Test 3

Question 1

Charles Meinhold, Thomas S. Tenforde

Answer 1

3rd and 4th president of NRCP

Question 2

NCRP stands for?

Answer 2

National Council on Radiation Protection

Question 3

What was NCRP 49? Replaced by what NCRP? What was updated?

Answer 3

Structural shielding design for medical use of x-rays and gamma rays. NCRP 147. Dose limits for medical shielding designs were defined.

Question 4

ICRP 26 and 60 dose limits for radiation workers and the public?

Answer 4

Previously, ICRP 26: 50mSv (5000mrem), 5mSv (500mrem). Replaced by ICRP 60: 20mSv (2000mrem), 1mSv (100mrem). Per year.

Question 5

NCRP 116 vs ICRP 60 on dose limits

Answer 5

To workers. Annual effective dose limit: 20mSv for both. Cumulative eff. dose: 10mSv x age VS 20mSv in 5 years. Equivalent dose: 150mSv/500mSv lens/rest for both.
Dose to public. Eff. dose limit: 1mSv if continuous, 5mSv if infrequent VS 1mSv, higher if needed provided 5yr avg <1mSv/yr. Equivalent dose: 15mSv/50mSv lens/rest for both.

Question 6

NCRP 49 dose limit (old and defunct)

Answer 6

Derived from ICRP 26 limits by assuming 50 week = 1 working year. So, eff. dose limit for radiation worker is 100mR/wk ~ 1mSv/wk, for public is 10mR/wk ~ 0.1mSv/wk.

Question 7

Limits NCRP 116 and NCRP 147?

Answer 7

5mSv (500mR) to account for pregnant worker, 1mSv to public. Changed to per week on NCRP 147: 0.1mSv/wk to worker, 0.02mSv/wk to public.

Question 8

NCRP 49 VS 147 x-ray machines occupational/public dose limit?

Answer 8

Occupational: 100mR/wk to 10mR/wk! (1mSv/wk to 0.1)
Public: 10mR/wk to 2mR/wk! (0.1mSv/wk to 0.02)

Question 9

Bremsstrahlung intensity direction and efficiency

Answer 9

Low energy: 90 degrees, high energy: forward direction. Efficiency ~1%, rest is converted to heat.

Question 10

Rotating anode disk ~1930s

Answer 10

Molybdenum disk used. ~3000rpm, ~10000rpm for cineangiography. 6mm x 1.5mm = 9 mm^2 region bombarded with e-, with 1900mm^2 total area bombarded per rotation. Coolant oil in the covering of the tube absorbs infrared light.

Question 11

Power dissipation in x-ray tube

Answer 11

P = V x I. Thus, the tube voltage multiplied by tube current gives the power deposited to anode.

Question 12

Thermal rating of x-ray tube, HU

Answer 12

Heat Unit = P x time = V x I x time. This is amount of energy deposited to anode, since power x time = energy. For alternating current SINGLE PHASE (tube voltage = absolute value of sine) use kVprms=kVp*0.707 to calculate HU. This is because kVp is the maximum amplitude of AC voltage.

Question 13

Aluminum filter restrictions

Answer 13

Under 50kVp: 0.5mm, 50~70kVp: 1.5mm, above 70: 2.5mm Aluminum.

Question 14

Heel Effect of x-ray Tubes

Answer 14

Cathode side of the tube has higher intensity due to the anode side requiring x-rays to travel through molybdenum/tungsten target.

Question 15

Beam hardening and effective/equivalent energy

Answer 15

Hardening = higher equivalent energy. Equivalent energy is monoenergetic beam that would go through same attenuation as the spectrum.

Question 16

Quality of x-ray beam

Answer 16

Quality determined by HVL (Half-value Layer)

Question 17

PDD defined in radiation protection

Answer 17

PDD = D(x)/D(0) where D(0) is SURFACE dose. PDD is a function of SSD, effective energy, and area of irradiation.

Question 18

What does having more filtration do to PDD at a reasonable depth?

Answer 18

More filtration = more hardening = more penetration. Thus PDD is higher at most depths.

Question 19

What does having larger field size do to PDD?

Answer 19

More field size = more scatter. Thus PDD is higher / depth profile is flatter.

Question 20

Rule of Thumb of HVL of the body for x-rays

Answer 20

HVL ~ 4cm. The body is around 20cm, or 5 HVL long.

Question 21

What is SIE?

Answer 21

Used for fluoroscopy. Surface integral exposure (R * cm^2) is a measure of total energy (NOT DOSE) delivered during a fluoroscopy. Since exposure can only give “dose”, or E/mass, we need to multiply by “mass” to get the actual radiation energy delivered.

Question 22

What is DAP?

Answer 22

Used for x-ray exposure. Dose area product (Gy * cm^2) good estimation of TOTAL energy delivered to body. Since “dose” is only E/mass, need to multiply by surface area to get better estimation of actual radiation energy delivered. Also, DAP is constant wherever the beam is measured, meaning DAP can be measured near the beam source to estimate dose to patients.

Question 23

What is IRP dose?

Answer 23

Interventional reference point, or 15 cm from isocenter towards the x-ray tube, is used for cardiac examinations.

Question 24

What is TAR and what is it used for?

Answer 24

TAR is tissue air ratio, or dose-EXPOSURE ratio between for tissue-dose versus air-exposure at a given depth x. TAR = D(x)/Xair(x). Xair is measured in air. TAR is used for calculating the dose to tissue given exposure in air.

Question 25

Build Factor for Broad Beam Geometry, B

Answer 25

I = Io * B * exp(-ux)

In application, we can obtain ux, which has B associated with it. With this new B, calculate new u’x. Repeat.

Question 26

NCRP 49 Shielding Terms

Answer 26

Distance, time, barrier, primary barrier, secondary barrier, workload (mA*min/wk), use factor (U), occupancy factor (T)

Question 27

Primary vs Secondary Barrier

Answer 27

Shielding for reducing main x-ray beam (primary) vs shielding for reducing scattered/leakage to acceptable level.

Question 28

Workload (W)

Answer 28

Measure of amount of x-ray generated per week (mA*min/wk). Assume 5 days of work days per week.

Question 29

Use Factor (U)

Answer 29

Fraction of the mA*s that is directed towards a particular barrier. Use factor for secondary wall is 1.

Question 30

Occupancy Factor (T)

Answer 30

Fraction of time at which the area is occupied by the personnel. For occupation in radiation people are assumed to have T=1 in any room.

Question 31

Full, partial, occasional occupancy

Answer 31

T=1, 1/4, 1/16. Office, children’s playground, labs, shops have occupancy factor T=1 since people are there all the time. Restroom, elevators for workers, parking lots, and corridors T=1/4. Others are T=1/16.

Question 32

Rule of Thumb for Dose for X-ray with certain kVp

Answer 32

Dose(1) / Dose(2) = [kVp(1) / kVp(2)]^2

Question 33

Tube current mA versus intensity spectrum

Answer 33

Spectrum increases in amplitude linearly with mA. ie raising mA by 2 increases intensity by 2.

Question 34

The filter used at Sands Building is:

Answer 34

4 filter, 0.1mm Cu with 2.5mm Al.

Question 35

If one gets closer to x-ray source, dose generally increases because?

Answer 35

Inverse square law - per mass, the energy received is larger, since fluence is larger.

Question 36

What is Dose to air given exposure?

Answer 36

Dair = 0.869 * X, where Dair is given in rad and exposure X is given in Roentgen = 2.58x10^-4C/kg.

Question 37

f-factor

Answer 37

Conversion directly from X to Dose to tissue. Dmed = f-factor * X. Formula: f-factor = [u(med)/u(air)] * 0.869 rad/R, where u is u(en), or mass absorption coefficient.
If exposure is given in C/kg, use f-factor = [u(med)/u(air)] * 33.7 J/C

Question 38

Why is f-factor low for fat at low photon energy?

Answer 38

Lower Zeff than soft tissue or bone. Since photoelectric effect occurs at lower E, and interaction probability goes as (Zeff)^3, lower Zeff = lower f-factor.

Question 39

Why is f-factor high for fat at intermediate photon energy?

Answer 39

Hydrogens have higher electron density (approx. 2 times) than others. So fat has higher Compton probability.

Question 40

Dose in terms of mass absorption coefficient?

Answer 40

D = [u(en)/p] * PSI, where PSI is energy fluence.

Question 41

Dose in bone-soft interface?

Answer 41

Dose is increased for soft tissues near interface due to charge build-up from bone.

Question 42

Dose in soft tissue INCLUDED within the bone? (for example, osteocytes?)

Answer 42

f-factor greatly increased for lower energies. Also, larger the cavity, lower the f-factor. Lowest f-factor in the center of the cavity (electron build-up less).

Question 43

Difference between TAR and f-factor?

Answer 43

Both converts exposure to air to dose to tissue. But, TAR used for depth-profiling of dose.

Question 44

X-ray THERAPY Leakage Limit for above 500 kVp

Answer 44

0.1% of useful beam exposure RATE

Question 45

X-ray THERAPY Leakage Limit for below 500 kVp

Answer 45

1 R / hr @ 1 meter (1000 mR / hr)

Question 46

X-ray DIAGNOSTICS Leakage Limit for below 500 kVp

Answer 46

0.1 R / hr @ 1 meter (100 mR / hr)

Question 47

Value of U (the use factor) for leakages

Answer 47

Always 1, since leaks happen even when x-ray tube is not facing the secondary wall.

Question 48

90 degree scatter is around what % of incident useful beam X_u?

Answer 48

a = 0.1% (1/1000). This means if a wall is being irradiated with exposure of X_u, 0.1% of the exposure is coming out at 90 degrees with incident beam.

Question 49

Scattered dose formula (X_S)

Answer 49

Arbitrarily chosen as:
X_S = [X’_U * t * a * F] / [d^2 * 400], where F is field size in cm^2. a is the ratio between scatter and useful (X_U) exposure at 1 m. Here d represents distance from scatter source (the wall, for example)

Question 50

Scatter dose from patient formula to determine K_UX

Answer 50

K_UX = P / aWT * (dsec)^2 * (dsca)^2 * 400 / FS.
Note U = 1 so it is gone. dsec = distance from source to surface of patient. dsca = distance from patient skin to point of interest.

Question 51

Scatter vs Leakage and TVL / HVL Rules of Thumb

Answer 51

Scatter is less penetrating, since leakage is very hardened beams from x-ray tube. Calculate thickness requirements for both scatter and leakage for occupational exposure. If thicknesses differ by less than 1 TVL (~3 HVL) for the two calculations, ADD ONE HVL to the THICKER.