7.2 Digital Acquisition, Roadmapping & Projections

Digital Acquisition, Roadmapping & Projections

ARRT CI A.2 digital acquisition is exposure technique and frame rate. A.3 roadmapping is digital subtraction, field of view (magnification, collimation), compensating filters (wedge, soft), and 3D imaging. A.4 is projections/positions. This is how a noisy live fluoro picture becomes a diagnostic angiogram the operator can wire from and the archive can defend.

Quick Answer: Record adult coronaries at about 15 frames per second (teaching, not an ARRT secret number); use 30 fps for some pediatric or very fast motion. DSA builds the peripheral, aortic, and graft roadmap; motion ruins it. Collimate tightly. Mag modes raise dose. Wedge/soft filters tame bright lung versus dense mediastinum. 3D rotational angiography sizes structural work (TAVR annulus, LAA). Learn the standard coronaries: RAO caudal for LCx/OM, RAO cranial and LAO cranial for LAD/diagonals, LAO caudal spider for the left-main bifurcation, plus AP cranial and lateral.

Digital acquisition: exposure technique and frame rate

A cine/digital-acquisition pedal is a short radiographic series. ABC still runs, but the dose-per-frame target is much higher than fluoro. Before you press it:

• Collimate so lung and unattenuated field do not drive ABC down and underexpose the heart.
• Drop unnecessary magnification.
• Park the detector close (small OID) and avoid extreme SID stretch — geometry details return in 8.3.
• Use spectral filtration the vendor provides; do not defeat it to “brighten” a run.
• Keep the run as short as the anatomy allows. A five-second left coronary at 15 fps is already 75 frames of cine-level dose.

Frame rate (typical teaching, not an unpublished ARRT mandate):

• About 15 fps for adult coronary cine — enough temporal samples for a 60–80 beat heart without doubling frames.
• About 30 fps for some pediatric cases or fast motion (infant rates, severe tachycardia) when 15 fps undersamples.
• Ventriculography is often in the same 15 fps adult neighborhood; some labs drop frames if the ventricle is large and slow.
• Peripheral DSA commonly uses lower rates (a few frames per second) because vessels do not dance like coronaries.

Lower frame rate lowers dose if mA per frame is unchanged. Too low, and you miss a dissection flap that was only open in systole. Do not run 30 fps on a still, 50-beat adult as a sharpness trick.

Digital subtraction and the roadmap overlay

Digital subtraction angiography (DSA) stores a mask (pre-contrast) and subtracts it from contrast-filled frames so bone and soft tissue drop out and the vessel remains. That is the workhorse for peripheral work, aortography, bypass grafts (especially when clips and sternal wires compete), and some structural aortograms.

Motion artifact is the DSA failure mode. A breath, a cough, a table bump, or cardiac motion shifts anatomy between mask and fill. The result is misregistration — black-white edges that mimic dissection or hide the true lumen. Suspend respiration when the patient can cooperate. For coronaries, cardiac motion often makes DSA less useful than unsubtracted cine; native coronary work is usually cine, not DSA.

A roadmap is the subtracted (or contrast-filled) vessel map overlaid on live fluoro so you can wire a chronic total occlusion, a femoropopliteal lesion, or a structural target without another full contrast injection every millimeter. If the patient moves after the map is built, the overlay lies — remask or repeat. Pixel-shift rescue lives in 7.3; large motion needs a new acquisition, not faith in software.

Field of view: magnification versus collimation

Magnification modes (smaller electronic FOV on a flat-panel detector) improve spatial resolution for a tiny dissection or a 2.25 mm diagonal. ABC then concentrates output into that smaller field, so mag increases patient dose. Use mag for the one run that needs it; do not live-fluoro a whole PCI in high mag.

Collimation (lead shutters, including virtual collimation on last-image-hold) shrinks the irradiated field without that mag penalty. Scatter falls, KAP falls, and ABC is less likely to be fooled by bright corners. Collimate to the heart, the graft, or the iliac — not to the full detector. Tight collimation is the default; mag is the expensive zoom.

Compensating filters: wedge and soft

The cardiac silhouette sits next to air-filled lung. Unfiltered, the lung is a bright flood that can drive ABC so the mediastinum and spine underexpose, or it can saturate the detector while the artery is still gray.

• Wedge filters (thick edge over lung, thin edge over heart/spine) even the beam.
• Soft filters (copper/aluminum equalization filters in the collimator assembly) similarly harden and shape the beam over the bright field.

Place the thick portion over the lucent lung, not over the vessel you came to see. Filters do not replace collimation. They stop ABC from exposing for the lung instead of the coronary.

3D imaging

3D rotational angiography spins the C-arm through a wide arc (often on the order of 180°+) during a contrast injection, then reconstructs a volume. Cardiac-interventional uses cluster around structural work:

• TAVR / TAVI: annulus area and perimeter, coronary heights, coaxial implant angles
• Left atrial appendage (LAA) closure: ostial diameter, depth, and shape
• Aortic root and, less often, complex congenital or graft anatomy

The run is a high-dose acquisition. The reconstruction (windowing, centerline, measurements) is 7.3. Motion, late contrast, or an incomplete rotation yields a pretty but wrong volume — do not size a valve from garbage.

Projections and positions

Angiographic projection is the C-arm angle that unforeshortens the segment and opens bifurcations. Position includes how the patient and table sit under that beam. Typical teaching maps (angles vary by habitus; these are not ARRT-mandated degree lists):

RAO caudal looks “down” the AV groove at the circumflex. RAO cranial and LAO cranial lift the LAD off the spine and open diagonals; add LAO cranial when you need the distal shaft of the left main. LAO caudal spider is the left-main bifurcation view — the one that shows whether a distal-LM lesion commits LAD, LCx, or both. AP cranial is a simple LAD workhorse. Lateral (about 90°) pulls overlapping left-system vessels apart.

Table panning keeps the vessel in the collimated field during a long injection. Pan with the contrast, not after it. A perfect spider view that clips the ostial left main is a wasted high-dose run.

Scenario and traps

Elective left-coronary cine in a 70 kg adult: 15 fps, no extra mag, shutters on the heart, a wedge over the left lung if the field is bright, RAO caudal for the circumflex, RAO or LAO cranial for the LAD, spider if the left main is in question. A 4-year-old with a rapid rate may need 30 fps for a few short runs — teaching, not a mandate to cine every child at 30.

Iliac DSA for access: suspend breath, build the roadmap, wire on the overlay, recapture if the table moved. Do not DSA a native coronary expecting a still mask.

Traps: mag as default zoom. Collimation and mag treated as the same thing. Wedge over the artery instead of the lung. 30 fps adult cine “for quality.” Sizing a TAVR annulus from a motion-corrupted 3D spin. Using RAO cranial when the question is the LCx in the AV groove.