Radiographic Production, Patient Positioning, and Documentation for Quality Images

The Exposure Triangle: kVp, mAs, SID

Three primary variables determine radiographic exposure and image quality.

kilovoltage peak (kVp)

kVp sets the maximum energy of the x-ray beam and therefore its penetrating power and contrast. Higher kVp gives shorter-scale contrast (more shades of gray, less black-and-white) because more scatter reaches the receptor; lower kVp gives long-scale (high) contrast. The 15% rule: increasing kVp by 15% and halving mAs keeps exposure constant while lowering patient dose.

milliampere-seconds (mAs)

mAs (tube current × time) controls the quantity of x-rays produced and therefore the overall image density. Doubling mAs roughly doubles image density; halving mAs darkens underexposed film or, on digital, increases noise. mAs primarily affects brightness/density; kVp primarily affects contrast and penetration.

Source-to-Image Distance (SID)

SID follows the inverse-square law: doubling SID quarters the beam intensity. Most small-animal radiography uses a fixed 40-inch (100 cm) SID. If SID changes, mAs must be adjusted by the formula mAs2 = mAs1 × (SID2/SID1)² to maintain constant exposure.

Grids for Tissue >10 cm

For body parts thicker than ~10 cm (canine abdomen, large canine thorax), scatter radiation degrades contrast. A grid (focused or parallel, ratio 6:1 to 12:1) is placed between the patient and the receptor to absorb scatter. Use of a grid requires a technique chart entry with higher mAs (often 2–3× non-grid technique) because the grid also absorbs some primary beam. The Bucky factor indicates how much to increase exposure.

Technique Charts

A technique chart is a table of kVp and mAs indexed by body part and patient thickness (or weight). VTNE-relevant points:

• Charts are calibrated for a specific SID, grid, and receptor system — change any of those and the chart must be re-calibrated.
• kVp typically scales with tissue thickness (about +2 kVp per cm of abdomen, +1–2 kVp per cm of thorax).
• For digital (CR/DR), kVp can be pushed higher and mAs lowered to reduce patient dose; the image processor corrects density.
• Always check the chart before exposure — guessing leads to retakes (which double patient and staff radiation dose).

Patient Positioning and the Thoracic Series

Principles of Positioning

• The area of interest must be closest to the receptor (the anode-heel effect and patient thickness reduce detail on the tube side).
• Center the beam on the anatomic point of interest, not the middle of the patient; this keeps anatomy from being distorted by the divergent beam.
• Keep the part parallel to the receptor to avoid foreshortening.
• Use positioning aids (troughs, foam wedges, sandbags) — never use staff to hold a position when sedation or a restraint device will do.

The Thoracic Series

At minimum, the thorax needs two orthogonal views: a lateral and either a VD or DV. Standard practice for thorough evaluation (e.g., pulmonary metastasis screening, cardiac assessment) uses the three-view thorax:

1. Right lateral — left lung lobes are dependent (closer to receptor), showing the left cranial and caudal lobes best.
2. Left lateral — right lobes are dependent, best shown.
3. VD or DV — differentiates left vs right, shows cardiac silhouette and pulmonary vasculature symmetrically.

VD vs DV: VD (patient in dorsal recumbency, beam goes ventral→dorsal) is used for cardiac assessment and most routine thoracic work. DV (sternal recumbency, beam dorsal→ventral) is preferred for dyspneic patients because sternal recumbency preserves ventilation better, and for evaluating the caudal lung lobes because the heart falls away from the diaphragm.

Oblique views may be added to profile specific lung lobes or rib lesions.

Abdominal Positioning

• Lateral: patient in right lateral recumbency; center beam at the level of the 13th rib (cranial abdomen) or umbilicus (full abdomen). Include diaphragm through the coxofemoral joint.
• VD: patient in dorsal recumbency; beam centered mid-abdomen. Padded troughs stabilize the patient.
• For pneumocystography or positive cystography, the bladder is centered.

Orthopedic Positioning Essentials

• Pelvis (ventrodorsal hip-extended view): limbs extended parallel, patellae centered over the trochlea, femurs not rotated; the pelvis must be perfectly symmetrical — asymmetry between the obturator foramina or coxofemoral joints indicates rotation and invalidates joint measurements.
• Stifle: lateral view with stifle flexed 90°, femoral condyles superimposed; caudocranial view with limb extended.
• Elbow: true lateral with elbow flexed 90°; craniocaudal with limb extended.
• Always include the joint above and below a long-bone fracture.

Beam Centering, Collimation, and the Anode-Heel Effect

• Center the beam on the anatomy of interest; use a ruler to set SID consistently.
• Collimate tightly to the area of interest — collimation reduces scatter (improving contrast), lowers patient dose, and protects staff by limiting the irradiated volume. Never use a collimation field larger than the receptor.
• The anode-heel effect: the cathode side of the beam has higher intensity than the anode side. Align the cathode with the thicker/denser part of the patient (e.g., the abdomen caudal end on a lateral thoraco-abdominal study) for more even exposure.

Documentation and Labeling

Every diagnostic radiograph must be unambiguously identifiable.

Markers must be placed before exposure — adding a marker afterward is a documentation failure. On digital systems, the L/R marker is burned in at acquisition, not added in post-processing.

Quality Pitfalls and the Retake

Common retake causes and their fixes:

• Too dark/too light: wrong mAs; check technique chart and patient thickness measurement.
• Poor contrast (gray, washed out): too high kVp, no grid when one was needed, or grid cutoff from off-center/off-focus alignment.
• Motion blur: too long exposure time (high mAs with low mA); increase mA, decrease time, or sedate the patient.
• Rotation (asymmetric pelvis, tilted thorax): reposition and re-shoot — never accept a rotated study for orthopedic measurements.

Each retake doubles radiation dose to patient and staff. The cheapest fix is to position and expose correctly the first time.