Pauline W. Worters

Metal-Induced Artifacts in MRI

OBJECTIVE The purpose of this article is to review some of the basic principles of imaging and how metal-induced susceptibility artifacts originate in MR images. We will describe common ways to reduce or modify artifacts using readily available imaging techniques, and we will discuss some advanced methods to correct readout-direction and slice-direction artifacts. CONCLUSION The presence of metallic implants in MRI can cause substantial image artifacts, including signal loss, failure of fat suppression, geometric distortion, and bright pile-up artifacts. These cause large resonant frequency changes and failure of many MRI mechanisms. Careful parameter and pulse sequence selections can avoid or reduce artifacts, although more advanced imaging methods offer further imaging improvements.

Compressed-sensing multispectral imaging of the postoperative spine.

To apply compressed sensing (CS) to in vivo multispectral imaging (MSI), which uses additional encoding to avoid magnetic resonance imaging (MRI) artifacts near metal, and demonstrate the feasibility of CS‐MSI in postoperative spinal imaging.

Accuracy and repeatability of fourier velocity encoded M-mode and two-dimensional cine phase contrast for pulse wave velocity measurement in the descending aorta

To assess the accuracy and repeatability of Fourier velocity encoded (FVE) M‐mode and two‐dimensional (2D) phase contrast with through‐plane velocity encoding (2D‐PC) for pulse wave velocity (PWV) evaluation in the descending aorta using five different analysis techniques.

Non‐contrast‐enhanced renal and abdominal MR angiography using velocity‐selective inversion preparation

Non‐contrast‐enhanced MR angiography is a promising alternative to the established contrast‐enhanced approach as it reduces patient discomfort and examination costs and avoids the risk of nephrogenic systemic fibrosis. Inflow‐sensitive slab‐selective inversion recovery imaging has been used with great promise, particularly for abdominal applications, but has limited craniocaudal coverage due to inflow time constraints. In this work, a new non‐contrast‐enhanced MR angiography method using velocity‐selective inversion preparation is developed and applied to renal and abdominal angiography. Based on the excitation k‐space formalism and Shinnar‐Le‐Roux transform, a velocity‐selective excitation pulse is designed that inverts stationary tissues and venous blood while preserving inferiorly flowing arterial blood. As the magnetization of the arterial blood in the abdominal aorta and iliac arteries is well preserved during the magnetization preparation, artery visualization over a large abdominal field of view is achievable with an inversion delay time that is chosen for optimal background suppression. Healthy volunteer tests demonstrate that the proposed method significantly increases the extent of visible arteries compared with the slab‐selective approach, covering renal arteries through iliac arteries over a craniocaudal field of view of 340 mm. Magn Reson Med, 2013.

Hexagonal undersampling for faster MRI near metallic implants

Slice encoding for metal artifact correction acquires a three‐dimensional image of each excited slice with view‐angle tilting to reduce slice and readout direction artifacts respectively, but requires additional imaging time. The purpose of this study was to provide a technique for faster imaging around metallic implants by undersampling k‐space.

Inversion-recovery-prepared dixon bSSFP: Initial clinical experience with a novel pulse sequence for renal MRA within a breathhold

To evaluate the capability of a new breathhold non–contrast‐enhanced MRA method (Non‐contrast Outer Radial Inner Square k‐space Scheme, NORISKS) to visualize renal arteries by comparing the method with a routine clinical but significantly longer non–contrast‐enhanced (non‐CE) MRA technique.

Balanced SSFP Transient Imaging using Variable Flip Angles for a Predefined Signal Profile

Variable flip angles are used in steady‐state free precession (SSFP) acquisitions (e.g., time‐of‐flight) but to a lesser extent than in spin echo acquisitions. In balanced steady‐state free precession, imaging is often assumed to occur during the steady state, which has been well described in the literature. However, in many cases, imaging occurs during the transient stage, and the use of variable flip angles can improve signal and thus image quality. Here, we present the calculation of flip angles in transient balanced steady‐state free precession to generate a predefined signal profile. The signal profile was iteratively optimized to maximize the integral of the signal versus time curve. The key contribution of this work is the formulation of the flip angle as a deterministic function of the preceding and desired magnetization. Catalyzation schemes, e.g., Kaiser‐windowed ramp, can be combined with variable flip angles balanced steady‐state free precession to reduce signal oscillations. A uniform signal profile was used as an example to demonstrate the variable flip angle algorithm. Accuracy of the algorithm and Bloch simulations were verified with MRI phantom acquisitions. Renal angiograms were acquired using an inflow‐based balanced steady‐state free precession MR angiography technique; improved small‐vessel depiction was observed in volunteer examinations. Magn Reson Med, 2010.

Accuracy of Phase Contrast, Black-Blood, and Bright-Blood Pulse Sequences for Measuring Compliance and Distensibility Coefficients in a Human-Tissue Mimicking Phantom

To assess the accuracy of MR‐derived luminal diameter variations and its implications for compliance (CC) and distensibility coefficients (DC) by comparison with high‐resolution digital photography (HRDP) in a tissue‐mimicking phantom with pulsatile flow.

A 3D balanced-SSFP Dixon technique with group-encoded k-space segmentation for breath-held non–contrast-enhanced MR angiography

A three-dimensional balanced steady-state free precession (b-SSFP)-Dixon technique with a novel group-encoded k-space segmentation scheme called GUINNESS (Group-encoded Ungated Inversion Nulling for Non-contrast Enhancement in the Steady State) was developed. GUINNESS was evaluated for breath-held non-contrast-enhanced MR angiography of the renal arteries on 18 subjects (6 healthy volunteers, 12 patients) at 3.0 T. The method provided high signal-to-noise and contrast renal angiograms with homogeneous fat and background suppression in short breath-holds on the order of 20 s with high spatial resolution and coverage. GUINNESS has potential as a short breath-hold alternative to conventional respiratory-gated methods, which are often suboptimal in pediatric subjects and patients with significant diaphragmatic drift/sleep apnea.

Intravoxel incoherent motion analysis of renal allograft diffusion with clinical and histopathological correlation in pediatric kidney transplant patients: A preliminary cross-sectional observational study

The purpose of this study was to compare IVIM values in pediatric renal transplants with histopathology and clinical management change. Fifteen pediatric renal transplant recipients (mean 15.7±2.9 years) were prospectively scanned on a 3T MR scanner with multi‐b DTI, prior to same‐day transplant biopsy. IVIM maps from 14 subjects were analyzed (one excluded due to motion). Mean values were computed from cortical ROIs and medullary ROIs corresponding to the biopsy site. Subjects were also grouped according to whether or not the biopsy resulted in a change in clinical management. Cortico‐medullary IVIM estimates and histopathologic Banff scores were correlated with KT. Cortico‐medullary IVIM differences between the “change” and “no change” groups was compared with Mann‐Whitney U test. Cortical Dp showed significant moderate negative correlation with Banff t and ci scores (KT=−0.497, P=.035 and KT=−0.46, P=.046) and moderate positive correlation with Banff i score (KT=0.527, P=.028). Cortical Pf showed significant moderate correlation with ci and ct scores (KT=0.489, P=.035 and KT=0.457, P=.043). Tissue diffusivity, Dt, estimated with IVIM was significantly different between the “change” and “no change” groups in medullary ROIs (U=6, P=.021). IVIM analysis has potential as a noninvasive biomarker in assessment of pediatric renal allograft pathology.