Comparing three-dimensional models of placenta accreta spectrum with surgical findings.

OBJECTIVE: Placenta accreta spectrum (PAS) is associated with significant maternal morbidity mainly related to blood loss. Pre-operative planning is aided by antenatal ultrasound and magnetic resonance imaging. We sought to assess whether three-dimensional (3D) models from MR images were accurate when compared with surgical and pathological findings. METHODS: Digital Imaging and Communications in Medicine files containing MR images with varying severity of PAS (n = 4) were modeled using 3D Slicer. Placenta, bladder, and myometrial defects were modeled. Myometrial defects at three different uterine locations were included-anterior, lateral and inferior. 3D models were used to identify the relationship between the myometrial defect and the internal cervical os. Findings were validated in a larger series of PAS cases (n = 14) where patterns of invasion were compared with estimated blood loss and distance from defect to the internal os. RESULTS: The defect illustrated in the four 3D models correlates to both surgical and pathological findings in terms of depth and pattern of invasion, location of defect, bladder involvement. Blood loss and topography of the defect from 3D modeling were examined in 14 further cases. Inferior defects were associated with increased blood loss compared with anterior defects. Increased distance from cervix was associated with reduced blood loss (R2  = 0.352, P = 0.01). CONCLUSION: Three-dimensional models of PAS provide an accurate preoperative description of placental invasion and should be investigated as a tool for selecting patients for uterine-conserving surgery. Accurate 3D models of placenta accreta spectrum are achievable and may provide additional information, such as distance of the defect from the internal os.

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

PURPOSE: A 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. METHODS: Three 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. RESULTS: Significant 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. CONCLUSIONS: This 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.

Heating and safety of a new MR-compatible guidewire prototype versus a standard nitinol guidewire.

Our purpose in this study was to examine heating of nitinol and polyetheretherketone (PEEK) guidewires during near-real-time MR imaging in an artificial vascular model an "aorta phantom". The first 100 cm of the nitinol- and PEEK-based guidewires both 145 × 0.08 cm were immersed in a saline-filled aorta phantom. The probes of a fiber-optic thermometer were positioned at the tips of both wires. Balanced steady-state free precession (bSSFP) [TE 1.6 ms; TR 3.5 ms; flip angle (FA) 60°; field of view (FOV) 40 cm; matrix 256 × 256; specific absorption rate (SAR); 1.15 Watt (W)/kg] and spoiled gradient-echo (SPGR) (TE 1.8 ms; TR 60 ms; FA 60°; FOV 40 cm; matrix 256 × 256; SAR 1.15 W/kg) pulse sequences were acquired in a 1.5-T MR scanner with use of an 8-channel array coil. Temperatures were recorded while the phantom was placed centrally in the bore of a MR scanner and in an off-center position (x = 24 cm, y = -5 cm, z = -10/10 cm). The temperature of the nitinol guidewire increased by 0.3 °C (center) and 1.1 °C (off-center position) with use of the bSSFP and by 9.6 and 13 °C (off-center position) with use of the SPGR sequence. Only minor temperature changes up to a maximum of 0.4 °C were observed with the MR-compatible PEEK guidewire when any position or sequence was applied. The PEEK guidewire showed substantially lower heating as compared to the nitinol guidewire in near-real-time imaging sequences in a phantom.

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

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.

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.

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 prototype 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.

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.