Simon Wildermuth

Dual-source CT in step-and-shoot mode: noninvasive coronary angiography with low radiation dose.

PURPOSE To prospectively investigate computed tomographic (CT) image quality parameters by using different protocols and to calculate radiation dose estimates for noninvasive coronary angiography performed with dual-source CT in the step-and-shoot (SAS) mode. MATERIALS AND METHODS This study was local ethics board approved; written informed consent was obtained from all patients. In the preliminary portion of the study, 40 patients underwent CT coronary angiography in the SAS mode: at 100 kV (protocol A) in 22 patients with a body mass index (BMI) of less than 25 kg/m(2) and at 120 kV (protocol B) in 18 patients with a BMI of 25-30 kg/m(2). Both protocols involved use of an attenuation-based tube current and 1 mL of contrast material per kilogram of body weight. The final portion of the study involved 50 additional patients: 21 patients with a BMI of 25-30 kg/cm(2) assigned to protocol B and 29 patients with a BMI of less than 25 kg/cm(2) assigned to protocol C, which was performed with 100 kV, an attenuation-based tube current, and a reduced contrast material dose of 0.8 mL/kg. Image quality was independently assessed. Attenuation in the aorta and coronary arteries and image noise were measured. Radiation dose was estimated. RESULTS Mean image noise was similar with protocols A and B. Mean attenuation in the aorta and coronary arteries with protocol A (444 HU) was significantly (P < .001) higher than that with protocol B (358 HU). The reduced contrast material dose in protocol C yielded attenuation similar to that with protocol B. Diagnostic image quality was achieved with all protocols in 1237 (97.9%) of 1264 coronary segments. No significant differences in image quality between the 100- and 120-kV protocols were found. Mean heart rate had a significant effect on motion artifacts (area under receiver operating characteristic curve [AUC] = 0.818; 95% confidence interval [CI]: 0.723, 0.892; P < .001), whereas heart rate variability had a significant effect on stair-step artifacts (AUC = 0.79; 95% CI: 0.687, 0.865; P < .001). The mean estimated effective dose was 1.2 mSv +/- 0.2 for protocols A and C and 2.6 mSv +/- 0.5 for protocol B. CONCLUSION Dual-source SAS-mode CT coronary angiography yielded diagnostic image quality for 97.9% of coronary segments at a low radiation dose.

Dual- and multi-energy CT: approach to functional imaging

The energy spectrum of X-ray photons after passage through an absorber contains information about its elemental composition. Thus, tissue characterisation becomes feasible provided that absorption characteristics can be measured or differentiated. Dual-energy CT uses two X-ray spectra enabling material differentiation by analysing material-dependent photo-electric and Compton effects. Elemental concentrations can thereby be determined using three-material decomposition algorithms. In comparison to dual-energy CT used in clinical practice, recently developed energy-sensitive photon-counting detectors sample the material-specific attenuation curves at multiple energy levels and within narrow energy bands; the latter allows the detection of element-specific, k-edge discontinuities of the photo-electric cross section. Multi-energy CT imaging therefore is able to concurrently identify multiple materials with increased accuracy. These specific data on material distribution provide information beyond morphological CT, and approach functional imaging. This article reviews the principles of dual- and multi-energy CT imaging, hardware approaches and clinical applications.

MR-Guided Percutaneous Angioplasty: Assessment of Tracking Safety, Catheter Handling and Functionality

AbstractPurpose: Magnetic resonance (MR)-guided percutaneous vascular interventions have evolved to a practical possibility with the advent of open-configuration MR systems and real-time tracking techniques. The purpose of this study was to assess an MR-tracking percutaneous transluminal angioplasty (PTA) catheter with regard to its safety profile and functionality. Methods: Real-time, biplanar tracking of the PTA catheter was made possible by incorporating a small radiofrequency (RF) coil in the catheter tip and connecting it to a coaxial cable embedded in the catheter wall. To evaluate potentially hazardous thermal effects due to the incorporation of the coil, temperature measurements were performed within and around the coil under various canning and tracking conditions at 1.5 Tesla (T). Catheter force transmission and balloon-burst pressure of the MR-tracking PTA catheter were compared with those of a standard PTA catheter. The dilatative capability of the angioplasty balloon was assessed in vitro as well as in vivo, in an isolated femoral artery segment in a swine. Results: The degree of heating at the RF coil was directly proportional to the power of the RF pulses. Heating was negligible with MR tracking, conventional spin-echo and low-flip gradient-echo sequences. Sequences with higher duty cycles, such as fast spin echo, produced harmful heating effects. Force transmission of the MR-tracking PTA catheter was slightly inferior to that of the standard PTA catheter, while balloon-burst pressures were similar to those of conventional catheters. The MR-tracking PTA catheter functioned well both in vitro and in vivo. Conclusion: The in vivo use of an MR-tracking PTA catheter is safe under most scanning conditions.

Computational Fluid Dynamics: Hemodynamic Changes in Abdominal Aortic Aneurysm After Stent-Graft Implantation

The aim of this study was to demonstrate quantitatively and qualitatively the hemodynamic changes in abdominal aortic aneurysms (AAA) after stent-graft placement based on multidetector CT angiography (MDCT-A) datasets using the possibilities of computational fluid dynamics (CFD). Eleven patients with AAA and one patient with left-side common iliac aneurysm undergoing MDCT-A before and after stent-graft implantation were included. Based on the CT datasets, three-dimensional grid-based models of AAA were built. The minimal size of tetrahedrons was determined for grid-independence simulation. The CFD program was validated by comparing the calculated flow with an experimentally generated flow in an identical, anatomically correct silicon model of an AAA. Based on the results, pulsatile flow was simulated. A laminar, incompressible flow-based inlet condition, zero traction-force outlet boundary, and a no-slip wall boundary condition was applied. The measured flow volume and visualized flow pattern, wall pressure, and wall shear stress before and after stent-graft implantation were compared. The experimentally and numerically generated streamlines are highly congruent. After stenting, the simulation shows a reduction of wall pressure and wall shear stress and a more equal flow through both external iliac arteries after stenting. The postimplantation flow pattern is characterized by a reduction of turbulences. New areas of high pressure and shear stress appear at the stent bifurcation and docking area. CFD is a versatile and noninvasive tool to demonstrate changes of flow rate and flow pattern caused by stent-graft implantation. The desired effect and possible complications of a stent-graft implantation can be visualized. CFD is a highly promising technique and improves our understanding of the local structural and fluid dynamic conditions for abdominal aortic stent placement.

Segmental Arterial Mediolysis: CTA Findings at Presentation and Follow-Up

OBJECTIVE Segmental arterial mediolysis is a rare noninflammatory vascular disease of the abdominal splanchnic arteries. The purpose of our study was to retrospectively describe the CT angiography (CTA) findings of this disease and the evolution of those findings over time in five patients. CONCLUSION Comparison of CTA and digital subtraction angiography suggests that CTA is useful to diagnose symptomatic segmental arterial mediolysis. Midterm CTA follow-up (median, 3 years) indicates that segmental arterial mediolysis lesions may resolve or remain unchanged.

A survey of interactive mesh‐cutting techniques and a new method for implementing generalized interactive mesh cutting using virtual tools‡

In our experience, mesh-cutting methods can be distinguished by how their solutions address the following major issues: definition of the cut path, primitive removal and re-meshing, number of new primitives created, when re-meshing is performed, and representation of the cutting tool. Many researches have developed schemes for interactive mesh cutting with the goals of reducing the number of new primitives created, creating new primitives with good aspect ratios, avoiding a disconnected mesh structure between primitives in the cut path, and representing the path traversed by the tool as accurately as possible. The goal of this paper is to explain how, by using a very simple framework, one can build a generalized cutting scheme. This method allows for any arbitrary cut to be made within a virtual object, and can simulate cutting surface, layered surface or tetrahedral objects using a virtual scalpel, scissors, or loop cautery tool. This method has been implemented in a real-time, haptic-rate surgical simulation system allowing arbitrary cuts to be made on high-resolution patient-specific models. Published in 2002 by John Wiley & Sons, Ltd.

Towards active guidewire visualization in interventional magnetic resonance imaging

Improving the visibility of interventional devices is of paramount importance if MRI-guided fluoroscopy is to become a reality. Passive visualization is problematic in that the susceptibility-induced artifacts are material- and orientation-dependent. Here a concept is presented for making interventional devices visible. It involves fitting a device with a straight-wire antenna. As the sensitivity of such an antenna is highest for signal sources in the immediate neighborhood, using the antenna for reception gives an outline image. In this manner a guidewire or other interventional device could be made MRI-visible. The image appearance of a straight-wire antenna depends on the orientation of the device with respect to the main magnetic field and imaging plane. This phenomena is discussed theoretically and documented with MR images.

Multi-detector computed tomography of acute abdomen.

Acute abdominal pain is one of the most common causes for referrals to the emergency department. The sudden onset of severe abdominal pain characterising the “acute abdomen” requires rapid and accurate identification of a potentially life-threatening abdominal pathology to provide a timely referral to the appropriate physician. While the physical examination and laboratory investigations are often non-specific, computed tomography (CT) has evolved as the first-line imaging modality in patients with an acute abdomen. Because the new multi-detector CT (MDCT) scanner generations provide increased speed, greater volume coverage and thinner slices, the acceptance of CT for abdominal imaging has increased rapidly. The goal of this article is to discuss the role of MDCT in the diagnostic work-up of acute abdominal pain.

Endovascular Therapeutic Options for Isolated Iliac Aneurysms with a Working Classification

The purpose of this paper is to demonstrate a variety of stent-grafting and embolization techniques and describe a new classification for endovascular treatment of isolated iliac artery aneurysms. A total of 19 patients were treated for isolated iliac aneurysms. Depending on the proximal iliac neck and the uni-/bilaterality of common iliac artery aneurysms (CIAA’s) the patient may be treated by a tube (Type Ia) or a bifurcated stent-graft (Type Ib) in addition to internal iliac artery embolization. Neck anatomy is also critical in determining therapeutical options for internal iliac artery aneurysms (IIAA’s). These are tube stent-grafting plus internal iliac branch embolization (Type IIa), coiling of afferent and efferent internal iliac vessels (Type IIb) and IIAA packing (Type IIc). The average length of stay for these procedures was 3.8 days. During the mean follow-up of 20.9 months, aneurysm size remained unchanged in all but 4 patients. Reinterventions were necessary in option Type Ib (3/8 pat.) and Type Ia (1/7 pat.) due to extender stent-graft migration (n = 2) or reperfusion leaks (n = 2). We conclude that Iliac artery aneurysms may be successfully and safely treated by a tailored approach using embolization or a combination of embolization and stent-grafting. Long-term CT imaging follow-up is necessary, particularly in patients treated with bifurcated stent-grafts (Type Ib).

A Study on the Compliance of a Right Coronary Artery and Its Impact on Wall Shear Stress

A computational model incorporating physiological motion and uniform transient wall deformation of a branchless right coronary artery (RCA) was developed to assess the influence of artery compliance on wall shear stress (WSS). Arterial geometry and deformation were derived from modern medical imaging techniques, whereas the blood flow was solved numerically employing a moving-grid approach using a well-validated in-house finite element code. The simulation results indicate that artery compliance affects the WSS in the RCA heterogeneously, with the distal region mostly experiencing these effects. Under physiological inflow conditions, coronary compliance contributed to phase changes in the WSS time history, without affecting the temporal gradient of the local WSS nor the bounds of the WSS magnitude. Compliance does not cause considerable changes to the topology of WSS vector patterns nor to the localization of WSS minima along the RCA. We conclude that compliance is not an important factor affecting local hemodynamics in the proximal region of the RCA while the influence of compliance in the distal region needs to be evaluated in conjunction with the outflow to the myocardium through the major branches of the RCA.