Radiation Dosimetry and Safety

Thomas H. HauserMD, MMSc, MPH, FACC

Director of Nuclear CardiologyBeth Israel Deaconess Medical Center

Assistant Professor of MedicineHarvard Medical School

Boston, MA

• A study published in May by Yale University researchers found that radiologists and emergency room physicians were largely unaware of how much radiation the scans delivered, and now a study by Columbia University researchers highlights the risk of dying from radiation-induced cancer. A single full-body scan delivers a radiation dose nearly 100 times that of a screening mammogram and only slightly lower than the dose received by atomic bomb survivors a mile and a half from the burst sites. That's not a huge added risk (only a 1-in-1,250 risk of dying from cancer), but people worried about hidden disease would presumably want repeated, regular screenings. Full-body scans administered annually for 30 years starting at age 45 could cause one cancer death in every 50 patients, the new study estimates. That is a disturbingly high risk for a procedure of dubious benefit.

Annual Number of CTs in USA

Brenner and Hall, NEJM 357 (22): 2277

Outline

• Radiation dosimetry• Radiation safety

Outline

• Radiation dosimetry• Radiation safety

Measures of Radiation• Activity: Becquerel (= 2.7*10-11 Curie)• Absorbed dose: Gray (= 100 rad)

– 1 joule of energy deposited per kg of absorber

• Biologically effective dose: Sievert (= 100 rem)– Sievert = Gray * Q * N

• Q , quality factor– Photons, electrons, = 1– Alpha particles, = 20

• N , depends on type of body tissue– For most tissues, = 0.05– For gonads, = 0.2– For bone marrow, colon, lung, stomach, = 0.12

Example Radiation Exposures

http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/radexp.html

Your Exposure Increases by 0.01 mSv from…

• Three days of living in Atlanta • Two days of living in Denver • About seven hours in some spots in the

Espirito Santo State of Brazil. • An average year of TV watching (CRT) • A year of wearing a luminous dial watch • A coast-to-coast airline flight

http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/radexp.html

Radiation Biology (Radiobiology)

Radiation Biology (Radiobiology)

Radiation Dose Ranges for Medical Imaging

Stochastic Processes

• A stochastic process is one whose behavior is non-deterministic in that a state does not fully determine its next state. Stochastic crafts are complex systems whose practitioners, even if complete experts, cannot guarantee success. Classical examples of this are medicine: a doctor can administer the same treatment to multiple patients suffering from the same symptoms, however, the patients may not all react to the treatment the same way. This makes medicine a stochastic process.

www.wikipedia.org

Health Effects of Radiation• Stochastic health effects

– Long term, low level radiation exposure

– Increased exposure increases likelihood of the effect• Increased exposure does not affect severity

• Effects typically occur late after exposure

– Cancer, teratogenic effects, genetic (germ line) effects

• Non-stochastic health effects– Short term, high level radiation exposure

– Increased exposure increases both the likelihood and severity of the effect

• Effects typically occur shortly after exposure

– Burns, radiation sickness, organ failurewww.epa.gov

Health Effects of Radiation

• The health effects of radiation from medical imaging are stochastic– Low level radiation exposure– Typically multiple discrete exposures– Cancer is the main health effect of concern

Models to Determine Radiation Risk• Linear no threshold

– Risk from stochastic health effects decreases linearly with biologically effective absorbed dose

– Implies that there is risk to even low levels of radiation

– Most widely accepted model

• Linear with threshold– Risk from stochastic health effects decreases linearly with dose

until the radiation level crosses a threshold beyond which there is no risk

• Hormesis– The hypothesis that low doses of radiation are beneficial while high

doses are harmful

– Widely rejected

Cancer Risk Estimates

• Atomic weapon survivors– 25,000 survivors with exposures of <50 mSv– Significantly increased risk of cancer

• Nuclear power plant employees– 400,000 studied with mean dose of 20 m Sv– Significantly increased risk of cancer

• Information from both cohorts quantitatively similar

Brenner and Hall, NEJM 357 (22): 2277

Cancer Risk Estimates

Brenner and Hall, NEJM 357 (22): 2277

CT Radiation Doses

Nuclear Radiation Doses

Toohey et al, Radiographics. 2000;20:533-546

Coronary CT Radiation Dose

Einstein et al, JAMA. 2007;298:317-323.

Radiation Exposure

Einstein et al, JAMA. 2007;298:317-323.

Radiation Exposure

Einstein et al, JAMA. 2007;298:317-323.

LAR for 80 yo man with heart only ~1/5000

Outline

• Radiation dosimetry• Radiation safety

Radiation Safety

• Time• Distance • Shielding

Time

• Spend as little time as possible near radiation– Delivered dose is a function of time

Distance• The dose of radiation decreases as the square

of the distance between you and the source– Increases as the square of the distance as you get

closer

Shielding

• If you must spend a significant amount of time near a radiation source, use as much shielding as possible– Lead– Plastic

Lead Shielding

Plastic Shielding

• Best for β emitters– Prevents bremsstrahlung

Why is the Dose So High for Coronary CTA?

• Pitch– Typical pitch for helical/spiral acquisitions is 0.2

• Each part of body is irradiated 5 times

– Increases time of exposure

• kV, mA– Relatively high values to obtain high quality

images with high spatial resolution– Increases intensity of exposure

Retrospective Gating: Helical/Spiral

• Data acquired throughout the cardiac cycle– Reconstruction of entire dataset for evaluation of

LV function– Radiation dose of 15 to 21 mSv

Prospective Gating: Dose Modulation• Tube current is varied over the cardiac cycle to

produce high quality images during ventricular diastole (coronary quiescent period).

• Optimal image quality only during diastole– Evaluation of LV function still possible, but not optimal

• Radiation dose 7 to 12 mSv (40% reduction)

Prospective Gating: Step and Shoot• Tube voltage turned on prospectively only

during ventricular diastole• Data not acquired during systole

– Limited range of reconstruction– No assessment of LV function

• Radiation dose of 3 to 5 mSv (70% reduction)

Dual Source CT

McCollough, C. H. et al. Radiology 2007;243:775-784

Radiation Exposure

Hausleiter et al. Circulation, 113 (10): 1305. (2006)

Summary• Radiation dosimetry

– The sievert is the preferred measure of radiation exposure– Medical imaging results in low dose exposure with stochastic health

effects– The principle adverse outcome for low dose exposure is cancer– Coronary CTA delivers a relatively high dose of 15 to 21 mSv that

results in a non-trivial risk of cancer

• Radiation safety– Time, distance, shielding– For coronary CTA, pitch and exposure intensity are the primary

determinants of radiation dose– Employing dose modulation, prospective gating, dual source CT and

lower kV may reduce the radiation dose