Liều giới hạn liệu bức xạ cho bệnh nhân (Radiation dosimetry)

Radiation Dosimetry of the Patient – Chapter 24, BushbergKalpana Kanal, Ph.D., DABR Lecturer, Diagnostic Physics Dept.. Dosimetry dose quantities serve as indicators of the risk of biolo

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Radiation Dosimetry of the Patient – Chapter 24, Bushberg

Kalpana Kanal, Ph.D., DABR Lecturer, Diagnostic Physics Dept of Radiology

UW Medicine

a copy of this lecture may be found at:

http://courses.washington.edu/radxphys/PhysicsCourse04-05.html

Kalpana M Kanal, Ph.D., DABR

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1 Dosimetry

dose quantities serve as indicators of the risk of biologic damage to the patient

caused by cell killing

its function can be impaired

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1 Dosimetry

skin damage and cataracts

occur

effect increases with the dose

from an imaging procedure, the dose to that organ is estimated

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1 Dosimetry

genetically transformed but reproductively viable descendants

with dose

to limit the risks of stochastic effects and to prevent the non-stochastic effects

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1 Dosimetry

exposure

patient risk

power of the x-ray beam, or the radiosensitivity of the exposed region

close to 1.0 so that dose is numerically equal to exposure

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1 Dosimetry

area of the x-ray beam (exposed area)

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1 Dosimetry

this says nothing about the total mass of tissue exposed and the distribution of the absorbed energy

20 mGy to the finger?

ionizing energy absorbed for a 70-kg person with a 35 g finger

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1 Dosimetry

imparted energy (Joules), is the product of the dose (Gray) and the mass (Kg) over which the energy is imparted

dose to the tissue in the slice

the imparted energy is approx 15 times that of a single slice (you also have to consider scatter from adjacent slices, about 10-25%)

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1 Dosimetry

for the different sensitivities of the exposed tissue to biologic damage

data and incorporate significant uncertainties

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1 Dosimetry

skin exposure (ESE)

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←

→

c.f Bushberg, et al The Essential Physics of Medical

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1 Dosimetry

calibration, when a comprehensive analysis of effective dose is unnecessary

c.f Bushberg, et al

The Essential Physics of Medical Imaging, 2 nd ed., p

797.

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1 Risk

estimates the risk of fatal cancer for exposures to adults of working age to be 0.004 deaths per Sv or 0.0004 per rem

an effective dose of 10 mSv (1 rem)

estimates, risk is presumed to be proportional to the effective dose

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1 Risk

would result from an effective dose of 1 mSv (0.1 rem), or

of 50 mSv (5 rem)

for infants and children and substantially lower for adults older than 50 years of age

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1 Typical Absorbed and Effective doses

Imaging, 2 nd ed., p 798.

→

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1 Risks

6214 2540

348.0 8.7

Barium Enema (with fluoro)

5571 2277

312.0 7.8

CT Chest

5429 2219

304.0 7.6

CT Abdomen

5071 2073

284.0 7.1

CT Pelvis

3000 1226

168.0 4.2

Intravenous Urography

1500 613

84.0 2.1

Lumbar Spine

1286 526

72.0 1.8

CT Head

857 350

48.0 1.2

Abdomen

786 321

44.0 1.1

Pelvis

714 292

40.0 1.0

Thoracic Spine

71 29

4.0 0.1

Mammography

71 29

4.0 0.1

Skull Exam

29 12

1.6 0.04

Chest Radiograph

Equivalent to Number of Highway Miles Driven

Equivalent to Number of Cigarettes Smoked

Risk of Fatal Cancer (per million)

Effective Dose (mSv) Procedure

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1 Interventional Radiologic Procedures

←

c.f Bushberg, et al

799.

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2 Radiographic Procedures

Geometry for measuring the output free-in-air of a radiographic system

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Geometry for measuring the output free-in-air of a radiographic system when phototiming is used

c.f Bushberg, et al

804.

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4.5 years 500

10-20

CT Abdomen or

Pelvis

3.6 years 400

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CT Chest

50 35 1

Equivalent no

of chest x-rays

6 months 1.0

Abdomen

4 months 0.7

Pelvis

3 days 0.02

Chest PA

Approx period

of background radiation Eff Dose (mSv)

Procedures

3 Effective Dose Comparison with Chest PA Exam

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Question

rad, the dose for a 10 slice examination would be approximately _ rad and the imparted energy would be rad (ignore scatter)

A 5, 15

B 15, 5

D 50, 5

E 5, 50

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Question

contiguous slices are taken, then dye is injected and 10 slices are repeated The total entrance skin exposure is about _ R

A 2.0

B 2.2

D 5.0

E 20.0 You have to consider scatter 25% of 2 R = 0.5 So 2.5 per scan is the rad exp For two scans, 2.5*2 = 5.0

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Question

film with a 400 speed screen-film system is:

A 13 mR

B 150 mR

C 300 mR

D 850 mR

E 3000 mR

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Question

A 15 mR

B 40 mR

C 5 R

D 10 R

E 50 mrem

1 CT head scan ESE

2 Lateral chest ESE

3 10 min fluoro (thin patient)

4 Monthly limit for a pregnant technologist

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Question

A 15 mR

B 40 mR

C 5 R

D 10 R

E 50 mrem

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