How a conventional CT detector works
In a conventional scanner, X-rays that pass through the body strike a scintillator crystal, which converts them into visible light. A photodiode behind the crystal measures the light and produces an electrical signal proportional to the total energy arriving over a short time. Information about individual X-ray photons, and their energies, is lost in that averaging step, and electronic noise is added.
What photon-counting changes
A photon-counting detector uses a semiconductor (cadmium telluride) that converts each X-ray photon directly into an electrical pulse. The electronics count every pulse and sort it by energy. Three practical things follow.
- Higher spatial resolution. The detector elements are smaller and there is no light spreading between them, so images resolve finer detail. In the coronary arteries this means sharper edges on calcified plaque and inside stents, where conventional CT often over-estimates narrowing because calcium "blooms".
- Lower radiation dose. Electronic noise is removed and low-energy photons that carry little information are weighted appropriately, so the same image quality needs fewer X-rays. Contrast dose can often be reduced too.
- Spectral information in every scan. Because photons are sorted by energy, the scanner can separate materials such as iodine contrast, calcium and soft tissue in a single acquisition. This helps characterise the plaque itself, not just the narrowing it causes.
Why it matters for the coronary arteries
The coronary arteries are only two to four millimetres wide and move with every heartbeat. Any improvement in resolution translates directly into fewer scans that are "non-diagnostic" because of calcium or a stent, and fewer people sent on to an invasive angiogram that turns out to be unnecessary. Early clinical studies have reported improved assessment of heavily calcified vessels and of stent patency compared with conventional CT.
Photon-counting CT in Adelaide
Jones Radiology at SAHMRI on North Terrace operates a photon-counting CT scanner, and A/Prof Verjans refers CT coronary angiograms there. The scan itself feels the same to the patient as a conventional CTCA: the same cannula, heart-rate tablet, GTN spray and breath-holds. The difference is in the pictures.
Limits
Photon-counting CT does not remove the need for a slow, regular heart rhythm, and it still uses iodine contrast and ionising radiation, albeit less. It is one tool among several; for some patients a stress echocardiogram or an invasive angiogram remains the right test, and for questions about the heart muscle itself, such as myocarditis or cardiomyopathy, cardiac MRI is the better modality.