Imaging Procedures and Equipment
Imaging Procedures and Equipment in CT tests protocol selection, patient positioning, contrast timing, and safe operation of the gantry, power injector, and detector system. The exam favors the sequence of steps that produces diagnostic images without overscan.
Core rules
- Use the scout/topogram to set the scan range; include the entire organ or vascular segment and avoid scanning beyond it.
- Match collimation and pitch to the protocol. Pitch = table feed per gantry rotation ÷ total beam width. Pitch >1 shortens scan time and lowers dose but can increase noise if mA is fixed.
- Give clear breath-hold instructions and use bolus tracking for arterial studies. Place the ROI in the target vessel, set a threshold, then add the protocol delay.
Worked example
For CT angiography of the abdominal aorta, you place the ROI in the descending aorta at the level of the diaphragm. You trigger at 120 HU and add a 6–8 second post-threshold delay so the contrast column reaches the iliac arteries. Scanning too early opacifies the aorta but leaves the runoff poorly enhanced; too late allows venous contamination.
Traps
- Centering the patient low in the gantry increases dose and can distort AEC.
- Oral contrast is not routine for every abdominal indication; it can obscure or delay the study.
- Do not confuse injector flow rate with catheter gauge limits.
- Reconstructing only thick slices can hide small lesions and prevents good MPR.