8.3 Patient Radiation Protection

Patient Radiation Protection

Outline Image Production 1.D.1 is patient radiation protection: collimation (shutters and virtual collimation), magnification, frame rates, geometry (SID, OID, tube angle), pulsed or continuous fluoroscopy, last image hold, and dose rate. The CI technologist is not a spectator of automatic brightness control. Every second of beam-on, every extra centimeter of air gap, and every cine run is a patient-dose decision.

Quick Answer: Use the inverse-square law. Keep the detector close (small OID, small air gap). Raise the table so the patient is not sitting on the tube. Collimate rather than magnify. Use pulsed fluoro at the lowest clinically useful frame rate. Talk off the pedal with last image hold (LIH). Save cine/acquisition for pictures you truly need. Children and young women's breasts and eyes are high-priority extra-sensitive targets.

Inverse square and geometry (SID, OID, tube angle)

X-ray intensity falls with the square of distance from the source. Double the distance, quarter the intensity. For a typical under-table tube:

• Raise the table (patient farther from the tube) to reduce entrance skin dose, within the limits of isocenter and the operator's reach. Parking the table on the housing to make someone taller more comfortable is how you cook the back.
• Lower the image receptor / detector onto the patient to decrease OID (object-to-image distance) and the air gap. A detector high in the air magnifies anatomy, increases scatter reaching the detector, and drives automatic brightness control (ABC) to raise output — patient dose goes up.
• SID (source-to-image distance) is tube-to-detector. The usual CI mistake is a large gap between patient and detector, not a “better SID.” Bring the detector down; do not invent geometry that leaves the receptor at the ceiling.

Thoughtful table and detector moves. Do not slam the table to the floor if that parks the patient's back against the tube. Do not leave the detector high “so the C-arm can spin” through a long chronic-total-occlusion case. After steep angles, re-check that the detector is still close.

Steep caudal and cranial angles (and heavy LAO/RAO) increase path length through the thorax. ABC raises kV/mA. Entrance skin dose at the beam-entry surface climbs, and the same skin patch can be re-irradiated as you return to a favorite steep caudal. That is how deterministic skin injury (hair loss, erythema, rare necrosis) happens after a long PCI. Flatten the angles when the working view is no longer needed. Use the shallowest angle that answers the question.

Cath labs use an under-table tube. If a stem ever describes an over-table tube, the table-height advice reverses for source-to-skin distance — but the detector-close rule does not.

Collimation versus magnification

Collimation uses lead shutters to irradiate only the anatomy of interest. Smaller field → less dose-area product (DAP), less scatter (better image and less staff dose), and often a lower chance of overlapping skin maps. Virtual collimation shows shutter positions on the last image without live fluoro so you can frame the next shot before you step on the pedal. Use it. Aiming shutters on live fluoro wastes the feature.

Magnification (smaller FOV, electronic or geometric mag) makes vessels look larger but increases dose rate because ABC pushes more photons into a smaller detector area. Mag when you must see a 2 mm dissection. Collimate rather than mag when you only needed to crop out lung. Mag is not a harmless zoom button. Geometric mag from a large OID is the worst of both worlds: blur plus dose.

Frame rates, pulsed versus continuous, dose rate

Frame rate (pulses or frames per second) is roughly linear with dose if pulse width and mA stay similar: 15 fps costs about twice 7.5 fps. Diagnostic coronary fluoro often lives at 7.5–15 pps; many labs default lower for non-coronary fluoro. Drop the rate during positioning and wire work when motion blur is acceptable.

Pulsed fluoroscopy delivers discrete pulses. Continuous fluoro is an older, high-dose mode that should not be the CI default. If a stem still offers continuous, treat it as the high-dose trap.

Dose-rate modes (low/normal/high fluoro, cine/acquisition) change pulse width and current. High-level or “boost” fluoro is for a few seconds of a bad image, not for the entire case. Cine / digital acquisition can be an order of magnitude higher dose than fluoro. Fluoro store (storing the LIH or a fluoro loop) is the low-dose way to keep a picture of a balloon inflation. Do not cine a memory shot when fluoro store would do.

Last image hold

Last image hold (LIH) freezes the last fluoro frame on the monitor when you come off the pedal. Talk, point, and teach on LIH, not on live fluoro. “Hold the shot while I explain the diagonal” is an occupational and patient-dose error. LIH is free. Live fluoro is not.

Who is extra-sensitive

Pediatric patients have longer remaining lifetime for stochastic cancer risk, smaller mass, and less room to collimate carelessly. Use pulsed low frame rate, aggressive collimation, no decorative mag, and the shortest cine list that answers the question. ABC will otherwise treat a small child like a thick adult if you leave adult defaults.

Young women: breasts in the primary beam during caudal and steep LAO/RAO are a breast-dose problem. Collimate, avoid unnecessary caudal that parks breast in the field, and keep arms out of the beam.

Eyes (lens) of the patient matter when cranial angles and a large field include the face — more often a neuro setup, but CI structural work with cranial angulation still deserves shutters off the orbits.

A shield that sits in the primary beam can increase automatic output. Patient shielding helps only when it is not fighting ABC. Under-table lead is primarily a staff-scatter tool, not a substitute for collimation on the patient's skin map.

Deterministic versus stochastic (conceptual)

Deterministic (tissue) effects have a practical threshold. Skin is the CI organ of concern: transient erythema, epilation, and, at high peak skin doses, dermal necrosis. These track peak skin dose / air kerma at the reference point, not fluoro minutes alone. Teaching discussion of possible skin effects often sits in the few-gray range (labs commonly talk around 2–5 Gy as an NCRP-style substantial-dose conversation). That magnitude is radiation-protection teaching, not an ARRT pass/fail cutoff.

Stochastic effects (cancer, heritable risk) have no threshold in the linear no-threshold teaching model. Risk rises with effective dose and is more concerning in children and young adults. Collimation, fewer cine runs, and lower frame rates cut both stories, but they are not the same injury.

Action / effect on patient dose

Worked case

A 16-year-old and a 78-year-old both need a diagnostic left-heart catheterization. Same room. For the adolescent: lowest pulsed rate that is diagnostic, shutters in with virtual collimation before the pedal, detector down, table not on the tube, no extra caudal “because that is our lab's favorite,” LIH for the huddle, fluoro store for a balloon picture, cine only for the runs that change therapy, breasts and eyes out of the open field. For the older adult the stochastic story is smaller, but a two-hour CTO in steep caudal still writes a skin-dose story — same geometry rules, because deterministic injury does not care that the patient is 78.

Exam traps

• Equating fluoro time with gray of skin dose.
• Magnifying to crop instead of shutting collimators.
• Teaching on live fluoro.
• Leaving the detector high “for room to work.”
• Running adult 15 fps cine lists on children because “the machine auto-exposes.”