ACVR Veterinary Radiology cheat sheet

Roentgen (R) is the SI unit of exposure; 1 R = 2.58 × 10⁻⁴ C/kg of air.

Absorbed dose is measured in gray (Gy); 1 Gy = 1 J/kg = 100 rad.

Effective dose (Sv) = absorbed dose × radiation weighting factor × tissue weighting factor; diagnostic x-rays use weighting factor 1.

ALARA: dose reduction follows time-distance-shielding; doubling distance from a point source reduces exposure to one quarter (inverse square law).

Opaque vs. lucent: bone/metal are radiopaque (white); gas is radiolucent (black); fat is more lucent than soft tissue.

Silhouette sign: loss of a border between two structures of the same opacity localizes a lesion to the contacting surface.

Radiographic signs of malignancy in bone: aggressive periosteal reaction, cortical destruction, ill-defined margins, soft-tissue mass.

Thoracic vertebral heart score (VHS) in dogs: normal ≤ 10.5 vertebrae (long axis + short axis on lateral view).

Pneumoperitoneum, loss of serosal detail, and gas within a wall suggest hollow-viscus rupture or necrosis.

Biologic effective dose depends on total dose, dose per fraction, overall treatment time, and tissue type (the 4 R's of radiobiology).

The 4 R's: repair of sublethal damage, redistribution through the cell cycle, repopulation, and reoxygenation.

α/β ratio: late-responding tissues have low α/β (~2-3 Gy); early-responding tumors and acute tissues have high α/β (~8-10 Gy).

Linear-quadratic model: surviving fraction = exp(−(αD + βD²)); the α/β ratio is the dose where α and β contributions are equal.

Fractionation spares late-responding normal tissues more than tumors because of greater repair capacity and low α/β.

Tumor control probability rises with dose; normal tissue complication probability rises with dose and volume irradiated.

Palliative protocols often use coarse fractions (e.g., 4-8 Gy per fraction) to reduce visit number while accepting higher late-effect risk.

Radiation-induced acute effects (mucositis, dermatitis) appear during or shortly after treatment; late effects (fibrosis, necrosis) occur months to years later.

Chemotherapy and radiation can interact; radiosensitizers increase tumor kill but may also increase normal tissue effects.

Navicular syndrome: radiographic changes include enlarged synovial invaginations, sclerosis of the navicular bone, and flexor cortical irregularity.

Distal phalanx rotation in laminitis is assessed on lateromedial radiographs by the angle between the dorsal hoof wall and dorsal aspect of P3.

Equine cervical vertebral malformation (wobbler) is evaluated with lateral cervical radiographs; stenosis is judged by minimum sagittal diameter and intravertebral ratios.

Metacarpophalangeal joint: dorsal osteochondral fragments and palmar osteochondral disease are common; skyline and flexed views improve detection.

Equine thorax radiography is limited by patient size; ultrasound is preferred for pleural and peripheral pulmonary lesions.

Sesamoid fracture, suspensory desmitis, and stress fractures of the third metacarpal bone are common causes of forelimb lameness evaluated radiographically.

Nuclear scintigraphy is highly sensitive for early stress fractures and occult bone injury before radiographic changes appear.

Computed tomography of the equine head delineates dental roots, sinuses, and the guttural pouch better than survey radiography.