Erwin Immel

Liver displacement during ventilation in Thiel embalmed human cadavers - a possible model for research and training in minimally invasive therapies

Abstract Respiration-related movement of organs is a complication in a range of diagnostic and interventional procedures. The development and validation of techniques to compensate for such movement requires appropriate models. Human cadavers embalmed with the Thiel method remain flexible and could provide a suitable model. In this study liver displacement during ventilation was assessed in eight Thiel embalmed cadavers, all of which showed thoracic and abdominal motion. Four cadavers displayed realistic lung behaviour, one showed some signs of pneumothorax after prolonged ventilation, one had limited filling of the lungs, and two displayed significant leakage of air into the thorax. A coronal slice containing the largest section through the liver was imaged with a real-time Fast Gradient Echo (FGR) MRI sequence: Craniocaudal displacement of the liver was then determined from a time-series of slices. The maximum liver displacement observed in the cadavers ranged from 7 to 35 mm. The ventilation applied was comparable to tidal breathing at rest and the results found for liver displacement are similar to values in the literature for respiratory motion of the liver under similar conditions. This indicates that Thiel embalmed cadavers have potential as a model for research and training in minimally invasive procedures.

Improvement of the MR imaging behavior of vascular implants

Vascular implants can cause significant MR image artifacts due to the material (susceptibility artifact) or the electromagnetic characteristics (RF artifact). These artifacts are caused by the distortion of the magnetic field and interferences with the radio frequency (RF) waves of the MR imaging process. Void or complete vanishing of signals occurs in close proximity or inside implants. The artifacts can be minimized by using a material with low magnetic susceptibility and a design of the implant which avoids electrical conductive loops. But not all designs can be made loop‐free and non conductive. A resonant circuit tuned to the Larmor frequency of the MR tomography overcomes the RF artifact and thus improves the visualization of the implant lumen. The paper reviews the state‐of‐the‐art technology of the MR‐signal improvement in implants lumen, with particular regard to the use of resonant circuits such as stents or Vena Cava Filter (VCF), with resonators in 1.0 Tesla and 1.5Tesla MRT.

Evaluation of an Active Vena Cava Filter for MR Imaging in a Swine Model

PURPOSE To evaluate the feasibility of magnetic resonance (MR) imaging-guided placement of an active vena cava filter (AVCF) in a swine model, the effectiveness of the system in filtering thrombi, and the detection of thrombi with MR imaging. MATERIALS AND METHODS This study was approved by the government committee on animal investigations. An AVCF tuned to the Larmor frequency of a 1.5-T MR unit was placed in the inferior vena cava (IVC) of seven pigs under real-time MR imaging guidance. Steady-state free precession sequences with four different flip angles (90°, 40°, 25°, and 15°), T1-weighted turbo spin-echo sequences with two flip angles (90° and 15°), and black-blood proton-density-weighted sequences with a flip angle of 90° were performed before and after filter placement. In six cases, extracorporeally produced thrombi were injected through the femoral access to test filter function. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were assessed before and after filter deployment and compared by using the signed-rank test. RESULTS All AVCFs were successfully deployed. Significant differences (P < .05) in the SNR and CNR of the IVC were found before and after AVCF placement and between sequences with different flip angles. Intravenous thrombi were caught in all cases and clearly depicted with MR imaging. On black-blood proton-density-weighted images, high-signal-intensity thrombi inside the filter were clearly detectable without any overlaying artifacts. CONCLUSION MR imaging-guided deployment and monitoring of an AVCF is feasible. The AVCF enhances the SNR and CNR, resulting in clear depiction of thrombi inside the filter without the need for contrast material. Design modifications for improved intracaval fixation and retrieval of the prototype AVCF will be required.

MR-Guided Interventions and Surgery

This chapter covers the use of magnetic resonance imaging (MRI) for guidance of interventional and operative procedures, including the basics on MRI systems design, integration, workflow principles, device visualization, and instrument positioning and tracking. MRI provides soft tissue contrast superior to x-ray technology and ultrasound. The contrast can be weighted towards water or fat, and images acquired in arbitrary, multiplanar orientations and three-dimensional (3-D) volumes. MRI’s flow and temperature sensitivity alongside its lack of ionizing radiation and nephrotoxic iodinated contrast agents renders it a suitable imaging technique for vascular and percutaneous interventions. Intraoperative MR imaging allows detection of hidden structures of the tissue volume in the surgical field. Although MRI has been applied since the beginning of the 1990s during operation and intervention, the lack of approved MRI-compatible tools and the technical hurdles in integrating MRI systems into clinical applications have hampered its wider distribution. These problems can be overcome through appropriate technical solutions and the use of suitable nonmagnetic and nonconductive materials.

Experimental MRI visible resonant prosthetic heart valves

Artificial heart valves comprising metal can cause significant MR image artifacts due to the material (susceptibility artifact) and/or electromagnetic characteristics (RF artifact). The purpose of our study was to examine current commercially available heart valve prostheses, integrate resonant circuits and to produce a protoype self-expanding heart valve for MRI-guided placement. Different types of commercially available heart valves were tested in MRI. Freshly excised porcine heart valves were sutured with 6–0 prolene into a 21 mm Nitinol stent. Resonant circuits were integrated into the heart valves and tuned to the Larmor frequency of the MRI (42.58 MHz for 1.0 T and about 64 MHz for 1.5 T and 128 MHz  for 3T). The artifacts caused by the non-ferromagnetic heart valves and the Nitinol stent could be overcome. MRI signal could be enhanced using low flip angles <40 and visualization improved in 1T, 1.5T and 3T MRI. MRI-guided implantation was facilitated. A resonant circuit tuned to the Larmor frequency of the MR tomography can overcome the RF artifacts and thus improve the visualization of prosthetic heart valves.

Comparative ergonomic workflow and user experience analysis of MRI versus fluoroscopy-guided vascular interventions: an iliac angioplasty exemplar case study

PurposeA methodological framework is introduced to assess and compare a conventional fluoroscopy protocol for peripheral angioplasty with a new magnetic resonant imaging (MRI)-guided protocol. Different scenarios were considered during interventions on a perfused arterial phantom with regard to time-based and cognitive task analysis, user experience and ergonomics.MethodsThree clinicians with different expertise performed a total of 43 simulated common iliac angioplasties (9 fluoroscopic, 34 MRI-guided) in two blocks of sessions. Six different configurations for MRI guidance were tested in the first block. Four of them were evaluated in the second block and compared to the fluoroscopy protocol. Relevant stages’ durations were collected, and interventions were audio-visually recorded from different perspectives. A cued retrospective protocol analysis (CRPA) was undertaken, including personal interviews. In addition, ergonomic constraints in the MRI suite were evaluated.ResultsSignificant differences were found when comparing the performance between MRI configurations versus fluoroscopy. Two configurations [with times of 8.56 (0.64) and 9.48 (1.13) min] led to reduce procedure time for MRI guidance, comparable to fluoroscopy [8.49 (0.75) min]. The CRPA pointed out the main influential factors for clinical procedure performance. The ergonomic analysis quantified musculoskeletal risks for interventional radiologists when utilising MRI. Several alternatives were suggested to prevent potential low-back injuries.ConclusionsThis work presents a step towards the implementation of efficient operational protocols for MRI-guided procedures based on an integral and multidisciplinary framework, applicable to the assessment of current vascular protocols. The use of first-user perspective raises the possibility of establishing new forms of clinical training and education.

MR Enhancing Implants

Implantation of cardiovascular implants such as stents, prosthetic heart valves or vena cava filters (VCF) are usually performed using X-ray-based imaging techniques (fluoroscopy) in a cath lab. Possible migration and intraluminal changes in the implants require a post interventional follow-up diagnostic procedure. For example, restenosis (up to 25 %) and late in-stent thrombosis (up to 2 % by in drug-eluting stents and despite dual anti-platelet therapy with aspirin and thienopyridine up to 6 % in diabetes patients) frequently occur after the implantation of a vascular stent. Vena cava filter is by nature subject of thrombus capturing. For explantation and retrieval, it must be assured that no large blood clot formations are still present in the filter. Prosthetic heart valves and the recently introduced balloon and self-expanding heart valves for transapical and TAVI procedure require follow-up imaging of possible complications such as paravalvular leakages, valve thrombosis or malfunction of the valve leaflets which are made of bovine pericardium or porcine heart valves. Therefore, post interventional non-invasive follow-up diagnostic is needed. Transthoracic ultrasound (TUS) is the method of choice for routine follow-up but due to the sound scattering at the valve scaffold, diagnosis is compromised. More thorough diagnosis can be achieved by invasive techniques such as transesophageal endoscopic ultrasound (TEE) or intravascular ultrasound (IVUS) reveals more information but still the sound scattering leaves gaps in the diagnoses of valve function. X-ray-based invasive imaging with iodinated contrast agents can be applied but the disadvantage of this diagnostic method is, additional to the ionising radiation, adverse reactions to iodinated contrast agents. This includes general (acute and delayed) and organ-specific adverse effects (contrast-induced nephrotoxicity and cardiovascular, pulmonary and neurotoxicity) and occurs, for example, in 17–35 % of patients with history of previous adverse reaction (some of the predisposing factors for an adverse reaction are: infants and elderly, history of asthma or allergy, dehydration and heart disease).

MR enhancing vascular and cardiovascular Implants

Introduction: MRI-guided implantation of cardiovascular implants such as vascular stents, Vena Cava Filters (VCF), prosthetic heart valves (HV) and PFO occluders is limited because the materials conventionally used in these implants and other interventional tools cause significant MR image artefacts or produce very low image contrast. Resonant circuits tuned to the Larmor frequency of the MRI were used to enhance the MR signal and to improve the visualization of the implants.