Image Production and Evaluation
Image chain basics
Fluoroscopy provides real-time low-dose imaging; cine and digital angiography acquire higher-dose sequences for review. Spatial resolution depends on pixel size, matrix, and focal spot; contrast resolution depends on bit depth and photon statistics. Noise (quantum mottle) decreases as dose increases. Digital subtraction angiography (DSA) subtracts a mask image from contrast-filled frames, so patient motion produces misregistration artifact.
Acquisition choices
Frame rate drives dose and temporal resolution: 15 fps is often adequate for coronaries, 30 fps for rapid motion or ventriculography. Pulsed fluoroscopy at 7.5 pps roughly halves dose compared with 15 pps. Last-image hold and stored fluoroscopy loops reduce screening time. Magnification improves perceived detail only when the focal spot is small; it always raises patient skin dose and often lowers receptor dose.
Evaluation tasks
Ventriculography estimates ejection fraction by area-length or Simpson methods; EF = (EDV - ESV) / EDV. Stenosis quantification compares minimal lumen diameter with a reference segment, and QCA reduces visual bias. ECG gating helps select end-diastolic frames. Collimation and table height affect scatter and geometric unsharpness: keep the receptor close to the patient and the tube far away.
Traps
- Believing magnification always improves resolution.
- Confusing kVp effects on contrast with mA effects on noise.
- Forgetting that panning the table during DSA destroys registration.
Rule of thumb
If the image is noisy, raise dose or reduce magnification and frame rate; if contrast is poor in a large patient, raise kVp modestly rather than pushing mA to its limit.