PC-CT in non-cardiac applications

by Alan Barclay

The objective of Prof. Tamandl’s talk was to illustrate the performance of the new PC-CT system on non-cardiac applications by showing examples of images acquired under real-life clinical conditions. The majority of the images were acquired in his institution over the last year and a half. But first Prof. Tamandl briefly summarized the advantages of PC-CT over conventional EID CT as being

◾ Increased spatial resolution, with no dose penalty.

The in-plane resolution is 0.151 mm × 0.176 mm at the isocenter.

◾ Improved Contrast to Noise Ratio, with improved iodine sensitivity. Less noise and the possibility of lower dose of contrast agents.

◾ The possibility of spectral imaging in every scan, enabling virtual mono-energetic reconstructions, iodine maps and virtual non-contrast reconstructions.

A) Lung Patients

Prof. Tamandl showed the comparison of images acquired by PC-CT and EID CT from a patient with interstitial lung disease. In this study the PC-CT examination was carried out applying a straightforward protocol — no special tweaking / set-up was required. The EID images were acquired using a high-end conventional EID CT scanner, which used to be the flagship scanner in the hospital. The images for PC-CT were much clearer than those from EID CT at approximately the same dose. When quantitative analysis of image quality is carried out, the PC-CT images show statistically significant improvement in terms of quality compared to images acquired by EID CT.

Figure 3. Effect of varying keV: a patient with Hepatocellular Carcinoma (HCC) having had an administration of 90 ml of IOMERON 400. It can be seen that as the chosen keV level is decreased, e. g. to 40 keV or 50 keV, the attenuation and clarity of smaller structures are improved.

In long-COVID patients, i. e., patients with persistent Covid symptoms, it was found that, when compared with EID CT, PC-CT detected additional findings, e. g. bronchio - ectasis in about 50 % of cases, for examinations carried out at approximately the same radiation dose. Such findings have direct clinical significance.

Systemic sclerosis – Interstitial lung Disease. In a study from Dr. Alkadhi’s group in Zurich, comparisons were made between by conventional EID CT and PC-CT, which for this study was carried out at lower doses, namely 100 %, 66 % and 33 % of the dose used in EID CT. Even at 33 % of the dose used in the conventional scanner, the images were equivalent in terms of diagnostic performance. Efforts are on-going to reduce the dose even further with no loss of diagnostic performance.

B) MSK patients

It was found that PC-CT was clearly superior to EID CT in detecting osteolysis in patients with multiple myeloma in studies carried out at approximately the same dose.

Temporal bone imaging. In a young patient with recurrent middle ear infections who had a stapes prosthesis inserted, it was found that the PC-CT images were less noisy, and clearer than EID CT even at half the dose. More extensive studies confirmed these findings.

C) Oncology patients / abdominal imaging

Patients with hepatocellular carcinoma were imaged, using the spectral imaging capabilities of the new photon counting CT system which can provide virtual monoenergetic images (VMI), e.g. at 40 keV, 50 kEV, 60 keV, 70 keV & 80 keV [Figure 3]. Such mono-energetic images do not have to be set up prior to the PC-CT examination; they can be extracted from the data during or after the examination. At low keV levels, the iodine attenuation is of course much greater. The price to pay for this, namely increased noise, is still acceptable, i. e. the contrast-to-noise ratio is increased compared to EID CT. Several broader studies have been carried with the same conclusion, namely that with low-keV VMI, PC-CT yields significantly improved objective and subjective quality of arterial phase oncological imaging compared with EID CT. This advantage may translate into higher diagnostic confidence and lower radiation dose protocols. Interestingly in such stud- ies, although the average dose with PC-CT was 18 % lower than that with EID CT, there were nevertheless some patients who actually received higher dose than with EID CT. However, this might be due to ongoing protocol optimizations.

Obese patients: The question of noise in the imaging of obese patients is an issue — the usual tradeoff using EID CT is either accepting very noisy images at reasonable radiation doses or significantly increasing the dose to reduce the noise. With PC-CT there is still an increase in noise with increasing BMI of obese patients but studies have shown that the dose used is 25 % less than that with EID CT. Thus the use of PC-CT is preferred with obese patients.

One reason behind the high quality of the PC-CT images is that a new iterative reconstruction algorithm, specially designed for PC-CT is incorporated in the new system. Known as quantitative iterative reconstruction (QIR), this algorithm was first evaluated in phantoms where it was found that the loss of accuracy as dose is reduced was much less than the loss of accuracy with even high level EID CT. Such findings have been confirmed in human patients where it was shown that the use of QIR decreased noise by 44 % and increased the CNR by 74 % in portal venous phase examinations.

Iodine quantification: It was found that the error in iodine quantification was roughly equivalent to that obtained with even high-level EID CT systems, but as the size of the patient or phantom increases, the increase on the iodine error quantification was much less than that with EID CT If it is desired to go in the opposite direction, i.e. removing the iodine, so creating Virtual Non Contrast (VNC) images, there has always been a small residual iodine attenuation in VNC images compared to true non-contrast (TNC) images. This difference is constantly being reduced with development of the software. Although the quality of VNC images in photon counting CT is still slightly less good than in TNC, they are nevertheless still acceptable for qualitative purposes.

One main use of VNC is in steatosis assessment. It was found that the VNC images from the PC-CT were reliable for the assessment of hepatic steatosis, although there may be a tendency to slightly overestimate the number of patients with steatosis, because of a small error of approximately 8 – 10. Hounsfield Units between the VNC and the TNC. Successive software updates are reducing this error.

TAKE-HOME MESSAGES

Prof. Tamandl’s main conclusions were:

◾ Higher resolution with less noise in non-contrast images was shown in:-Interstitial Lung Disease patients; Post-Covid investigations ; Patients with Temporal bone prostheses; MSK patients

◾ Spectral information similar to dual energy CT but available all the time

◾ Higher Contrast-Noise-Ratio less noise: Better performance in low dose scans; Virtual mono-energetic reconstructions are now standard Iodine quantification and virtual non-contrast images easily available

More Information

A video recording of this Braccosponsored symposium is available at: https://connect.myesr.org/course/ photon-counting-ct-a-new-challenge-for-contrast-administration/