Felix G. Meinel

State of the Art: Iterative CT Reconstruction Techniques

Owing to recent advances in computing power, iterative reconstruction (IR) algorithms have become a clinically viable option in computed tomographic (CT) imaging. Substantial evidence is accumulating about the advantages of IR algorithms over established analytical methods, such as filtered back projection. IR improves image quality through cyclic image processing. Although all available solutions share the common mechanism of artifact reduction and/or potential for radiation dose savings, chiefly due to image noise suppression, the magnitude of these effects depends on the specific IR algorithm. In the first section of this contribution, the technical bases of IR are briefly reviewed and the currently available algorithms released by the major CT manufacturers are described. In the second part, the current status of their clinical implementation is surveyed. Regardless of the applied IR algorithm, the available evidence attests to the substantial potential of IR algorithms for overcoming traditional limitations in CT imaging.

Metal artifact reduction by dual-energy computed tomography using energetic extrapolation: a systematically optimized protocol.

ObjectivesEnergetic extrapolation is a promising strategy to reduce metal artifacts in dual-source computed tomography (DSCT). We performed this study to systematically optimize image acquisition parameters for this approach in a hip phantom and assess its value in a clinical study. Materials and MethodsTitanium and steel hip prostheses were placed in a standard hip phantom and a water tank and scanned on a DSCT scanner. Tube spectra, tube current ratio, collimation, pitch, and rotation time were optimized in a stepwise process. Artifacts were quantified by measuring the standard deviation of the computed tomography density in a doughnut-shaped region of interest placed around the prosthesis. A total of 22 adult individuals with metallic implants referred for computed tomography for a musculoskeletal indication were scanned using the optimized protocol. Degree of artifacts and diagnostic image quality were rated visually (0–10) and maximum streak intensity was measured. ResultsSn140/100 kVp proved superior to Sn140/80 kVp. There was a benefit for increasing tube current ratio from 1:1 to 3:1, but not beyond, in favor of the Sn140 kVp spectrum. Artifacts were less severe for a collimation of 32 × 0.6 mm as compared with 40 × 0.6 mm. A pitch of 0.5 at a rotation time of 0.5 seconds per rotation was preferable to other combinations with comparable scanning times. In the clinical study, increasing the extrapolated photon energy from 64 to 120 keV decreased the severity of artifacts from 8.0 to 2.0 (P < 0.001) and decreased streak intensity from 871 to 153 HU (P < 0.001). The median diagnostic image quality rating improved from 2.5 to 8.0 (P < 0.001). The median energy level visually perceived as optimal for diagnostic evaluation was 113 keV (range, 100–130 keV). ConclusionsSn140/100 kVp with a tube current ratio of 3:1, a collimation of 32 × 0.6 mm, and extrapolated energies of 105 to 120 keV are optimal parameters for a dedicated DSCT protocol that effectively reduces metal artifacts by energetic extrapolation. The protocol effectively reduces metal artifacts in all types of metal implants. The optimized reconstructions yielded relevant additional findings.

Emphysema diagnosis using X-ray dark-field imaging at a laser-driven compact synchrotron light source

In early stages of various pulmonary diseases, such as emphysema and fibrosis, the change in X-ray attenuation is not detectable with absorption-based radiography. To monitor the morphological changes that the alveoli network undergoes in the progression of these diseases, we propose using the dark-field signal, which is related to small-angle scattering in the sample. Combined with the absorption-based image, the dark-field signal enables better discrimination between healthy and emphysematous lung tissue in a mouse model. All measurements have been performed at 36 keV using a monochromatic laser-driven miniature synchrotron X-ray source (Compact Light Source). In this paper we present grating-based dark-field images of emphysematous vs. healthy lung tissue, where the strong dependence of the dark-field signal on mean alveolar size leads to improved diagnosis of emphysema in lung radiographs.

First–Arterial-Pass Dual-Energy CT for Assessment of Myocardial Blood Supply: Do We Need Rest, Stress, and Delayed Acquisition? Comparison with SPECT

PURPOSE To compare the relative contributions of rest, stress, and delayed acquisitions with the accuracy of dual-energy (DE) computed tomography (CT) for the assessment of myocardial blood supply. MATERIALS AND METHODS With institutional review board approval and HIPAA compliance, 55 consecutive patients (10 women, 45 men; mean age, 62 years ± 10) clinically referred for cardiac single photon emission computed tomography (SPECT) who were known to have or were suspected of having coronary artery disease were prospectively enrolled. DE CT studies were acquired during adenosine stress, at rest, and after 6-minute delay. The DE CT iodine distribution maps were visually assessed for perfusion deficits or late iodine enhancement. Per-segment agreement between modalities was investigated with κ statistics. Test characteristics for the detection of perfusion deficits were calculated for combinations of rest, stress, and delayed DE CT acquisition, with SPECT as reference standard. RESULTS At SPECT, 714 segments were considered normal, 192 showed fixed perfusion defects, and 29 showed reversible perfusion deficits. Sensitivity of rest-only DE CT was 92%, and specificity was 98%. Stress-only, rest-stress, stress and delayed, and the combination of all three had a sensitivity of 99% and a specificity of 97%. Of 29 segments with reversible perfusion deficits at SPECT, 13 (45%) were misclassified by using rest-stress DE CT as fixed perfusion deficits. With stress DE CT plus delayed acquisition, 13 of 192 (7%) segments with fixed perfusion deficits at SPECT were misclassified as reversible. CONCLUSION Rest-stress acquisition should be the protocol of choice for assessment of the myocardial blood supply in DE CT. The accuracy of DE CT is not increased by the addition of a delayed DE CT acquisition, which may therefore be omitted to reduce radiation exposure. With rest-stress DE CT, almost one-half of defects that are reversible at SPECT were classified as fixed; radiologists and clinicians need to be aware of this incongruence when they interpret DE CT myocardial perfusion studies.

Pulmonary Emphysema Diagnosis with a Preclinical Small-Animal X-ray Dark-Field Scatter-Contrast Scanner

PURPOSE To test the hypothesis that the joint distribution of x-ray transmission and dark-field signals obtained with a compact cone-beam preclinical scanner with a polychromatic source can be used to diagnose pulmonary emphysema in ex vivo murine lungs. MATERIALS AND METHODS The animal care committee approved this study. Three excised murine lungs with pulmonary emphysema and three excised murine control lungs were imaged ex vivo by using a grating-based micro-computed tomographic (CT) scanner. To evaluate the diagnostic value, the natural logarithm of relative transmission and the natural logarithm of dark-field scatter signal were plotted on a per-pixel basis on a scatterplot. Probability density function was fit to the joint distribution by using principle component analysis. An emphysema map was calculated based on the fitted probability density function. RESULTS The two-dimensional scatterplot showed a characteristic difference between control and emphysematous lungs. Control lungs had lower average median logarithmic transmission (-0.29 vs -0.18, P = .1) and lower average dark-field signal (-0.54 vs -0.37, P = .1) than emphysematous lungs. The angle to the vertical axis of the fitted regions also varied significantly (7.8° for control lungs vs 15.9° for emphysematous lungs). The calculated emphysema distribution map showed good agreement with histologic findings. CONCLUSION X-ray dark-field scatter images of murine lungs obtained with a preclinical scanner can be used in the diagnosis of pulmonary emphysema. SUPPLEMENTAL MATERIAL http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.13122413/-/DC1.

The novel inhibitor of histone deacetylase resminostat (RAS2410) inhibits proliferation and induces apoptosis in multiple myeloma (MM) cells

Inhibition of histone deacetylase (HDAC) is a promising mechanism for novel, anti‐myeloma agents. We investigated the effects of the novel HDAC inhibitor resminostat on multiple myeloma (MM) cells in vitro. Resminostat is a potent inhibitor of HDACs 1, 3 and 6 [50% inhibitory concentration (IC50) = 43–72 nmol/l] representing HDAC classes I and II and induces hyperacetylation of histone H4 in MM cells. Low micromolar concentrations of resminostat abrogated cell growth and strongly induced apoptosis (IC50 = 2·5–3 μmol/l in 3 out of 4 MM cell lines) in MM cell lines as well as primary MM cells. At 1 μmol/l, resminostat inhibited proliferation and induced G0/G1 cell cycle arrest in 3 out of 4 MM cell lines accompanied with decreased levels of cyclin D1, cdc25a, Cdk4 and pRb as well as upregulation of p21. Resminostat decreased phosphorylation of 4E‐BP1 and p70S6k indicating an interference with Akt pathway signalling. Treatment with resminostat resulted in increased protein levels of Bim and Bax and decreased levels of Bcl‐xL. Caspases 3, 8 and 9 were activated by resminostat. Furthermore, synergistic effects were observed for combinations of resminostat with melphalan and the proteasome inhibitors bortezomib and S‐2209. In conclusion, we have identified potent anti‐myeloma activity for this novel HDAC inhibitor.

Coronary Artery Computed Tomography Scanning

A 46-year-old woman with diabetes mellitus and a body mass index of 32 kg/m2 reports repeated episodes of chest pain after moderate activity. single photon emission computed tomography myocardial perfusion imaging demonstrates no electrocardiographic (EKG) abnormalities and a small fixed perfusion defect in the posterior left ventricle. Prospectively EKG-triggered coronary CT angiography (CCTA), performed with a radiation dose of 3.5 mSv, demonstrates unremarkable coronary arteries with no evidence of stenosis or atherosclerosis (Figure 1). Figure 1. Unremarkable coronary CT angiography study with no evidence of stenosis or atherosclerosis. Cx indicates circumflex artery; D1, first diagonal; LAD, left anterior descending; LM, left main; OM1, first obtuse marginal; and RCA, right coronary artery. A 63-year-old man with treated hypertension and hyperlipidemia experiences diffuse chest pain and shortness of breath after a long distance flight. Catheter angiography 2 years earlier was normal. At emergency department arrival, his EKG demonstrates no signs of myocardial injury. Cardiac troponin I is 0.04 ng/mL. An EKG-synchronized acute chest pain CCTA examination demonstrates extensive noncalcified plaque of the mid left anterior descending coronary artery causing severe stenosis with signs of acute myocardial hypoperfusion in the anterior and apical left ventricle (Figure 2). The patient undergoes successful revascularization with a drug eluting stent. Figure 2. Coronary CT angiography examination ( A–C ) demonstrates extensive noncalcified plaque of the mid left anterior descending coronary artery causing severe stenosis (arrows) with signs of acute myocardial hypoperfusion in the anterior and apical left ventricle (arrowheads). The obstructive lesion is confirmed by cardiac catheterization ( D ). CCTA has left the early stages of clinical evaluation and matured into a robust diagnostic technique in both elective and emergent settings. Technological innovations are continuously improving the diagnostic performance and decreasing the radiation dose associated with this test. In this Clinician Update, we provide an updated summary on the state-of- …

Xenon-Enhanced Dual-Energy CT Lung Ventilation Imaging: Techniques and Clinical Applications

OBJECTIVE The purpose of this article is to review the techniques, clinically relevant potential applications, and limitations of xenon-enhanced dual-energy CT of the chest. CONCLUSION The functional evaluation of lung ventilation may be of great importance to patients with pulmonary disease. Many measures are used to assess pulmonary function, but the results are estimates of only global status rather than the regional distribution of disease. With the introduction of dual-energy CT, regional lung ventilation function can be assessed with inhaled xenon gas. This technique yields not only high-spatial-resolution anatomic information but also information about regional ventilation.

CT Myocardial Perfusion Imaging

OBJECTIVE. CT myocardial perfusion imaging is rapidly becoming an important adjunct to coronary CT angiography for the anatomic and functional assessment of coronary artery disease with a single modality. Existing techniques for CT myocardial perfusion imaging include static techniques, which provide a snapshot of the myocardial blood pool, and dynamic techniques. CONCLUSION. This review provides a systematic overview of the presently available approaches for the assessment of myocardial perfusion at CT, including diagnostic accuracy and limitations.

Contrast-Induced Acute Kidney Injury: Definition, Epidemiology, and Outcome

Contrast-induced acute kidney injury (CI-AKI) is commonly defined as a decline in kidney function occurring in a narrow time window after administration of iodinated contrast material. The incidence of AKI after contrast material administration greatly depends on the specific definition and cutoff values used. Although self-limiting in most cases, postcontrast AKI carries a risk of more permanent renal insufficiency, dialysis, and death. The risk of AKI from contrast material, in particular when administered intravenously for contrast-enhanced CT, has been exaggerated by older, noncontrolled studies due to background fluctuations in renal function. More recent evidence from controlled studies suggests that the risk is likely nonexistent in patients with normal renal function, but there may be a risk in patients with renal insufficiency. However, even in this patient population, the risk of CI-AKI is probably much smaller than traditionally assumed. Since volume expansion is the only preventive strategy with a convincing evidence base, liberal hydration should be encouraged to further minimize the risk. The benefits of the diagnostic information gained from contrast-enhanced examinations will still need to be balanced with the potential risk of CI-AKI for the individual patient and clinical scenario.