Willi A. Kalender’s research while affiliated with Friedrich-Alexander-University Erlangen-Nürnberg and other places

Purpose Contrast‐enhanced imaging of the breast is frequently used in breast MRI and has recently become more common in mammography. The purpose of this study was to make single‐scan contrast‐enhanced imaging feasible for photon‐counting breast CT (pcBCT) and to assess the spectral performance of a pcBCT scanner by evaluating iodine maps and virtual non‐contrast (VNC) images. Methods We optimized the settings of a pcBCT to maximize the signal‐to‐noise ratio between iodinated contrast agent and breast tissue. Therefore, an electronic energy threshold dividing the x‐ray spectrum used into two energy bins was swept from 23.17 keV to 50.65 keV. Validation measurements were performed by placing syringes with contrast agent (2.5 mg/ml to 40 mg/ml) in phantoms with 7.5 cm and 12 cm in diameter. Images were acquired at different tube currents and reconstructed with 300 μm isotropic voxel size. Iodine maps and VNC images were generated using image‐based material decomposition. Iodine concentrations and CT values were measured for each syringe and compared to the known concentrations and reference CT values. Results Maximal signal‐to‐noise ratios were found at a threshold position of 32.59 keV. Accurate iodine quantification (average root mean square error of 0.56 mg/ml) was possible down to a concentration of 2.5 mg/ml for all tube currents investigated. The enhancement has been sufficiently removed in the VNC images, so they can be interpreted as unenhanced CT images. Only minor changes of CT values compared to a conventional CT scan were observed. Noise was increased by the decomposition by a factor of 2.62 and 4.87 (7.5 cm and 12 cm phantoms) but did not compromise the accuracy of the iodine quantification. Conclusions Accurate iodine quantification and generation of VNC images can be achieved using contrast‐enhanced pcBCT from a single CT scan in the absence of temporal or spatial misalignment. Using iodine maps and VNC images, pcBCT has the potential to reduce dose, shorten examination and reading time, and to increase cancer detection rates.

Objectives: The purpose of this work is to present the data obtained from the first clinical in vivo application of a new dedicated spiral breast computed tomography (B-CT) equipped with a photon-counting detector. Materials and methods: The institutional review board approved this retrospective study. Twelve women referred for breast cancer screening were included and underwent bilateral spiral B-CT acquired in prone position. Additional sonography was performed in case of dense breast tissue or any B-CT findings. In 3 women, previous mammography was available for comparison. Soft tissue (ST) and high-resolution (HR) images were reconstructed. Two independent radiologists performed separately the readout for subjective image quality and for imaging findings detection. Objective image quality evaluation was performed in consensus and included spatial resolution, contrast resolution, signal-to-noise ratio (SNR), and contrast-to-noise ratio. All women were asked to report about positioning comfort and overall comfort during data acquisition. Results: The major pectoral muscle was included in 15 breast CT scans (62.5%); glandular component was partially missing in 2 (8.3%) of the 24 scanned breasts. A thin "ring artifact" was present in all scans but had no influence on image interpretations; no other artifacts were present. Subjective image quality assessment showed excellent agreement between the 2 readers (κ = 1). Three masses were depicted in B-CT and were confirmed as simple cysts in sonography. Additional 5 simple cysts and 2 solid benign lesions were identified only in sonography. A total of 12 calcifications were depicted with a median size of 1.1 mm (interquartile range, 0.7-1.7 mm) on HR and 1.4 mm (interquartile range, 1.1-1.8 mm) on ST images. Median SNRgl, SNRfat, and contrast-to-noise ratio were significantly higher in ST than in HR reconstructions (each, P < 0.001). A mild discomfort due to positioning of the rib cage on the table was reported by 2 women (16.7%); otherwise, no discomfort was reported. Conclusions: The new dedicated B-CT equipped with a photon-counting detector provides high-quality images with potential for screening of breast cancer along with minor patient discomfort.

Since their discovery in 1895, x-rays have been widely used for imaging humans. Recently they have also gained on importance in small-animal imaging (SAI). Most techniques known from clinical medicine, including single- and dual-energy x-ray imaging, have been successfully ported to SAI and are the subject of this chapter. As trivial as it is, simple x-ray examinations may bring diagnostically valuable information in a variety of applications. Unenhanced radiography reveals skeletal anatomy, contrast-enhanced imaging allows improved visualization of the vasculature and strongly vascularized areas, and dedicated methods such as bone densitometry deliver quantitative information. In analogy to clinical x-ray imaging, we will separately describe standard two-dimensional (2D) projection imaging and the more advanced three-dimensional (3D) computed tomography (CT) imaging techniques. Also in analogy to clinical applications, CT is considered to be of significantly higher importance as it provides more information and possibilities than conventional 2D approaches. It will therefore be covered in much more detail.

Objectives: The performance of metal artifact reduction (MAR) methods in x-ray computed tomography (CT) suffers from incorrect identification of metallic implants in the artifact-affected volumetric images. The aim of this study was to investigate potential improvements of state-of-the-art MAR methods by using prior information on geometry and material of the implant. Materials and methods: The influence of a novel prior knowledge-based segmentation (PS) compared with threshold-based segmentation (TS) on 2 MAR methods (linear interpolation [LI] and normalized-MAR [NORMAR]) was investigated. The segmentation is the initial step of both MAR methods. Prior knowledge-based segmentation uses 3-dimensional registered computer-aided design (CAD) data as prior knowledge to estimate the correct position and orientation of the metallic objects. Threshold-based segmentation uses an adaptive threshold to identify metal. Subsequently, for LI and NORMAR, the selected voxels are projected into the raw data domain to mark metal areas. Attenuation values in these areas are replaced by different interpolation schemes followed by a second reconstruction. Finally, the previously selected metal voxels are replaced by the metal voxels determined by PS or TS in the initial reconstruction. First, we investigated in an elaborate phantom study if the knowledge of the exact implant shape extracted from the CAD data provided by the manufacturer of the implant can improve the MAR result. Second, the leg of a human cadaver was scanned using a clinical CT system before and after the implantation of an artificial knee joint. The results were compared regarding segmentation accuracy, CT number accuracy, and the restoration of distorted structures. Results: The use of PS improved the efficacy of LI and NORMAR compared with TS. Artifacts caused by insufficient segmentation were reduced, and additional information was made available within the projection data. The estimation of the implant shape was more exact and not dependent on a threshold value. Consequently, the visibility of structures was improved when comparing the new approach to the standard method. This was further confirmed by improved CT value accuracy and reduced image noise. Conclusions: The PS approach based on prior implant information provides image quality which is superior to TS-based MAR, especially when the shape of the metallic implant is complex. The new approach can be useful for improving MAR methods and dose calculations within radiation therapy based on the MAR corrected CT images.

PURPOSE Using dedicated breast computed tomography (bCT) for detection and diagnosis of breast cancer is a novel approach in breast imaging. Existing bCT systems showed comparable performance to digital mammography (DM) and breast tomosynthesis (BT) in detection of lesions especially if contrast media were applied but do not have sufficient resolution to detect microcalcifications (µCa) smaller than 300 µm. The purpose of the study was to compare a novel high-resolution low-dose bCT system to clinical DM and BT. METHOD AND MATERIALS 30 surgical specimens were evaluated for this study. 14 of the specimens were lumpectomies, 16 total mastectomies. All women had a pre-operatively diagnosed breast cancer or DCIS. Specimens were investigated directly after surgery with DM, BT, bCT and pathology examination (ground truth). DM and BT were used with standard clinical settings, bCT with a tube voltage of 60 kV. Dose was kept below 5 mGy for bCT. 3 breast imaging experts examined the randomized data sets. Time for image viewing was recorded. Sensitivity and specificity for detection of lesions and calcifications were calculated. RESULTS Histology revealed 17 invasive cancers and 10 DCIS in the specimens (27 lesions in total). 16 of the specimens contained calcifications. 73 % of the specimens were rated as heterogeneously or extremely dense in DM. Mean time for image viewing was 77 s for DM, 122 s for BT and 131 s for BCT. Sensitivity for lesions was 41 % for DM, 52 % for BT and 70 % for bCT. Sensitivity for calcifications was 75 % for DM, 69 % for BT and 94 % for bCT. Specificity for lesions was 71 % for DM, 29 % for BT and 71 % for bCT. Specificity for calcifications was 67 % for all modalities. CONCLUSION For detection of lesions as well as calcifications, bCT showed superior sensitivity compared to DM and BT. Radiologists are not used to inspect bCT images in clinical routine, viewing times nevertheless were still comparable to those of BT. Sensitivity and specificity for lesion detection could potentially be increased further using contrast media. CLINICAL RELEVANCE/APPLICATION Dedicated high-resolution low-dose bCT proved to be superior to DM and BT especially for detection of calcifications and lesions in dense breasts.

Rationale and objectives: This study compared a novel photon-counting breast computed tomography (pcBCT) system with digital mammography (DM) and digital breast tomosynthesis (DBT) systems. For this reason, surgical specimens were examined with all three techniques and rated by three observers. Materials and methods: A total of 30 surgical specimens were investigated with DM, DBT, and pcBCT; the associated images were shown to three experienced radiologists. Findings (22 microcalcifications and 23 mass lesions) were recorded and compared to the results of the pathological examination. Sensitivity and specificity for detection of microcalcifications and lesions were calculated and displayed using receiver operating characteristic curves. Results: Sensitivity for microcalcifications was 82% for DM, 70% for DBT, and 85% for pcBCT. Specificity for microcalcifications was 71% for DM, 75% for DBT, and 83% for pcBCT. Sensitivity for lesions was 45% for DM, 62% for DBT, and 65% for pcBCT. Specificity for lesions was 76% for DM, 62% for DBT, and 76% for pcBCT. Conclusions: pcBCT showed a comparable or superior performance compared to the clinically approved DM and DBT systems. Mass lesion detectability can be increased further by the use of contrast media.

The increase in the radiation exposure from CT examinations prompted the investigation on the various dose-reduction techniques. Significant dose reduction has been achieved and the level of radiation exposure of thoracic CT is expected to reach the level equivalent to several chest X-ray examinations. With more scanners with advanced dose reduction capability deployed, knowledge on the radiation dose reduction methods has become essential to clinical practice as well as academic research. This article reviews the history of dose reduction techniques, ongoing changes brought by newer technologies and areas of further investigation.

Introduction: Recent studies have shown a substantial reduction of radiation dose from computed tomography (CT) scans down to 0.1 mSv for lung cancer screening and cardiac examinations, when applying optimization techniques. Hence, CT localizer radiographs (LRs) might now be considered a significant contributor to the total dose of the CT examination. We investigated in our study the potential for reducing dose of the LRs by adapting the patient-specific acquisition parameters of the LR. Materials and methods: Localizer radiographs covering the lungs were acquired on 2 clinical scanners (64 slices, conventional detector [CD]; 96 slices, fully integrated detector [ID]) for 3 semianthropomorphic phantoms, representing a slim, a normal, and an obese adult. Starting at 120-kV tube voltage and 250-mA current were reduced until the image quality of the LR, and thereby the accuracy of the automatic exposure control was compromised; this was defined as a deviation of measured attenuation values in the center of the LR of more than 5% from the reference values measured at the highest tube voltage and current. Subsequent Monte Carlo calculations on anthropomorphic phantoms were performed to calculate organ and effective dose values for the respective optimal settings. In addition, effective dose values normalized to CTDIvol for tube voltages ranging from 60 to 160 kV were determined for the different combinations of phantom sizes, sexes, and LR views to evaluate dose efficiency. Results: For the CD scanner, the optimal LR settings depended strongly on phantom size. Higher tube voltage and current were necessary for the larger phantoms. The ID scanner showed uncompromised LR quality for all phantoms using the lowest possible tube voltage-tube current combination of 80 kV and 20 mA. Depending on patient size and LR direction, effective dose values for the optimal settings ranged from 6 to 53 μSv and 3 to 11 μSv for the CD and ID scanner, respectively. For the example of an anterior-posterior LR on a normal patient, using the optimal settings instead of the standard settings on the ID scanner reduced LR dose from 53 μSv (120 kV, 30 mA) to 10 μSv (80 kV, 20 mA). The simulations for the different tube voltages show that effective dose and CTDIvol behave similarly for different views and patient sizes. However, the tube voltage level itself impacts the relationship between CTDIvol and effective dose, by up to a factor of 2. Discussion: Dose from LRs may contribute significantly to the total effective dose of low-dose CT examinations such as lung cancer screening. Optimal LR settings can reduce LR dose substantially, but adaptations have to consider scanner characteristics, detector technology, and patient size. Thus, for low-dose CT examinations, such as cardiac examinations and lung cancer screening, LR optimization may result in a significant dose reduction and thereby in a substantial reduction of total dose.

This final chapter of the book discusses possible or likely developments in technology needed for the thoracic region, i.e. for lung, cardiac and even breast imaging by CT. Respective activities related to hardware, software and applications are addressed in the next two sections. Considerations on image quality and dose will point out some basic physics constraints that have to be kept in mind. Application-related aspects are taken into account throughout the chapter.