Photon-Counting Detector CT: Key Points Radiologists Should Know

Abstract

Photon-counting detector (PCD) CT is a new CT technology utilizing a direct conversion X-ray detector, where incident X-ray photon energies are directly recorded as electronical signals. The design of the photon-counting detector itself facilitates improvements in spatial resolution (via smaller detector pixel design) and iodine signal (via count weighting) while still permitting multi-energy imaging. PCD-CT can eliminate electronic noise and reduce artifacts due to the use of energy thresholds. Improved dose efficiency is important for low dose CT and pediatric imaging. The ultra-high spatial resolution of PCD-CT design permits lower dose scanning for all body regions and is particularly helpful in identifying important imaging findings in thoracic and musculoskeletal CT. Improved iodine signal may be helpful for low contrast tasks in abdominal imaging. Virtual monoenergetic images and material classification will assist with numerous diagnostic tasks in abdominal, musculoskeletal, and cardiovascular imaging. Dual-source PCD-CT permits multi-energy CT images of the heart and coronary arteries at high temporal resolution. In this special review article, we review the clinical benefits of this technology across a wide variety of radiological subspecialties.

Keywords: Clinical applications; Computed tomography; Diagnostic imaging; Photon counting X-ray detectors; Spectral tomography.

Fig. 1. Schematic comparison of conventional EIDs and PCDs.

A. EIDs use a scintillator to generate visible light when an incident X-ray photon hits them, then the light is recorded by a photodiode with reflective septa in between detector elements to reduce crosstalk. B. While PCD-CT uses a semiconductor to directly generate positive and negative charges, with negative charges going to pixelated anodes to record each individual photon and its energy. EID = energy integrating detector, PCD = photon-counting detector

Fig. 2. A 74-year-old male clinically diagnosed with idiopathic non-specific interstitial pneumonia was scanned on conventional energy-integrating detector CT (A) and investigational PCD-CT (B) using a clinical routine protocol.

A, B. PCD-CT demonstrates fine reticulations (arrowheads, B) in the right subpleural right lower lobe, compared to conventional CT, which appears to show ground glass opacities in this region (arrowheads, A). PCD-CT more sharply displays traction bronchiectasis than conventional CT (arrows). PCD = photon-counting detector

Fig. 3. A 56-year-old male with multiple myeloma.

A, B. Axial energy integrating detector-CT (A) and PCD-CT (B) slices through the thoracic spine. Lytic lesions in the thoracic spine are more clearly seen on the PCD-CT image. A lytic lesion in the posterior aspect of the vertebral body with breach of the posterior cortex is more clearly dealinated (dashed arrows). A smaller lytic lesion in the vertebral body (arrows) is more conspicuous on the PCD-CT image. PCD = photon-counting detector

Fig. 4. The incudostapedial joint (arrows), shown on energy integrating detector-CT (A) and PCD-CT (B) images.

The joint was one of several anatomic structures specifically graded using a 5-point Likert score, with higher scores favoring the quality of the PCD-CT images. Adapted from Benson et al. AJNR Am J Neuroradiol 2022;43:579-584 [18]. PCD = photon-counting detector

Fig. 5. A 70-year-old female with a history of resected intrahepatic cholangiocarcinoma, gastric bypass, and a side-to-side jejunojejunostomy.

A, B. Both studies were taken at a tube voltage of 120 kV. Coronal energy integrating detector-CT (A) shows the jejunojejunostomy (dashed arrow) but photon-counting detector-CT (B) improves the visualization of the contrast and the sharpness of the folds in the jejunojejunostomy (arrow).

Fig. 6. A 67-year-old female with pancreatic adenocarcinoma.

A-D. Compared with axial and coronal 2-mm images acquired with energy integrating detector-CT (A, B), axial and coronal 1-mm images acquired with PCD-CT (C, D) provide better visualization of the hypodense tumor in the uncinate owing to the ability of PCD-CT to highlight iodine contrast; also in these images, the ability of PCD-CT to display thinner slices without substantial increase in image noise is demonstrated. Thinner slices reduce partial volume averaging for small structures and pathologies. PCD = photon-counting detector

Fig. 7. A 64-year-old patient with metastatic pancreatic cancer.

A, B. Photon-counting detector-CT image (A) shows a very small liver metastasis in the right posterior section (arrows) confirmed on subsequent MRI (B).

Fig. 8. A 73-year-old female with peritoneal dissemination of ovarian cancer.

A. Energy integrating detector-CT demonstrates irregularity of the serosa of the sigmoid colon and questionable nodularity along the anterior peritoneal reflection. B. Photon-counting detector-CT clearly demonstrates small tumor implants causing irregular and nodular-like thickening of the anterior peritoneal reflection (arrows).

Fig. 9. A 69-year-old female patient with known peripheral arterial disease.

A, B. 3-dimensional reconstruction images reconstructed from energy integrating detector-CT angiography (A) and 145 mL of iodinated contrast appear similar to PCD-CT angiography images (B) using only 55 mL of the same iodinated contrast agent. This example illustrates the ability to leverage improved iodine signal from PCD-CT for reduced need for iodine contrast. PCD = photon-counting detector

Fig. 10. A 36-year-old male patient with gout arthritis with tophi.

A-D. Images obtained using PCD-CT with subsequent material classification show monosodium urate deposition in green at the great toe interphalangeal joint.

Fig. 11. A 6-year-old female clinically diagnosed with cystic fibrosis was scanned on a PCD-CT (CT dose index: 0.05 mGy inspiration [shown] and 0.05 mGy expiration).

PCD-CT demonstrates cylindrical bronchiectasis in the right middle lobe (arrow). PCD = photon-counting detector

Fig. 12. A 74-year-old male patient with known peripheral arterial disease.

A, B. Axial reconstructions in a patient with peripheral arterial disease show calcium blooming in the anterior tibial artery on energy integrating detector-CT (arrow, A). Compared with energy integrating detector-CT reconstruction (A), photon-counting detector-CT reconstruction (B) in the same patient at the same level shows significantly improved visualization of the calcium plaque in the anterior tibial artery (arrow, B) because of which the luminal caliber can be better assessed.

Fig. 13. Calcium separation algorithm.

A. Axial energy integrating detector-CT reconstruction in a patient with peripheral arterial disease shows a dense calcific plaque in the right common femoral artery (arrow). B. Axial photon-counting detector-CT reconstruction of the same patient with the use of a dedicated calcium separation algorithm shows subtraction of the calcified plaque (arrow).