Test 3 Flashcards
Charles Meinhold, Thomas S. Tenforde
3rd and 4th president of NRCP
NCRP stands for?
National Council on Radiation Protection
What was NCRP 49? Replaced by what NCRP? What was updated?
Structural shielding design for medical use of x-rays and gamma rays. NCRP 147. Dose limits for medical shielding designs were defined.
ICRP 26 and 60 dose limits for radiation workers and the public?
Previously, ICRP 26: 50mSv (5000mrem), 5mSv (500mrem). Replaced by ICRP 60: 20mSv (2000mrem), 1mSv (100mrem). Per year.
NCRP 116 vs ICRP 60 on dose limits
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.
NCRP 49 dose limit (old and defunct)
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.
Limits NCRP 116 and NCRP 147?
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.
NCRP 49 VS 147 x-ray machines occupational/public dose limit?
Occupational: 100mR/wk to 10mR/wk! (1mSv/wk to 0.1)
Public: 10mR/wk to 2mR/wk! (0.1mSv/wk to 0.02)
Bremsstrahlung intensity direction and efficiency
Low energy: 90 degrees, high energy: forward direction. Efficiency ~1%, rest is converted to heat.
Rotating anode disk ~1930s
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.
Power dissipation in x-ray tube
P = V x I. Thus, the tube voltage multiplied by tube current gives the power deposited to anode.
Thermal rating of x-ray tube, HU
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.
Aluminum filter restrictions
Under 50kVp: 0.5mm, 50~70kVp: 1.5mm, above 70: 2.5mm Aluminum.
Heel Effect of x-ray Tubes
Cathode side of the tube has higher intensity due to the anode side requiring x-rays to travel through molybdenum/tungsten target.
Beam hardening and effective/equivalent energy
Hardening = higher equivalent energy. Equivalent energy is monoenergetic beam that would go through same attenuation as the spectrum.
Quality of x-ray beam
Quality determined by HVL (Half-value Layer)
PDD defined in radiation protection
PDD = D(x)/D(0) where D(0) is SURFACE dose. PDD is a function of SSD, effective energy, and area of irradiation.
What does having more filtration do to PDD at a reasonable depth?
More filtration = more hardening = more penetration. Thus PDD is higher at most depths.
What does having larger field size do to PDD?
More field size = more scatter. Thus PDD is higher / depth profile is flatter.
Rule of Thumb of HVL of the body for x-rays
HVL ~ 4cm. The body is around 20cm, or 5 HVL long.
What is SIE?
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.
What is DAP?
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.
What is IRP dose?
Interventional reference point, or 15 cm from isocenter towards the x-ray tube, is used for cardiac examinations.
What is TAR and what is it used for?
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.
Build Factor for Broad Beam Geometry, B
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.
NCRP 49 Shielding Terms
Distance, time, barrier, primary barrier, secondary barrier, workload (mA*min/wk), use factor (U), occupancy factor (T)
Primary vs Secondary Barrier
Shielding for reducing main x-ray beam (primary) vs shielding for reducing scattered/leakage to acceptable level.
Workload (W)
Measure of amount of x-ray generated per week (mA*min/wk). Assume 5 days of work days per week.
Use Factor (U)
Fraction of the mA*s that is directed towards a particular barrier. Use factor for secondary wall is 1.
Occupancy Factor (T)
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.
Full, partial, occasional occupancy
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.
Rule of Thumb for Dose for X-ray with certain kVp
Dose(1) / Dose(2) = [kVp(1) / kVp(2)]^2
Tube current mA versus intensity spectrum
Spectrum increases in amplitude linearly with mA. ie raising mA by 2 increases intensity by 2.
The filter used at Sands Building is:
4 filter, 0.1mm Cu with 2.5mm Al.
If one gets closer to x-ray source, dose generally increases because?
Inverse square law - per mass, the energy received is larger, since fluence is larger.
What is Dose to air given exposure?
Dair = 0.869 * X, where Dair is given in rad and exposure X is given in Roentgen = 2.58x10^-4C/kg.
f-factor
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
Why is f-factor low for fat at low photon energy?
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.
Why is f-factor high for fat at intermediate photon energy?
Hydrogens have higher electron density (approx. 2 times) than others. So fat has higher Compton probability.
Dose in terms of mass absorption coefficient?
D = [u(en)/p] * PSI, where PSI is energy fluence.
Dose in bone-soft interface?
Dose is increased for soft tissues near interface due to charge build-up from bone.
Dose in soft tissue INCLUDED within the bone? (for example, osteocytes?)
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).
Difference between TAR and f-factor?
Both converts exposure to air to dose to tissue. But, TAR used for depth-profiling of dose.
X-ray THERAPY Leakage Limit for above 500 kVp
0.1% of useful beam exposure RATE
X-ray THERAPY Leakage Limit for below 500 kVp
1 R / hr @ 1 meter (1000 mR / hr)
X-ray DIAGNOSTICS Leakage Limit for below 500 kVp
0.1 R / hr @ 1 meter (100 mR / hr)
Value of U (the use factor) for leakages
Always 1, since leaks happen even when x-ray tube is not facing the secondary wall.
90 degree scatter is around what % of incident useful beam X_u?
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.
Scattered dose formula (X_S)
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)
Scatter dose from patient formula to determine K_UX
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.
Scatter vs Leakage and TVL / HVL Rules of Thumb
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.
