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Radiation Protection > Test 3 - NCRP 49 Terms > Flashcards

Test 3 - NCRP 49 Terms Flashcards

1
Q

Weekly design exposure rate (limit on exposure on person)

A

P, unit is in Roentgen

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2
Q

Weekly workload X-ray

A

W, unit is in mA*min

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3
Q

Weekly workload MV and gamma ray

A

W, unit is in R, measured at 1 meter from source

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4
Q

Use factor

A

U. 1 for floor, 1/4 for walls, since most x-rays tubes point downward.

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5
Q

Occupancy factor

A

T. 1 for commonly occupied spaces, 1/4 for medium, 1/16 for sparsely occupied.

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6
Q

Normalized output

A

X_n. Exposure per current output. Unit given as R/mA at 1 meter from source.

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7
Q

Exposure rate at 1 m from source of 1. useful beam, 2. leakage radiation, 3. scattered radiation

A

X’_u, X’_L, X’_s. (primes represent time derivative) Unit given as R/min at 1 meter from source.

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8
Q

Quotient of exposure at 1 m and workload

A

K_UX. Unit given as R/mA-min at 1 meter from source. Also known as exposure per workload.

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9
Q

Transmission factor for useful beam for 1. x-rays 2. gamma rays 3. leakage x-rays 4. leakage gamma 5. scattered x-ray 6. scattered gamma ray.

A

B_UX, B_UG, B_LX, B_LG, B_SX, B_SG. Transmission factor

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10
Q

Barrier Thickness 1. primary wall 2. secondary wall

A

S_p, S_s. Primary wall is the actual irradiated wall for useful beams. Secondary wall is the wall irradiated by leakage and scattered radiations.

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11
Q

Exposure in terms of X’_u and distance d in meters

A

X = X’_u*t/(d)^2. Since X’_u is defined as exposure rate at 1 meter, d in meter is suffice to obtain true exposure at point of interest.

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12
Q

Solving for B using P, X’_u, t and d.

A

Since P is what we want to achieve, we can solve for B given the exposure limit, P. P = B * X, and we know X = X’_u * t / d^2. So, P = B * X’_u * t / d^2. Note P = 10 mR/week, as of NCRP 147.

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13
Q

X’_u in terms of X_n, t, and workload W.

A

X’_u is exposure rate at 1 m. X_n is exposure at 1 m per workload W. Thus X_n * W / t = X’_u.

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14
Q

Solving for B using P, X_n, and workload W.

A

Since P = B * exposure / d^2, and exposure = X’_n * W (since X’_n is R at 1 meter per workload 1 mA*min), P = B * X_n * W / d^2.

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15
Q

Solving for K_UX given exposure limit P

A

Since K_UX is exposure / mAmin at 1 meter, K_UX = P * d^2 / W. Why? P is desired exposure at point of interest d meters away. P / W gives exposure per workload. Multiplying by d^2 will give R / mAmin AT 1 METER, or K_UX.

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16
Q

Solving for K_UX with U and T, use factor and occupancy factor.

A

U and T will give leniency to allowed K_UX (specifically will raise K_UX). This means more workload is needed to reach P, so more leniency. Specifically, K_UX = P * d^2 / WUT.

17
Q

General equation relating B (transmission factor), X’_n (exposure per workload) to P (exposure limit), d, and WUT in the presence of shielding

A

K_UX = B*X’_n = P * d^2 / WUT
Note that B is missing if no shielding exists. Note that when one calculates P * d^2 / WUT, the value can be used on a table to search for right lead thickness for acceptable value.

18
Q

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

A

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

19
Q

NCRP 116 vs ICRP 60 on dose limits

A

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.

20
Q

NCRP 49 dose limit (old and defunct)

A

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.

21
Q

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

A

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

22
Q

Rotating anode disk ~1930s

A

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.

23
Q

Aluminum filter restrictions

A

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

24
Q

Beam hardening and effective/equivalent energy

A

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

25
Q

PDD defined in radiation protection

A

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

26
Q

What does having larger field size do to PDD?

A

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

27
Q

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

A

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

28
Q

What is DAP?

A

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.

29
Q

NCRP 49 Shielding Terms

A

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

30
Q

Use Factor (U)

A

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

31
Q

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

A

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

32
Q

The filter used at Sands Building is:

A

4 filter, 0.1mm Cu with 2.5mm Al.

33
Q

f-factor

A

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

34
Q

Dose in terms of mass absorption coefficient?

A

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

35
Q

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

A

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).

36
Q

X-ray THERAPY Leakage Limit for above 500 kVp

A

0.1% of useful beam exposure RATE

37
Q

X-ray DIAGNOSTICS Leakage Limit for below 500 kVp

A

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

Radiation Protection (4 decks)

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