CAMRT RTR Exam cheat sheet

mAs controls photon quantity and receptor dose; kVp controls beam penetrability and subject contrast. A 15% kVp increase roughly doubles receptor dose at constant mAs.

Inverse square law: I1/I2 = (d2/d1)^2, so doubling the distance from a point source reduces intensity to one quarter.

Magnification = SID/SOD, where SOD = SID minus OID. Reducing object-to-image distance reduces magnification and improves recorded detail.

Photoelectric absorption varies roughly with Z^3/E^3 and dominates at low kVp and with high-atomic-number contrast media; Compton scatter depends on electron density and dominates at higher kVp.

Grid ratio = lead strip height divided by interspace width. Higher ratios remove more scatter and improve contrast but require more mAs; grid cut-off follows lateral decentring or tube angulation.

Typical half-value layer in general radiography is about 2.5 mm Al at 70 kVp; minimum total filtration is 2.5 mm Al equivalent.

CT pitch = table travel per gantry rotation divided by total beam collimation. DLP = CTDIvol multiplied by scan length; effective dose is estimated as DLP times a region-specific k-factor.

Display mapping: window level sets the CT number shown as mid-grey (brightness); window width sets the range mapped to the grey scale (contrast). Narrowing width raises contrast but lowers latitude.

Occupational dose limit is 20 mSv per year averaged over five years, with no more than 50 mSv in any single year; the public limit is 1 mSv per year.

Larmor equation: precessional frequency = gyromagnetic ratio times field strength. For hydrogen-1, 42.58 MHz/T gives 63.9 MHz at 1.5 T and 127.7 MHz at 3 T.

TR governs T1 weighting and TE governs T2 weighting: short TR/short TE = T1-weighted, long TR/long TE = T2-weighted, long TR/short TE = proton density.

T2* combines true T2 dephasing with static field inhomogeneity. A 180-degree refocusing pulse (spin echo) removes the inhomogeneity component; gradient echo does not.

IEC normal operating mode limits whole-body SAR to 2 W/kg averaged over 6 minutes; first-level controlled mode permits 4 W/kg averaged over 15 minutes.

Gadolinium chelates shorten both T1 and T2; at low concentration T1 shortening dominates, giving high signal on T1-weighted images. Avoid in severe renal impairment because of nephrogenic systemic fibrosis risk.

STIR suppresses fat by inversion timing and is insensitive to field inhomogeneity; spectral (chemical) fat saturation requires good shimming and fails at low field strength.

Pixel size = FOV divided by matrix; SNR scales with voxel volume, so doubling the matrix in both directions at fixed FOV cuts SNR by about a factor of four.

K-space centre lines encode image contrast and signal-to-noise; peripheral lines encode edges and fine detail. Half-Fourier and echo-planar trajectories exploit this ordering.

Chemical shift displacement in hertz equals about 3.5 ppm times 42.58 times field strength, approximately 220 Hz at 1.5 T; the 5-gauss line defines the controlled access area.

Tc-99m has a 6.02-hour half-life and a 140 keV gamma emission and is eluted from a Mo-99/Tc-99m generator (Mo-99 half-life 66 hours); test eluate for Mo-99 breakthrough and aluminium.

Decay law: activity = initial activity times exp(-0.693 t / half-life); activity halves with every elapsed half-life.

Effective half-life combines physical and biological clearance: 1/Te = 1/Tphysical + 1/Tbiological.

PET positron annihilation produces two 511 keV photons emitted about 180 degrees apart; coincidence timing windows are typically a few nanoseconds and random events rise with the square of activity.

Counting follows Poisson statistics: standard deviation of a count equals the square root of the count, so quadrupling counts halves the relative uncertainty.

A pulse-height analyser window of about plus or minus 10% around the photopeak accepts primary photons while rejecting most scattered events.

Common Tc-99m agents: MDP for bone, DTPA or MAG3 for renal function, sulfur colloid for liver and spleen, pertechnetate for thyroid and Meckel diverticulum imaging; I-123 for thyroid imaging and I-131 for therapy.

Dose calibrator quality control includes daily constancy measurement, accuracy within plus or minus 10%, and checks that readings are independent of sample volume and geometry.

Absorbed dose: 1 Gy = 1 J/kg = 100 rad. Equivalent and effective dose use the sievert, where 1 Sv = 100 rem.

BED = n x d x (1 + d/(alpha/beta)), where n is fraction number and d is dose per fraction. Alpha/beta is about 10 Gy for tumours and early effects, about 3 Gy for late effects, and about 1.5 Gy for prostate.

Typical curative regimen: 2 Gy per fraction over 30 to 35 fractions, total 60 to 70 Gy; palliative schedules use 8 to 30 Gy in 1 to 10 fractions.

Percentage depth dose increases with beam energy, field size and source-to-surface distance; TMR and TPR are independent of SSD and are used for isocentric (SAD) setups.

ICRU volumes: GTV is visible disease, CTV adds microscopic spread, ITV adds internal motion, PTV adds set-up margin; PRV is the planning margin applied to an organ at risk.

Megavoltage photon beams show skin sparing, with maximum dose at about 1.5 cm for 6 MV and about 3 cm for 18 MV; bolus restores surface dose.

Electron beam therapeutic depth in centimetres is approximated by energy in MeV divided by 3 to 4, with rapid dose fall-off beyond that depth.

IMRT and VMAT use inverse planning against dose-volume objectives; VMAT modulates the multileaf collimator during continuous gantry rotation, shortening treatment time relative to fixed-field IMRT.

Brachytherapy dose falls roughly as the inverse square of distance from the source; the TG-43 formalism uses air-kerma strength, dose-rate constant and anisotropy functions.

mAs sets photon quantity and receptor dose, kVp sets penetrability.

Inverse square law: intensity falls with the square of distance.

Grid ratio compares strip height to interspace width.

Photoelectric absorption rises with atomic number cubed.

Magnification equals SID divided by SOD.

CT pitch equals table travel divided by collimation.

Window level controls brightness; width controls contrast.

Occupational dose limit averages 20 mSv per year.

Larmor frequency equals gyromagnetic ratio times field strength.

TR and TE determine T1, T2 or proton density weighting.

T2 star decay includes field inhomogeneity dephasing.

Ernst angle maximises signal for a given TR and T1.

STIR suppresses fat despite field inhomogeneity.

SAR rises with field strength squared and flip angle.

Gadolinium shortens T1 and brightens T1-weighted images.

K-space centre lines govern image contrast.

Tc-99m has a 6-hour half-life and 140 keV gamma.

Effective half-life combines physical and biological clearance.

Annihilation yields two 511 keV photons at 180 degrees.

Energy windows are set near the photopeak.

Random coincidences increase with the square of activity.

Counting precision improves with the square root of counts.

Generators are eluted and tested for breakthrough.

FDG uptake reflects regional glucose metabolism.

Gray equals one joule per kilogram.

BED compares schedules using alpha-beta.

PTV includes set-up and motion margins.

PDD rises with energy, field size and SSD.

Alpha-beta near 10 gray marks early-responding tissue.

Bolus builds up dose on the skin surface.

Inverse planning optimises fluence maps.

Brachytherapy dose falls as inverse square.