Radiography Mnemonics: 12 Memory Tricks for the ARRT Exam

Cardinal principles of radiation protection (ALARA)

“TDS: Time, Distance, Shielding”

1. TTime: minimize beam-on time (pulsed fluoro, last-image hold, avoid repeats)
2. DDistance: step back or out; intensity falls with the square of distance
3. SShielding: protective barriers, lead aprons and thyroid shields for personnel

Use it when: When a stem asks how personnel should protect themselves during mobile or fluoroscopic work, or which action best applies ALARA.

Careful: Distance is the most effective and cheapest of the three for staff. For patients the levers are collimation, correct technique and avoiding repeats: routine gonadal and fetal shielding is no longer recommended (AAPM 2019, NCRP Statement No. 13, 2021), and ARRT has said its exams may not refer to those shielding practices.

Inverse square law

“Double the distance, one quarter the intensity”

Beam intensity is inversely proportional to the square of the distance from the source: I1 / I2 = (D2 / D1) squared. Moving from 1 m to 2 m cuts intensity to one quarter; moving to 3 m cuts it to one ninth.

Use it when: For personnel dose at a new distance, or patient entrance exposure when SID changes and technique does not.

Careful: To keep receptor exposure the same when SID changes, use the direct square law (exposure maintenance formula): new mAs = old mAs x (new SID / old SID) squared. Doubling SID needs four times the mAs, not twice.

Radiosensitivity (law of Bergonie and Tribondeau)

“YUM-R cells are radiosensitive”

1. YYoung: immature cells and tissues (fetus, children)
2. UUndifferentiated: stem and precursor cells
3. MMetabolically active
4. RRapidly dividing: high mitotic rate (bone marrow, intestinal crypt cells, gonads)

Use it when: When an item asks which tissue or cell type is most or least radiosensitive, or why the fetus is at higher risk.

Careful: Lymphocytes are the classic exception: they are among the most radiosensitive cells even though mature lymphocytes do not divide rapidly. Nerve and muscle cells are the least radiosensitive.

Radiation quantities and SI units

“Coulombs count the air, grays get absorbed, sieverts say the risk”

1. C/kgExposure (ionization in air); old unit roentgen. Air kerma is given in gray (Gya)
2. GyAbsorbed dose; old unit rad. 1 Gy = 100 rad
3. SvEquivalent and effective dose; old unit rem. 1 Sv = 100 rem
4. BqRadioactivity; old unit curie

Use it when: When an item asks which unit measures a quantity, or converts between SI and conventional units.

Careful: ARRT uses SI units as primary. For diagnostic x-rays the radiation weighting factor is 1, so absorbed dose in Gy equals equivalent dose in Sv numerically; effective dose then applies tissue weighting factors and is the quantity dose limits are written in.

Contrast reactions: physiologic versus allergic-like

“Warm and queasy, watch; hives and wheeze, treat; airway or pressure crash, call”

Mild physiologic reactions (warmth, flushing, metallic taste, limited nausea) and mild allergic-like reactions (a few hives, itching) usually need observation and reassurance. Moderate reactions (diffuse hives, bronchospasm without hypoxia, facial swelling) need treatment under physician direction. Severe reactions (laryngeal edema, hypotension, arrest) need the emergency team and full resuscitation.

Use it when: When a stem describes symptoms after contrast and asks for the best technologist action.

Careful: Vasovagal reactions cause hypotension with a slow pulse, while anaphylactoid reactions cause hypotension with a fast pulse; the treatments differ. Keep IV access in place, stop the injection if symptoms are more than mild, and remember the technologist does not give epinephrine without an order.

Rights of drug administration

“The six rights: Patient, Drug, Dose, Route, Time, Documentation”

1. PRight patient: two identifiers
2. DRight drug: check the label and expiration against the order
3. DRight dose
4. RRight route (IV, oral, rectal)
5. TRight time
6. DRight documentation

Use it when: When an item asks what to verify before giving contrast or a medication, or what was missed in an error scenario.

Careful: Two identifiers means name plus date of birth or medical record number, never the room number. Some programs teach seven or more rights (adding right reason or right to refuse), so read the stem for what it counts.

Airborne precautions

“My Chicken Hez TB”

1. MMeasles (rubeola)
2. ChChickenpox (varicella)
3. HezHerpes zoster, disseminated (or localized in an immunocompromised patient)
4. TBTuberculosis

Use it when: When a stem names an infection and asks for the room, mask or transport precautions.

Careful: Airborne precautions need an airborne infection isolation room and an N95 respirator for staff; the patient wears a surgical mask during transport. Influenza and pertussis are droplet, not airborne. C. difficile is contact precautions with soap-and-water hand hygiene, because alcohol rub does not kill spores.

The 15% rule

“Up 15 percent, double; down 15 percent, half”

A 15% increase in kVp roughly doubles exposure to the image receptor, the same as doubling mAs; a 15% decrease halves it. So raising kVp 15% and halving mAs keeps receptor exposure about the same.

Use it when: When a stem changes kVp and asks what happens to receptor exposure, or how to adjust mAs to compensate.

Careful: It is 15% of the current kVp, not 15 kVp: from 60 kVp it is about 9 kVp, from 100 kVp about 15. Raising kVp and lowering mAs cuts patient dose but lowers subject contrast; in digital imaging the processing hides much of that contrast change.

Exposure indicators and deviation index

“DI zero is the hero; plus 3 is double, minus 3 is half”

The deviation index compares the exposure index of an image with the target for that body part: DI = 10 x log10(EI / target EI). DI 0 is on target, +1 is about 26% over, -1 about 20% under, +3 is twice and -3 half the target exposure.

Use it when: When a stem gives an exposure indicator or DI and asks whether the image was over- or underexposed, or what to do.

Careful: Digital processing makes overexposed images look fine, which leads to dose creep, so check the indicator, not just the image. Vendor S numbers (for example Fuji) are inverse: a higher S number means less exposure.

Which joints the spine obliques show

“CAD and LPD”

1. CADCervical intervertebral foramina: Anterior obliques show the Downside (side closest to the IR)
2. LPDLumbar zygapophyseal joints: Posterior obliques show the Downside

Use it when: When an item names an oblique (RAO, LPO and so on) and asks which side's foramina or facet joints are demonstrated.

Careful: The reverse also holds: cervical posterior obliques show the upside foramina, lumbar anterior obliques the upside joints. Sacroiliac joints are the trap: a posterior oblique (25-30 degrees) shows the side up, so an LPO demonstrates the right SI joint.

Skull and sinus central-ray angles

“Towne 30/37, Caldwell 15, Waters 37”

1. 30/37AP axial (Towne): CR 30 degrees caudad to the OML or 37 degrees to the IOML
2. 15PA axial (Caldwell): CR 15 degrees caudad with the OML perpendicular to the IR
3. 37Parietoacanthial (Waters): OML forms 37 degrees with the IR, MML perpendicular

Use it when: When a stem names a skull or facial projection and asks for the CR angle or the positioning line.

Careful: The Towne angle depends on the line used: the OML and IOML differ by about 7 degrees, so 30 and 37 are the same projection. Do not confuse the Waters 37-degree OML with the modified Waters (55 degrees) or the PA axial Haas (25 degrees cephalad).

Sacrum and coccyx angles

“Sacrum 15 up, coccyx 10 down”

For the AP axial sacrum, angle the CR 15 degrees cephalad, centered midway between the symphysis pubis and the ASIS level. For the AP axial coccyx, angle 10 degrees caudad, centered about 2 inches above the symphysis.

Use it when: When an image shows a foreshortened sacrum or superimposed coccyx and asks for the CR correction.

Careful: For PA axial projections of the same structures the directions reverse (sacrum caudad, coccyx cephalad). A foreshortened sacrum with closed foramina on an AP axial view means the cephalad angle was too small.