Photon-counting detectors in computed tomography: from quantum physics to clinical practice.

Over the last decade, a fundamentally new type of computed tomography (CT) detectors has proved its superior capabilities in both physical and preclinical evaluations and is now approaching the stage of clinical practice. These detectors are able to discriminate single photons and quantify their energy and are hence called photon-counting detectors. Among the promising benefits of this technology are improved radiation dose efficiency, increased contrast-to-noise ratio, reduced metal artifacts, improved spatial resolution, simultaneous multi-energy acquisitions, and the prospect of multi-phase imaging within a single acquisition using multiple contrast agents. Taking the conventional energy-integrating detectors as a reference, the authors demonstrate the technical principles of this new technology and provide phantom and patient images acquired by a whole-body photon-counting CT. These images serve as a basis for discussing the potential future of clinical CT.

Potential of ultra-high-resolution photon-counting CT of bone metastases: initial experiences in breast cancer patients.

Conventional CT scanners use energy-integrating detectors (EIDs). Photon-counting detector (PCD) computed tomography (CT) utilizes a CT detector technology based on smaller detector pixels capable of counting single photons and in addition discriminating their energy. Goal of this study was to explore the potential of higher spatial resolution for imaging of bone metastases. Four female patients with histologically confirmed breast cancer and bone metastases were included between July and October 2019. All patients underwent conventional EID CT scans followed by a high resolution non-contrast experimental PCD CT scan. Ultra-high resolution (UHR) reconstruction kernels were used to reconstruct axial slices with voxel sizes of 0.3 mm × 0.3 mm (inplane) × 1 mm (z-direction). Four radiologists blinded for patient identity assessed the images and compared the quality to conventional CT using a qualitative Likert scale. In this case series, we present images of bone metastases in breast cancer patients using an experimental PCD CT scanner and ultra-high-resolution kernels. A tendency to both a smaller inter-reader variability in the structural assessment of lesion sizes and in the readers' opinion to an improved visualization of lesion margins and content was observed. In conclusion, while further studies are warranted, PCD CT has a high potential for therapy monitoring in breast cancer.

[Safety aspects of ultrasound diagnostics]. / Sicherheitsaspekte der Ultraschalldiagnostik.

Ultrasonography is free of ionizing radiation but during the examination it is accompanied by energy deposition in the tissue. Therefore, users should be familiar with the mechanisms of action and possible risks. Thermal and non-thermal (e.g. cavitation) effects are related to the intensity and sound pressure of ultrasound waves and are therefore also dependent on the ultrasound modality used, e.g., B­mode, color Doppler and/or pulsed wave (pw) Doppler. With B­mode ultrasound no dangerous thermal effects are to be expected. In embryos and foetuses as well as febrile patients caution should be exercised. The pw Doppler mode can cause temperature spikes and the risks increase with the duration of use. Ultrasound contrast media are pathogenic for cavitation and should be avoided during the 24 h prior to shock wave lithotripsy. In ultrasound modalities with high local energy deposition, the values for the thermal index (TI) and mechanical index (MI) displayed on the screen should be observed and as with ionizing radiation, the ALARA (as low as reasonably achievable) principle should be adhered to.

[Quantitative methods in sonography]. / Quantitative Verfahren in der Sonographie.

B­mode and color Doppler ultrasound are standard radiological methods to quantify tissue echo texture and tissue perfusion. Microstructure and composition of tissue influence echo texture parameters and acoustic parameters, such as speed of sound, attenuation and backscatter and quantitative color Doppler image parameters are influenced by the hemodynamics in depictable vessels. Dynamic contrast-enhanced ultrasound enables quantification of tissue perfusion and ultrasound elastography assists in assessing tissue stiffness. B­mode texture analysis, analysis of high-frequency echo signals and quantitative color Doppler image analysis are able to contribute to the assessment of tissue microstructure but have so far not been implemented clinically due to their complexity. Dynamic contrast-enhanced ultrasound and ultrasound elastography have proven to be robust under clinical conditions.

[Imaging of medullary thyroid carcinoma]. / Bildgebung beim medullären Schilddrüsenkarzinom.

BACKGROUND: About 10% of thyroid cancers are medullary thyroid carcinoma (MTC) and can occur sporadically, familially and in the context of type II multiple endocrine neoplasia (MEN). Imaging plays a pivotal role in screening family members and in diagnosis. DIAGNOSTIC METHODS: Diagnosis is based on ultrasound (US), thyroid scintigraphy, serum calcitonin and carcinoembryonic antigen (CEA) as well as fine needle biopsy. High-resolution US is the most important imaging method for locoregional staging, combined with computed tomography (CT) of the mediastinum. Positron emission tomography (PET-CT) using 18-F-DOPA is particularly suited for suspected occult metastases in case of rising tumor markers in serum. FINDINGS AND COURSE OF DISEASE: Diagnosis is made based on cytologic findings in a hypoechoic, cold thyroid nodule, combined with an elevation of serum calcitonin and CEA. US is the most important imaging modality during routine follow-up. CT is indicated for suspected mediastinal, lung, or liver metastases. CT should be replaced by MRI as early as possible to prevent significant cumulative radiation doses over time. RECENT CLINICAL DEVELOPMENTS: Although MTC is curable by surgery only, owing to its radio- and chemoresistance, the disease will often progress only slowly, and even patients with metastases will frequently survive 10 years or longer. For more aggressive variants and late symptomatic stages, targeted drugs that have the potential to indicate stabilization or even a partial remission of the disease are under clinical investigation or already approved.