How CT works, in four ideas
Each pixel is a number. The window decides which numbers you can see. Blur makes small things lie. Orientation is fixed by convention.
Every voxel is an attenuation number
The scanner measures how much each small block of tissue (a voxel) attenuates the X-ray beam and scales it against water and air.
water = 0 · air = −1000
Iodinated contrast raises blood from about 40 HU to 300–450 HU on a well-timed CTA. Calcium usually reads far higher, which is why calcium and contrast can still be told apart when the window is chosen well.
A screen shows ~256 grays; CT holds ~4000 numbers
The window picks a slice of the HU range to spread across the grays. Below L − W/2 is pure black (cyan outline), above L + W/2 is pure white (amber outline).
Why calcified iliacs look smaller than they are
Every CT image is slightly blurred by slice thickness and reconstruction kernel. Around bright calcium, the blur spills into the lumen. With a narrow window that spill saturates to white and the lumen looks narrower. This synthetic vessel has a true lumen of 6.0 mm with calcium on both sides.
The profile shows HU across the vessel. The amber line is the HU where the edge appears on screen with this window; cyan dashes mark the true lumen. Practical rule: measure calcified vessels in a wide window with a raised level, on thin slices.
You are standing at the patient's feet
- Axial slices stack head to feet. Coronal and sagittal images are rebuilt from the same voxels (multiplanar reformation, MPR).
- A double-oblique plane is tilted in two directions to sit exactly across a structure, such as the aortic annulus.
- MIP shows the brightest voxel along each ray (good for vessels, exaggerates calcium). Volume rendering is a 3D picture, not a measuring tool.
One contrast bolus, root to groin
- ECG-gated scan of the aortic root and heart, reconstructed in several phases so the annulus can be measured in systole.
- CTA of chest, abdomen and pelvis down past the femoral heads to show the whole access path. Usually from the same bolus.
- Contrast is timed with bolus tracking: the scan starts when a monitoring ROI in the aorta crosses a set HU threshold.
- Arms raised above the head to reduce streak artifact across the chest.
- Thin slices (about 0.5–1 mm) so oblique reformats stay sharp.
- Rate control with beta-blockers is generally used cautiously or avoided in severe aortic stenosis, unlike routine coronary CTA.
Cardiac CT for valve planning
Three skills: choose the phase, find the annular plane, and turn the annulus outline into numbers.
Pick the moment in the cardiac cycle
Gating ties each image to a percentage of the R–R interval. The annulus is usually a few percent larger in systole (around 30–40% R–R), so it is measured there. Raise the heart rate and watch the quiet part of diastole disappear.
Retrospective gating keeps the tube on through the whole cycle, so any phase can be reconstructed later (useful for picking the systolic annulus and for leaflet cine). Dose modulation drops the tube current outside the chosen window. Prospective triggering only exposes a window, which saves dose but limits which phases exist. Exposure numbers are illustrative.
Find the annular plane: line up three hinge points
The annulus is a virtual ring through the lowest attachment points (hinge points or nadirs) of the three cusps. It is tilted relative to the body, so an axial slice cuts it at an angle. Tilt the plane in the coronal and sagittal views, and move it up or down, until all three hinge points sit on it.
From an elliptical outline to sizing inputs
The annulus is usually an ellipse, not a circle. Trace it in systole and the software reports area and perimeter, then converts each into an "equivalent" circular diameter. Balloon-expandable valves are commonly sized by area and self-expanding valves by perimeter; always use the manufacturer's sizing chart.
Perimeter-derived diameter is always at least the area-derived one, and the gap widens as the annulus gets more elliptical. Calcium, especially in the LVOT, and bicuspid anatomy change how these numbers should be used.
Coronary obstruction screen
Coronary height is measured from the annular plane to the lower edge of each coronary ostium, perpendicular to the plane. Risk of coronary obstruction is commonly flagged with heights under about 10–12 mm, especially when the sinuses of Valsalva are narrow (under about 30 mm), and in valve-in-valve cases.
Will the sheath fit?
Compare the sheath's outer diameter with the smallest lumen on each side. Measurements you take in the viewer can be sent here.
Sheath labels often state the inner diameter; use the outer diameter from the manufacturer, and the largest expanded size for expandable sheaths. The defaults are examples; measure the scan in the viewer and send your values here.
The ratio is only the start
Calcium and tortuosity stop a vessel from stretching around a sheath. Tick what applies to the side you are favoring.
Reading the iliofemoral path
- Follow each side on a centerline from the aortic bifurcation to below the femoral bifurcation. Record the minimal luminal diameter, measured perpendicular to the vessel, inner wall to inner wall.
- Grade calcium by its arc (circumferential or horseshoe is worst) and note where it sits.
- Judge tortuosity on a 3D view; flexible, non-calcified tortuous vessels often straighten, calcified ones do not.
- Mark the puncture zone: common femoral artery over the femoral head, above the bifurcation, anterior wall free of calcium.
- If neither side works, assess alternative access: subclavian/axillary, transcarotid, transcaval.
Further reading: Hayashida et al., SFAR and vascular complications (2011) · SCCT expert consensus on CT for TAVI/TAVR (2019) · Ribeiro et al., coronary obstruction (2013)
Check yourself
Nine questions drawn from the other sections.