Radiation and Medical Imaging

 

I.                  Basic Tenants Regarding Radiation

a.      All ionizing radiation is generally regarded as potentially harmful

b.      Data regarding the health effects of low dose radiation exposure (the dose ranges typical of medical imaging) are indirect, and have been derived from extrapolation from populations exposed to high doses of radiation (i.e., atomic bomb survivors, radium dial painters, patients with mastitis treated with radiation, and patients with ankylosing spondylitis treated with radiation). This method of calculating the effects of low-dose radiation exposure is known as the linear-no-threshold model (LNT) (1).

                                                              i.      BIER VII (2) report bases its reliance on the LNT model on the observation that more than 60% of exposed atomic bomb survivors received a dose of 100 mSv or less and an excess of cancers has been observed in this cohort

1.      25000 female atomic bomb survivors have been followed for over 50 years and 173 have developed breast carcinoma, of which 41 cancers have been attributed to radiation exposure (2).

                                                            ii.      Low doses are defined by BIER VII as 0 – 100mSv (2).

                                                          iii.      LNT model assumes no threshold or minimum radiation dose is required to produce damage, and risk varies linearly with radiation dose (2).

                                                           iv.      The LNT model has been challenged as an inappropriately conservative measure of the effects of low level radiation. In fact, considerable evidence shows beneficial effects (diminished cancer rates, diminished mortality) of low level radiation (1).

c.      Theoretical radiation exposure risks

                                                              i.      Induction of hematological malignancy within 2 - 5 years of radiation exposure

                                                            ii.      Development of solid organ malignancy (lung, breast, thyroid, sarcoma) often 20 years or more following radiation exposure

                                                          iii.      Fetal effects- pregnancy loss, developmental delay, microcephaly, mental retardation, growth retardation

 

II.               Radiation Terms, Units, and Effects

a.      Exposure

                                                              i.      Defined as the ability of radiation (x-rays) to ionize air

                                                            ii.      Roentgen- quantity of x-rays needed to produce 2.58 x 10-4 charge / kg air

                                                          iii.      Measured with an ionization chamber

                                                           iv.      Does not indicate tissue energy absorption

b.     Absorbed dose

                                                              i.      Measures amount of radiation energy deposited within a given mass of tissue

                                                            ii.      Gray (Gy - Systθme Internationale units) or rad

1.      Gy = 1 joule / kg

2.      1 Gy = 100 rads

                                                          iii.      Used to quantify organ doses, effects of acute radiation injury, total body irradiation, and fetal doses

                                                           iv.      Does not reflect radiation sensitivity of individual tissues

c.      Effective dose (aka dose equivalent)

                                                              i.      Sievert (Sv – Systθme Internationale units) or rem

1.      1 Sv = 1 joule / kg

2.      1 Sv = 100 rem

3.      10 mSv = 1 rem

                                                            ii.      Does account for individual tissue radiation sensitivity (i.e., lung is more sensitive to radiation-induced damage than brain)

                                                          iii.      Reflects the equivalent whole-body dose needed to produce the stochastic risk (see below) resulting from the actual dose

d.      Radiation – induced injuries

                                                              i.      Stochastic effects- no known threshold for induction, no dose – response relationship. Risk is cumulative over time.

1.      Examples- cancer induction, teratogenesis

                                                            ii.      Deterministic effects- have a threshold for induction, dose – response relationship

1.      Examples- cataracts, skin epilation

 

III.            Radiation Benchmarks

a.      Average yearly background dose: worldwide- 2.4 mSv / year,          3 mSv / year in US (3).

b.      Average lifetime background dose worldwide- 0.15 Sv (0.15 rem) – 5 Sv

                                                              i.      Approaching 5 Sv in Iran due to natural springs (3).

c.      Average background dose received by fetus during a 9 – month gestation- 1.5 mGy

d.      1 mSv is delivered from 140 – 350 transatlantic flying hours in a subsonic aircraft (4).

 

       Patient (Maternal) Radiation Doses with Common Imaging Procedures†

Modality

Dose

Screening mammo

0.6 – 3 mSv

Chest radiograph (2 views)

0.05 - 0.1 mSv

Abdominal radiograph (1 view)

0.55 - 1.7 mSv

Lumbar spine (3 views)

1.8 mSv

CT Head

1.3 - 2 mSv

UGI

3.6 mSv

Small bowel series

15 mSv

Barium enema

3 – 8 mSv

Chest CT (routine)

5 mSv

Chest CTA for PE

2.2 -  6 mSv

(breast absorbed dose: 20 – 50 mGy)

Coronary artery calcium scoring

0.6 – 1.6 mSv

Coronary CTA

6 – 12 mSv

Coronary angiography

3 – 6 mSv

Abdomen CT

5 – 7 mSv

Routine enhanced abdominal - pelvic CT

8 – 11 mSv

Abdominal - pelvic CT for flank pain

10 mSv

Whole - body CT

10 – 23 mSv

Ventilation – perfusion scintigraphy

Perfusion only scintigraphy

1.4 mSv

0.8 mSv

Pulmonary angiography

2.3 – 4.1 mSv

Whole – body PET

14 mSv

Sestamibi myocardial perfusion imaging (per injection)

6 – 9 mSv

Thallium myocardial perfusion imaging (per injection)

26 – 35 mSv

†Sources- (5-8)

 

IV.            Radiation  - Induced Injuries

a.      BIER VII estimates that an effective dose of 10 mSv (1 rem) in an adult produces a 1/1000 lifetime risk of radiation – induced cancer (9). By comparison, about 420 people of a cohort of 1000 are expected to develop some kind of cancer over their lifetimes.

b.      ICRP (International Commission on Radiation Protection) estimates risk of fatal radiation – induced cancer at 5% per Sv. Others have suggested that 1 mSv will produce 5 excess cancers in 100,000 radiosensitive patients.

c.      BIER: Childhood cancer risk is estimated at 0.06% / 10 mSv, and 1.2 – 1.5% for 100 mSv (10). Brenner (11) estimates 0.18% excess cancer mortality for abdominal CT and 0.07% for head CT performed in a 1-year-old.

d.      USDA (www.fda.gov/cdrh/ct) estimates cancer risk from single body CT scan is 1/2000

e.      Lifetime breast carcinoma risk (and breast cancer mortality)  according to age for a 2.5 mSv mean glandular dose (3, 12):

                                                              i.      At age 20: 11 (2.5) per 100,000

                                                            ii.      At age 30: 6 (1.5) per 100,000

                                                          iii.      At age 40: 3.5  (1) per 100,000

f.        45 – year – old adult who undergoes 30 annual (screening) body CT scans- lifetime risk of cancer is 1.9% (1 in 50). A 60 – year – old man undergoing 15 annual body CT scans has a lifetime adjusted risk of cancer of 1 in 220 (13).

g.      Radiation – induced malignancy

                                                              i.      Solid tumors have a latency of 10 - 40 years following exposure

                                                            ii.      Hematologic malignancy occurs within 5 years of exposure

 

V.               Radiation and Pregnancy

a.      Background information for perspective

                                                              i.      Spontaneous pregnancy loss rate- 15% (1).

                                                            ii.      Congenital anomalies are seen in 6% of live births, major malformations in 3% (1).

                                                          iii.      Risk of intrauterine growth restriction- 4% (1).

                                                           iv.      Risk of maternal death during pregnancy- 1 / 170,000 (14).

                                                             v.      Recommended radiation exposure gestational limit- 50 mSv (5 rem).

                                                           vi.      Risk of childhood cancer is considered negligible with fetal exposures less than 0.05 Gy (50 mGy).

1.      Baseline frequency of childhood cancer: 1/600

                                                         vii.      Pregnancy termination- recommended with doses exceeding 100 mGy (risk of neurological damage with such doses is deemed sufficiently high)

                                                       viii.      Risk of PE in pregnancy: 0.5 – 3 / 1000 [up to 5x that of non-pregnant women (15, 16)].

b.      Radiation risks during pregnancy

                                                              i.      Teratogenic risk maximal during organogenesis (weeks 2 – 8) (1).

                                                            ii.      Up to 20 weeks, risks include microcephaly, growth retardation, mental retardation, developmental delay (1).

                                                          iii.      In utero exposure at any time has the potential to contribute to the development of childhood leukemia.

                                                           iv.      In utero exposure 50 mGy associated with negligible increase in childhood malignancy.

1.      0.1 mGy exposure in utero estimated to be associated with and excess cancer death rate ≤ 1 / 300,000 at 15 years of age.

2.      1/500 risk of malignancy induction in fetus exposed to 30 mGy.

 

                     Fetal Radiation Dose: CTPA vs. V/Q Scintigraphy

CT Exposure (mAs)†

Radiation

Dose (μGy)

V/Q Scan*

Radiation

Dose (μGy)

100

3.3 – 130.8

2 mCi Tc 99m, 10 mCi Xe 133

360

110

3.6 – 143.9

Full dose

380 - 508

150

5 – 196.2

Half –dose, no ventilation

140 - 250

200

6.6 – 261.6

Xe 133

< 0.01 mGy

 

 

110 MBq Tc 99m DTPA

0.9 mGy

* Xe133, Tc99m macroaggregated albumin

† 120 kVp, pitch = 1. Doses estimated during first trimester (17).

 

Fetal Radiation Doses Associated with

Pulmonary Embolism Imaging (note units)

 

Trimester

HCTPA

(120 kVp, 100 mAs)

Ventilation – Perfusion Scintigraphy

(2 – 5 mCi 99m Tc, 10 mCi Xe133

First

6 – 50 μGy (up to 600 μGy) †

100 - 370 μGy

Second

34 – 250 μGy

100 - 370 μGy

Third

0.28 – 0.8 mGy

100 - 370 μGy

† Other investigators have estimated much fetal doses from CT using the following paramters: 16 slice, 475 mAs, 1.25 mm detector width, 140 kVp (18).

 

c.      Maternal breast absorbed dose:

                                                              i.      CT- 20 - 60 mGy (19).

1.      Although female breast is radiosensitive, epidemiological studies have not shown an increased risk of breast carcinoma from radiation exposures less than 200 mGy (1).

2.      In contrast, others have estimated that a breast dose of 10 mGy in a female patient of age 20 increases the risk for breast carcinoma over baseline by nearly 14% (10, 20).

                                                            ii.      V/Q scan (half - dose perfusion scan only)- 0.28 mGy (21).

 

VI.            Radiation Dose Reduction

a.      Factors affecting radiation absorbed dose

                                                              i.      kVp

1.      increasing kVp from 120 – 140 kVp will increase radiation dose to patient by 35-40% [calculation based on: (140/120)2]

2.      decreasing kVp from 120 to 80 will decrease radiation dose by 60 - 70%

3.      Some still advise use of higher kVp, with lower mAs, because x-ray beam is more efficient and fewer low energy photons are deposited in the skin at higher kVp.

                                                            ii.      mA

1.      mA value is directly proportional to patient radiation dose

                                                          iii.      Tube rotation time

                                                           iv.      Pitch- dose is inversely proportional to pitch. Avoid pitches less than 1 (except cardiac CTA)

                                                             v.      Beam collimation

1.      overall, for single slice CT, narrower sections increase absorbed radiation dose

2.      With MSCT, wider beam collimation will increase dose

                                                           vi.      Scanner configuration (MSCT and detector configuration)

                                                         vii.      Part size scanned

1.      smaller parts and smaller patients have higher absorbed doses

b.      Avoid modalities employing radiation

a.      Start with clinical risk score, d – dimer

b.      Use lower extremity ultrasound when possible

c.      Consider MRI / MRA for pulmonary embolism assessment when appropriate.

c.      Decreasing dose due to CT:

                                                              i.      Decrease kVp or mA

1.      mA value is directly proportional to patient radiation dose

                                                            ii.      Use dose modulation (angular and z – axis modulation)

                                                          iii.      Limit scanning range

                                                           iv.      For chest CT-

1.      use bismuth breast shield for women

a.      May decrease dose to breast by 57 – 73% (22).

2.      use 315 cc oral 30 - 40% barium mixture to decrease fetal dose

a.      Decreases fetal radiation dose with CT by 91% (23).

d.      How other institutions handle imaging for pulmonary embolism in pregnancy (24):

                                                              i.      53% use CTA as the first – line study

                                                            ii.      60% obtain informed consent

                                                          iii.      40% modify imaging techniques in pregnancy

e.      Proposed imaging algorithm in the pregnant patient (14):

 

 

 

 

* PIOPED II investigators still recommend V/Q scan at this step

 

References

 

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2.         Academies NRCotN. Health risks from exposure to low levels of ionizing radiation: BIER VII. In:The National Acadamies Press. Washington, D.C., 2005; 1-4.

3.         Radiation. UNSCotEoA. Sources and Effects of Ionizing Radiation: UNSCEAR 2000 Report to the General Assembly

In. New York: United Nations, 2000.

4.         de Jong PA, Mayo JR, Golmohammadi K, et al. Estimation of cancer mortality associated with repetitive computed tomography scanning. Am J Respir Crit Care Med. 2006;173:199-203.

5.         Lockwood D, Einstein D, Davros W. Diagnostic imaging: radiation dose and patients' concerns. Cleve Clin J Med. 2006;73:583-586.

6.         Huda W. Effective doses to adult and pediatric patients. Pediatr Radiol. 2002;32:272-279.

7.         Cynthia H. McCullough PD, Mayo Clinic, Rochester, MN In.

8.         Curry III TS, Dowdey JE, Murray Jr RC. Christensen's Physics of Diagnostic Radiology. Philadelphia: Williams & Wilkins, 1990.

9.         radiation Ctathrfetlloi. BIER VII: Health risks from exposure to low levels of ionizing radiation. In:The National Acadamies Press, 2005; 1-4.

10.       El-Khoury GY, Madsen MT, Blake ME, Yankowitz J. A new pregnancy policy for a new era. AJR Am J Roentgenol. 2003;181:335-340.

11.       Brenner D, Elliston C, Hall E, Berdon W. Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR Am J Roentgenol. 2001;176:289-296.

12.       Brenner DJ. Radiation risk overstimated- reply. Radiology. 2006;240:614.

13.       Brenner DJ. Radiation risks potentially associated with low-dose CT screening of adult smokers for lung cancer. Radiology. 2004;231:440-445.

14.       Matthews S. Short communication: imaging pulmonary embolism in pregnancy: what is the most appropriate imaging protocol? Br J Radiol. 2006;79:441-444.

15.       Mallick S, Petkova D. Investigating suspected pulmonary embolism during pregnancy. Respir Med. 2006;100:1682-1687.

16.       Scarsbrook AF, Evans AL, Owen AR, Gleeson FV. Diagnosis of suspected venous thromboembolic disease in pregnancy. Clin Radiol. 2006;61:1-12.

17.       Winer-Muram HT, Boone JM, Brown HL, Jennings SG, Mabie WC, Lombardo GT. Pulmonary embolism in pregnant patients: fetal radiation dose with helical CT. Radiology. 2002;224:487-492.

18.       Hurwitz LM, Yoshizumi T, Reiman RE, et al. Radiation dose to the fetus from body MDCT during early gestation. AJR Am J Roentgenol. 2006;186:871-876.

19.       Hurwitz LM, Yoshizumi TT, Reiman RE, et al. Radiation dose to the female breast from 16-MDCT body protocols. AJR Am J Roentgenol. 2006;186:1718-1722.

20.       Allen C, Demetraides T. Radiation risk overstimated. Radiology. 2006;240:613-614.

21.       Cook JV, Kyriou J. Radiation from CT and perfusion scanning in pregnancy. Bmj. 2005;331:350.

22.       Ocker J. BIsmuth shields decrease radiation dose. In:Medical News Today, 2005.

23.       Yousefzadeh DK, Ward MB, Reft C. Internal barium shielding to minimize fetal irradiation in spiral chest CT: a phantom simulation experiment. Radiology. 2006;239:751-758.

24.       Schuster ME, Fishman JE, Copeland JF, Hatabu H, Boiselle PM. Pulmonary embolism in pregnant patients: a survey of practices and policies for CT pulmonary angiography. AJR Am J Roentgenol. 2003;181:1495-1498.