Ziwu Zhou

Cardiovascular MRI with ferumoxytol

The practice of contrast-enhanced magnetic resonance angiography (CEMRA) has changed significantly in the span of a decade. Concerns regarding gadolinium (Gd)-associated nephrogenic systemic fibrosis in those with severely impaired renal function spurred developments in low-dose CEMRA and non-contrast MRA as well as efforts to seek alternative MR contrast agents. Originally developed for MR imaging use, ferumoxytol (an ultra-small superparamagnetic iron oxide nanoparticle), is currently approved by the US Food and Drug Administration for the treatment of iron deficiency anaemia in adults with renal disease. Since its clinical availability in 2009, there has been rising interest in the scientific and clinical use of ferumoxytol as an MR contrast agent. The unique physicochemical and pharmacokinetic properties of ferumoxytol, including its long intravascular half-life and high r1 relaxivity, support a spectrum of MRI applications beyond the scope of Gd-based contrast agents. Moreover, whereas Gd is not found in biological systems, iron is essential for normal metabolism, and nutritional iron deficiency poses major public health challenges worldwide. Once the carbohydrate shell of ferumoxytol is degraded, the elemental iron at its core is incorporated into the reticuloendothelial system. These considerations position ferumoxytol as a potential game changer in the field of CEMRA and MRI. In this paper, we aim to summarise our experience with the cardiovascular applications of ferumoxytol and provide a brief synopsis of ongoing investigations on ferumoxytol-enhanced MR applications.

Self-gated 4D multiphase, steady-state imaging with contrast enhancement (MUSIC) using rotating cartesian K-space (ROCK): Validation in children with congenital heart disease.

To develop and validate a cardiac‐respiratory self‐gating strategy for the recently proposed multiphase steady‐state imaging with contrast enhancement (MUSIC) technique.

Rapid quantitative T2 mapping of the prostate using three-dimensional dual echo steady state MRI at 3T.

To develop and evaluate a rapid three‐dimensional (3D) quantitative T2 mapping method for prostate cancer imaging using dual echo steady state (DESS) MRI at 3T.

Accelerated ferumoxytol-enhanced 4D multiphase, steady-state imaging with contrast enhancement (MUSIC) cardiovascular MRI: validation in pediatric congenital heart disease: Accelerated 4D MUSIC

The purpose of this work was to validate a parallel imaging (PI) and compressed sensing (CS) combined reconstruction method for a recently proposed 4D non‐breath‐held, multiphase, steady‐state imaging technique (MUSIC) cardiovascular MRI in a cohort of pediatric congenital heart disease patients. We implemented a graphics processing unit accelerated CS‐PI combined reconstruction method and applied it in 13 pediatric patients who underwent cardiovascular MRI after ferumoxytol administration. Conventional breath‐held contrast‐enhanced magnetic resonance angiography (CE‐MRA) was first performed during the first pass of ferumoxytol injection, followed by the original MUSIC and the proposed CS‐PI MUSIC during the steady‐state distribution phase of ferumoxytol. Qualities of acquired images were then evaluated using a four‐point scale. Left ventricular volumes and ejection fractions calculated from the original MUSIC and the CS‐PI MUSIC were also compared with conventional multi‐slice 2D cardiac cine MRI. The proposed CS‐PI MUSIC reduced the imaging time of the MUSIC acquisition to 4.6 ± 0.4 min from 8.9 ± 1.2 min. Computationally intensive image reconstruction was completed within 5 min without interruption of sequential clinical scans. The proposed method (mean 3.3–4.0) provided image quality comparable to that of the original MUSIC (3.2–4.0) (all P ≥ 0.42), and better than conventional breath‐held first‐pass CE‐MRA (1.1–3.3) for 13 anatomical structures (all P ≤ 0.0014) with good inter‐observer agreement (κ > 0.46). The calculated ventricular volumes and ejection fractions from both original MUSIC (r > 0.90) and CS‐PI MUSIC (r > 0.85) correlated well with 2D cine imaging. In conclusion, PI and CS were successfully incorporated into the 4D MUSIC acquisition to further reduce scan time by approximately 50% while maintaining highly comparable image quality in a clinically practical reconstruction time.

Respiratory motion resolved, self-gated 4D-MRI using Rotating Cartesian K-space (ROCK).

Purpose To propose and validate a respiratory motion resolved, self‐gated (SG) 4D‐MRI technique to assess patient‐specific breathing motion of abdominal organs for radiation treatment planning. Methods The proposed 4D‐MRI technique was based on the balanced steady‐state free‐precession (bSSFP) technique and 3D k‐space encoding. A novel rotating cartesian k‐space (ROCK) reordering method was designed which incorporates repeatedly sampled k‐space centerline as the SG motion surrogate and allows for retrospective k‐space data binning into different respiratory positions based on the amplitude of the surrogate. The multiple respiratory‐resolved 3D k‐space data were subsequently reconstructed using a joint parallel imaging and compressed sensing method with spatial and temporal regularization. The proposed 4D‐MRI technique was validated using a custom‐made dynamic motion phantom and was tested in six healthy volunteers, in whom quantitative diaphragm and kidney motion measurements based on 4D‐MRI images were compared with those based on 2D‐CINE images. Results The 5‐minute 4D‐MRI scan offers high‐quality volumetric images in 1.2 × 1.2 × 1.6 mm3 and eight respiratory positions, with good soft‐tissue contrast. In phantom experiments with triangular motion waveform, the motion amplitude measurements based on 4D‐MRI were 11.89% smaller than the ground truth, whereas a −12.5% difference was expected due to data binning effects. In healthy volunteers, the difference between the measurements based on 4D‐MRI and the ones based on 2D‐CINE were 6.2 ± 4.5% for the diaphragm, 8.2 ± 4.9% and 8.9 ± 5.1% for the right and left kidney. Conclusion The proposed 4D‐MRI technique could provide high‐resolution, high‐quality, respiratory motion‐resolved 4D images with good soft‐tissue contrast and are free of the “stitching” artifacts usually seen on 4D‐CT and 4D‐MRI based on resorting 2D‐CINE. It could be used to visualize and quantify abdominal organ motion for MRI‐based radiation treatment planning.

Golden-ratio rotated stack-of-stars acquisition for improved volumetric MRI

To develop and evaluate an improved stack‐of‐stars radial sampling strategy for reducing streaking artifacts.

Segmented golden ratio radial reordering with variable temporal resolution for dynamic cardiac MRI.

Golden ratio (GR) radial reordering allows for retrospective choice of temporal resolution by providing a near‐uniform k‐space sampling within any reconstruction window. However, when applying GR to electrocardiogram (ECG)‐gated cardiac imaging, the k‐space coverage may not be as uniform because a single reconstruction window is broken into several temporally isolated ones. The goal of this study was to investigate the image artifacts caused by applying GR to ECG‐gated cardiac imaging and to propose a segmented GR method to address this issue.

Cardiac and respiratory self-gated 4D multi-phase steady-state imaging with ferumoxytol contrast (MUSIC)

Background Our recently proposed MUSIC approach acquires ferumoxytol-enhanced 4D images using ECG and airway pressure signal for motion gating. It has been routinely used in our institution and provides detailed information on vascular anatomy. However, the airway pressure signal is only available in patients with general anesthesia (GA) and ECG is often problematic in high field strength. Therefore, we propose the self-gated MUSIC (SGMUSIC) where cardiac and respiratory motion is compensated by retrospective data sorting base on derived SG signals. In this study, we tested SG-MUSIC on pediatric patients under GA so that the derived SG signal can be validated against the recorded physiological signal and the images compared with the original MUSIC.

Distortion-free diffusion MRI using an MRI-guided Tri-Cobalt 60 radiotherapy system: Sequence verification and preliminary clinical experience

Purpose: Monitoring tumor response during the course of treatment and adaptively modifying treatment plan based on tumor biological feedback may represent a new paradigm for radiotherapy. Diffusion MRI has shown great promises in assessing and predicting tumor response to radiotherapy. However, the conventional diffusion‐weighted single‐shot echo‐planar‐imaging (DW‐ssEPI) technique suffers from limited resolution, severe distortion, and possibly inaccurate ADC at low field strength. The purpose of this work was to develop a reliable, accurate and distortion‐free diffusion MRI technique that is practicable for longitudinal tumor response evaluation and adaptive radiotherapy on a 0.35 T MRI‐guided radiotherapy system. Methods: A diffusion‐prepared turbo spin echo readout (DP‐TSE) sequence was developed and compared with the conventional diffusion‐weighted single‐shot echo‐planar‐imaging sequence on a 0.35 T MRI‐guided radiotherapy system (ViewRay). A spatial integrity phantom was used to quantitate and compare the geometric accuracy of the two diffusion sequences for three orthogonal orientations. The apparent diffusion coefficient (ADC) accuracy was evaluated on a diffusion phantom under both 0 °C and room temperature to cover a diffusivity range between 0.40 × 10−3 and 2.10 × 10−3 mm2/s. Ten room temperature measurements repeated on five different days were conducted to assess the ADC reproducibility of DP‐TSE. Two glioblastoma (GBM) and six sarcoma patients were included to examine the in vivo feasibility. The target registration error (TRE) was calculated to quantitate the geometric accuracy where structural CT or MR images were co‐registered to the diffusion images as references. ADC maps from DP‐TSE and DW‐ssEPI were calculated and compared. A tube phantom was placed next to patients not treated on ViewRay, and ADCs of this reference tube were also compared. Results: The proposed DP‐TSE passed the spatial integrity test (< 1 mm within 100 mm radius and < 2 mm within 175 mm radius) under the three orthogonal orientations. The detected errors were 0.474 ± 0.355 mm, 0.475 ± 0.287 mm, and 0.546 ± 0.336 mm in the axial, coronal, and sagittal plane. DW‐ssEPI, however, failed the tests due to severe distortion and low signal intensity. Noise correction must be performed for the DW‐ssEPI to avoid ADC quantitation errors, whereas it is optional for DP‐TSE. At 0 °C, the two sequences provided accurate quantitation with < 3% variation with the reference. In the room temperature study, discrepancies between ADCs from DP‐TSE and the reference were within 4%, but could be as high as 8% for DW‐ssEPI after the noise correction. Excellent ADC reproducibility with a coefficient of variation < 5% was observed among the 10 measurements of DP‐TSE, indicating desirable robustness for ADC‐based tumor response assessment. In vivo TRE in DP‐TSE was less than 1.6 mm overall, whereas it could be greater than 12 mm in DW‐ssEPI. For GBM patients, the CSF and brain tissue ADCs from DP‐TSE were within the ranges found in literature. ADC differences between the two techniques were within 8% among the six sarcoma patients. For the reference tube that had a relatively low diffusivity, the two diffusion sequences provided matched measurements. Conclusion: A diffusion technique with excellent geometric fidelity, accurate, and reproducible ADC measurement was demonstrated for longitudinal tumor response assessment using a low‐field MRI‐guided radiotherapy system.

Ferumoxytol MRA and non-contrast CT fusion in TAVR candidates with renal failure

Background One of the advantages of CT angiography (CTA) over MRA for transcatheter aortic valve replacement (TAVR) planning is the ability to display aortic valve and arterial calcification accurately. However, in many patients who are TAVR candidates, renal impairment makes iodinated contrast media undesirable or contraindicated. Further, in patients with severe renal impairment, gadolinium based contrast agents may be problematic because of the perceived risk of NSF. We hypothesized that in patients where ferumoxytol-enhanced MRA (FEMRA) is a suitable alternative to CTA, vascular and valvular calcification may be accurately displayed over luminal anatomy by fusing non-contrast CT and FEMRA in these patients. Methods Nine patients who underwent FEMRA for assessment of arterial access anatomy prior to TAVR had additional non-contrast CT of the thorax and abdomen. CT and MR DICOM data were processed in a commercially available software (Mimics V17.0, Materialize) and the calcium from the CT was isolated and registered to the FEMRA data.