular had led to decades of non-healing ulcerations and lesions, over 100 surgical procedures and progressive amputations,  and death from radiation-induced neoplasms.
  • 1976: First mandated room shielding. NOW: Radiation protection, monitoring and usage rules at national, state and Institutional levels; mandated protection of individuals and adjacent rooms; mandated monitoring, inspection, licensing, quality assessment, performance reviews......We’ve come a long way!
  • 2006: ‘SENTINEL EVENT’: First time that medical/dental/human-generated exposure dose per capita on planet exceed natural/background radiation levels. Biggest contributor to this 600% increase over 25 preceding years:  CT.

1st Century: Analogue, film, hard copy”→ 2nd century: Digital imaging.  Principles of imaging remain the same:  x-rays pass through material (patient), attenuated variably by materials encountered land on imaging substrate carrying a meaningful pattern reflecting these varied attenuations converted to visible patterns of black/white grey.

Initial images were generated 100% by x-rays penetrating tissue.  We now have very sensitive phosphors lining the receiving cassette, and techniques which can ‘read’ the penetrating/attenuating pattern of a far tinier x-ray dose, reacting by generating visible light preserving the original x-ray penetration pattern, which in turn exposes the cassette and creates the image.  Current images are therefore are created by about 1% primary x-ray dose and 99% by the secondary visible light from x-ray excited phosphors.  This is the basis of the film or digital cassette for radiography, and of the image intensifier (invented by Russell H. Morgan, 1953, JHU SOM) for fluoroscopy.



  • Fluoroscopy tricks: image-capture (study static image on screen, not live image), after-image retention instead of constant fluoro                                           
  • Use shortest fluoro possible

DISTANCE: dose diminishes spatially as l/d2 so at 1 m (arm's length) , dose diminished to 1/1000th

  • Work with extended arms (decreases trunk dose)
  • Observe from distance, ‘live’ rooms ; step back/out before exposures, OR, EMed
  • Don't hold patients!

SHIELDING: ''lead'' aprons--care and feeding covered in Elective (DO NOT FOLD or CRUMPLE)

  • “Lead” thyroid shield, glove
  • Leaded eyeglasses (or real glass with photogrey coat)
  • See-through barriers on wheels
  • “Lead” drapes on image intensifier
  • Monitoring: confirms appropriate technique, observance of rules and regulations

SCATTER =s Primary source of dose to workers—know the enemy! ''Scatter'':  x-ray deflects from primary  path, decreases contrast, increases ''noise''

  • CONTRAST is useful difference in signal, subject/film
  • NOISE is useless info which diminishes image quality/spatial resolution
  • Primary source of dose =s scatter therefore =s PATIENT, if you avoid  primary beam.

REDUCING DOSE: DEVELOPE GOOD HABITS                                                    

  • Collimate, cone, center (i.e.—tell us precise one-finger area of interest, not just ‘Foot’!)            
  • Don't hold patients if possible; use gloves/thyroid, eye shields/barriers if must          
  • Decrease scattering volume in fluoro (lower the tower close to patient)
  • Fluoro in lieu of hard copy (20-30 sec fluoro =s one radiographic dose)              
  • Counterintuitive: in OR, face the primary beam/source (ask techs for help; confusing)
  • Use lowest possible exposures (Peds v. Adult, dose modified by weight, size, body part)

“BALL PARK'' DOSE - variables are TNTC, so quote with care and use salt.

  • CXR (Ur unit) 20-30mR or 0.2-0.3 mSv per view (x 2, PA/Lat)
  • Abdomen 3-500 mR or 3-5 mSv per view (x 2 vws, or 3 if CXR)
  • LS 600 mR or 6 mSv per view (x 3)--as of Oct. 2017, x2 at JHM
  • Mammo 70 mR or 0.7 mSv (x 3 vws.each side)
  • CT starts at 1000 mR or 10 msv (ACR website): As slices and cephalo-caudal scanned volumes increase, so does dose. Larger/heavier pts, denser/thicker body parts, need higher dose per volume Delivers higher deep/internal/organ dose than radiographs (i.e. #s misleading). “Doc in Box” prophylactic scans NOT YET dose cost/benefit effective! Over time will dose cause as many problems as are dx’d? ‘Over-treatment' issues, incidentalomas, and problems under investigation. Multi-center academic study and review (vs sell-to-consumer advertising)

“COST/BENEFIT RATIO'' ''Will the result affect management/outcome?'' Tissues of interest--gonads, marrow, thyroid, breast, lens, growing physes/ brain. Patient factors--age, fertility, life expectancy, informed consent, DNR, living will


  • Assesses utility of specific imaging procedures for ~200 specific clinical situations
  • Modalities ranked 1-9 (9 being most appropriate/informative/specific and/or sensitive) Lower-ranked tests add cost and dose without commensurate clinical/diagnostic impact
  • Still evolving (2x/yr); better for some DDx than other (e.g., excellent for 'R/O PE'')
  • ACR AC are found at under “Quality and Patient Safety'' - USE THEMI!

Each modality offers opportunities to optimize (reduce, limit or triage) dose to the patient and personnel. COST/BENEFIT considerations must be considered.  E.g., consider body CT:

Both # of CTs obtained annually and indications to get a CT  soared from 1970s to 2007. So did the number of slices, energies, and over-all exposures.

Evolving technology has dramatically raised the dose per study and #pts/day/machine.

Faster thinner ‘slices’ have markedly raised resolution and ability to ‘freeze’ motion, allowing huge advances in pulmonary, cardiac and vascular imaging; and decreasing artifact/noise, and making CT possible on pts with rapid respiratory or cardiac rates or trouble holding still (children, neuro dz, altered mental status).

SLICES'': transaxial CT often explained as 'slicing a loaf of bread. Earliest scanners started +2" thick slices and hrs acquisition per slice in pre-clinical CT, early 1970's 1.5 cm/1 minute in late 1970's (at which time scanners were commercially available), 2-3 mm thick/seconds in late '80's, allowing first 2D reformatting into other planes (coronal, sagittal, obliques) and first (simulated) 3D (e.g. Pixar 1986).

Dose also “progressed” from about 1.5 R or 15 mSv/study, to (3.5-5) R or (35-50) mSv,

Spiral/helical CT~1990 introduced acquisition of rapid total cephalo-caudal volume acquisition, ex post facto computer- processed into thin 'slices'

4-slice appeared~1998, 16- and 64-slice by 2003-5 (~2-4 second acquisition); rapidly rising into the 100s over the subsequent 10 years. Multi-slice technique produces simultaneous parallel ‘slices’ per rotation, hence allowing acquisition of a greater cephalo-caudal volume in decreased time (with obvious gains such as decreasing patient motion, decreasing need to hold breath, and  decreasing sedation needs for children) —but increased dose due to longer scan length (body volume) and higher dose along that Z axis than with older slice-by-slice scanners. 

256-slice was in clinical tests 2007 (JHU Neuro temporary test site, May 2007). The same year the Sentinel Event started a dose/over-use backlash, and by 2016 and 2017 High-Value Care assessments and mandates were balancing and slowing the 30 year drive for faster thinner higher resolution slices. Unclear where risk/benefit end point will be with rising concerns over dose and cost, but (IMHO) this drive for data-based balance and greater attention to individual patient considerations is a win-win for all.. 

JHU Rads* (July 2012):  (courtesy of Dr. M. Mahesh, personal communication)

  • Zayed 4: 3 FLASH, 128 slices per rotation (2 Body, 1 Peds)
  • Zayed 3: 64-slice (Neuro)
  • Zayed 1: FLASH 128-slice (Emergency Med)
  • Weinberg:  64-slice
  • JHOC 3: 128- and 64-slice *all subject to rapid change!
  • Greenspring Station: TBD

By 2010 CT accounted for 12% of all XR based exams in US but 2/3 the total medical dose. This is going to soar if/when virtual physicals/colonoscopy are scientifically validated.

  • 6.1 CTs per 1000 people 1970-1979 vs 44/1000 by 1988
  • 6 million CTs performed in US 1980 -> 33 million 1998-> 62 million, 2006
  • 2.7 million CTS on children < 15 yrs. old in 2000                                                     (kids) and (adults) looking to modify CT utilization to reduce dosage and unnecessary or redundant imaging.  For example: getting old studies from elsewhere reduces redundant scanning, and increasingly simple with CDs, jump drives, and electronic transfer.  Old machines had convenient but limited ‘Adult/Pediatric” pre-set buttons for different tissues or body parts (head, body, bone, lung, mediastinum,...) to set scanning parameters; newer systems (and many regulations!) fine-tune these settings further by requiring consideration of weight (fine-tuning settings for a 130 lb vs a 250 lb adult), body part size (ankle, leg vs. bony pelvis and abdomen), gender, etc.

OPTIMIZING CT DOSE: Takes time, thought, attention to each patient; worth it.

  • Reduce the # of CTS performed: “Will findings change management?”
  • Reduce the dose used, children (easier to penetrate, smaller...if not obese!)
  • Reduce adult dose proportional to weight/volume (95 vs 375 lb)
  • Reduce dose for pulmonary (tissue easier to penetrate)
  • Reduce volume (cephalocaudal field) studied (caveat-missed findings!)           

Evidence-based medicine: document best intervals for baseline/follow-up; document utility of information acquired by CT vs other means (e.g. MR, US)

Dose reduction counter-productive if it increases noise, decreases accuracy. Emphasize need to transport/digitize old studies, request old outside records, and give pts. CDs of new studies, to avoid repeat or unnecessary exams (JHU: Radiology Service Center-will digitize and load outside film or CDs)


Perform imaging requiring ionizing radiation during 1st 10 days of cycle from LMP; or document mitigating factors preventing pregnancy (BC, hysterectomy, hx)

Pre-implantation (1st 9d post-conception): inadvertent exposure believed to produce an ''all/none'' effect; fertilized egg either lost or, if implants, proceeds normally

Organogenesis (8-17 wks post-conception): ''ARTICLES'' multi-system impact of  any type insult at this stage (pharmaceutical, systemic illness, radiation, chemical,...); also intellectual compromise, reduced head circumference (HC). Radiation-associated anomalies tend to be CNS (microcephaly, reduced intellectual capacity) more than limb/organ system.

  • Ano
  • Rectal
  • Tracheal
  • Intestinal
  • Cardiac
  • Limb
  • Esophageal
  • Spine
  • (GU)

3rd trimester - bone marrow stem cells (leukemia-10 yr latency) 

POST-NATAL: assume “All radiation is bad'' (i.e. even tiny doses believed to make tiny tissue changes, many below observation thresholds or readily spontaneously  repaired) for safety’s sake, and then calculate  your patient’s actual risk/benefit. Children more vulnerable than adults, because of smaller size relative to dose; effects of early damage on growth, maturation, puberty, and tissue function; anticipated longevity to experience or allow eventual deleterious consequences; rapidity of cellular growth and turn-over magnifying change to even a small number of cells; and other factors TNTC. "Image Gently" guidelines; ACR AC guidelines, help make decisions.

Individual risk:

  • Leukemia, lymphomas in peds? (Chernobyl)
  • Mutagenic effects - lethal, non-lethal
  • Birth defects (2% incidence in healthy population)
  • Carcinomas (latency period 20-30 yrs), fibrosis; thyroid, cataracts
  • 2nd sarcoma- at radiation therapy, not imaging, dose levels

Population risk - mutagenic effects on gametes or normal tissues, contributing to later neoplastic or developmental  health care costs per unit #



*”The public's view of the dangers of diagnostic radiographs during pregnancy is greatly over-exaggerated.'' (Newsletter 2002)

*''5 R (5000 mR) considered cut-off for beginning fetal risk; malformations do not exceed control group until doses exceed 15 R; 25-80 R required to double the incidence of congenital malformations.''  Less than 5 R has not been associated w/ increased fetal anomalies or pregnancy loss (Am. Coll. Ob.Gyn, Nat'l Council on Rad Protection)

Radiation increases rate of known mutations; it (apparently) doesn't create new/bizarre ones. (X-Men: Fuggeddaboudit!)

Alcohol and/or smoking are associated w/ fetal abnormalities at a rate 40- 500 times greater than radiation. Don't forget that ubiquitous “2%” anomaly rate: in a population of normal healthy young pregnant women following all the rules, one can still expect approximately 2% rate of diagnosable ‘defects’ or anomalies, from trivial (accessory nipple) to severe .

Quotes one can find on-line:  many are about as reliable as Wikipedia, caveat emptor.  E.g. ”While it is undesirable to expose pregnant women to any radiation, the risk to the fetus is never high enough to contraindicate needed diagnostic radiographs. If a study is medically indicated, the benefits outweigh the risks to the fetus."

please note--I am not sure I agree with the wording of this one, especially that ‘never’! Wording from another site better though still debatable: ''Use reasonable caution, get informed consent, document such discussions, be sure information obtained would be necessary for/would modify patient care. Make sure such information could not be obtained by any other means (e.g., US)."


                                                         * * * * *

And a last word about those airport scanners to answer family queries (personal communication from Dr. M. Mahesh, Nov. 2010):

For all those who are traveling during these holidays, you must be bombarded with news of airport security including backscatter scanners, extended patdowns, etc. In this regard, I want to share with you my research regarding radiation doses from the backscatter scanners.

Generally, there are two types of scanners currently used in the United States:

  1. Millimeter radio wave scanners (appears like a glass booth) - do not use x-rays
  2. Backscatter x-ray scanners (appears like two refrigerators) - do use x-rays

The radiation dose from a single backscatter system is very low. A single backscatter scan is equivalent to about 30 minutes of natural background radiation or about 4-5 minutes of air-travel. One needs to undergo 1000-2000 backscatter scans to receive the radiation dose equivalent to a single chest x-ray dose.

For those who are further interested, please see an article that was published in the British Medical Journal earlier this year.

Have a safe travel!

Dr. Mahesh

Mahadevappa Mahesh, MS, PhD, FAAPM, FACR

Chief Physicist, Johns Hopkins Hospital

(NOTE-- in 2018, only the non-radiation radio wave scanners are in use.  NO radiation to passengers.  Luggage, however, is exposed to radiation. And there is that cosmic bombardment at 36,000 feet....)