Frank R. Korosec

Undersampled projection reconstruction applied to MR angiography

Undersampled projection reconstruction (PR) is investigated as an alternative method for MRA (MR angiography). In conventional 3D Fourier transform (FT) MRA, resolution in the phase‐encoding direction is proportional to acquisition time. Since the PR resolution in all directions is determined by the readout resolution, independent of the number of projections (Np), high resolution can be generated rapidly. However, artifacts increase for reduced Np. In X‐ray CT, undersampling artifacts from bright objects like bone can dominate other tissue. In MRA, where bright, contrast‐filled vessels dominate, artifacts are often acceptable and the greater resolution per unit time provided by undersampled PR can be realized. The resolution increase is limited by SNR reduction associated with reduced voxel size. The hybrid 3D sequence acquires fractional echo projections in the kx–ky plane and phase encodings in kz. PR resolution and artifact characteristics are demonstrated in a phantom and in contrast‐enhanced volunteer studies. Magn Reson Med 43:91–101, 2000.

Functional lung imaging using hyperpolarized gas MRI

The noninvasive assessment of lung function using imaging is increasingly of interest for the study of lung diseases, including chronic obstructive pulmonary disease (COPD) and asthma. Hyperpolarized gas MRI (HP MRI) has demonstrated the ability to detect changes in ventilation, perfusion, and lung microstructure that appear to be associated with both normal lung development and disease progression. The physical characteristics of HP gases and their application to MRI are presented with an emphasis on current applications. Clinical investigations using HP MRI to study asthma, COPD, cystic fibrosis, pediatric chronic lung disease, and lung transplant are reviewed. Recent advances in polarization, pulse sequence development for imaging with Xe‐129, and prototype low magnetic field systems dedicated to lung imaging are highlighted as areas of future development for this rapidly evolving technology. J. Magn. Reson. Imaging 2007.

Real-time MR imaging-guided passive catheter tracking with use of gadolinium-filled catheters.

PURPOSE To test the hypothesis that real-time magnetic resonance (MR) imaging-guided passive catheter tracking is feasible with use of dilute gadolinium (Gd)-filled catheters, to determine the optimal Gd concentration required for tracking, and to measure catheter tip tracking accuracy. MATERIALS AND METHODS The authors tested a real-time, T1-weighted, two-dimensional, spoiled gradient-recalled echo MR imaging sequence suitable for tracking catheters. In a yogurt phantom, the authors placed 5-F catheters filled with 2%-12% Gd solutions. MR imaging was performed with and without use of a projection dephaser that suppressed background signal. The authors measured signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and enhancement ratio to determine the optimal Gd concentration for catheter depiction. Catheter tip tracking accuracy was measured in an acrylic phantom with use of linear regression analysis, with goodness of fit assessed statistically with the F test. RESULTS Peak catheter SNR, CNR, and enhancement ratios were obtained with 4%-6% Gd concentrations. Tip tracking accuracy was determined to be +/- 0.41 mm (R2 = 0.99; P < .0001). MR imaging reconstructions were displayed up to 3.1 frames/sec. CONCLUSIONS Accurate MR imaging-guided passive catheter tracking was feasible in real-time with use of dilute Gd-filled catheters. This technique may have application in MR imaging-guided endovascular procedures.

Time‐resolved, undersampled projection reconstruction imaging for high‐resolution CE‐MRA of the distal runoff vessels

Imaging of the blood vessels below the knee using contrast‐enhanced (CE) MRI is challenging due to the need to coordinate image acquisition and arrival of the contrast in the targeted vessels. Time‐resolved acquisitions have been successful in consistently capturing images of the arterial phase of the bolus of contrast agent in the distal extremities. Although time‐resolved exams are robust in this respect, higher spatial resolution for the depiction of tight stenoses and the small vessels in the lower leg is desirable. A modification to a high‐spatial‐resolution T1‐weighted pulse sequence (projection reconstruction‐time resolved imaging of contrast kinetics (PR‐TRICKS)) that improves the through‐plane spatial resolution by a factor of 2 and maintains a high frame rate is presented. The undersampled PR‐TRICKS pulse sequence has been modified to double the spatial resolution in the slice direction by acquiring high‐spatial‐frequency slice data only after first pass of the bolus of contrast agent. The acquisition reported in the present work (PR‐hyperTRICKS) has been used to image healthy volunteers and patients with known vascular disease. The temporal resolution was found to be beneficial in capturing arterial phase images in the presence of asymmetric filling of vessels. Magn Reson Med 48:516–522, 2002.

MR-guided angioplasty of renal artery stenosis in a pig model: a feasibility study.

PURPOSE To test the hypothesis that magnetic resonance (MR) imaging can guide the percutaneous treatment of renal artery stenosis in a pig model. MATERIALS AND METHODS Ameroid constrictors were surgically placed around six renal arteries in four pigs. After 30-36 days, all stenoses were documented by conventional x-ray aortograms. MR-guided renal angioplasty was attempted for three stenoses. For these pigs, MR angiography was performed with use of contrast-enhanced three-dimensional (3D) techniques. The authors visualized catheters by filling them with dilute 4% gadolinium and imaging with two-dimensional (2D) and 3D MR fast spoiled gradient recalled echo techniques. Under MR guidance, the authors advanced a selective catheter into the affected renal artery and crossed the stenosis with a nitinol guide wire. Angioplasty was performed with a balloon catheter filled with dilute gadolinium. Stenosis and luminal diameter measurements were compared before and after angioplasty. RESULTS After ameroid constrictor placement, four significant stenoses, one mild stenosis, and one occlusion developed. Under MR guidance, the authors achieved technical success in performing three of three (100%) attempted dilations. After MR-guided angioplasty, the mean reduction in stenosis was 35% and the mean increase in luminal diameter was 1.6 mm. CONCLUSION Use of MR guidance for the angioplasty of renal artery stenosis in pigs is feasible.

Development of a Unique Phantom to Assess the Geometric Accuracy of Magnetic Resonance Imaging for Stereotactic Localization

OBJECTIVE To test the spatial accuracy of coordinates generated from magnetic resonance imaging (MRI) scans, using the Brown-Roberts-Wells head frame and localizer system (Radionics, Inc., Burlington, MA). METHODS An anthropomorphic head phantom, consisting of a two-dimensional lattice of acrylic spheres (4-mm diameter) spaced 10 mm apart and embedded in a brain tissue-mimicking gelatin-agar gel, was constructed. The intersphere distances for the target lattice positions in MRI and computed tomographic scan sets were compared. The data sets were fused, and differences in fiducial marker and intraphantom target positions were measured. RESULTS Intersphere distances were identical for the MRI and computed tomographic scan sets (10 +/- 0.1 mm). Differences in fiducial marker positions [maximal lateral difference, 0.97 mm; mean absolute lateral difference, 0.69 +/- 0.22 mm; maximal anteroposterior (AP) difference, 1.99 mm; mean absolute AP difference, 1.29 +/- 0.67 mm] were correlated with differences in intraphantom target positions (maximal lateral difference, 0.83 mm; mean absolute lateral difference, 0.28 +/- 0.24 mm; maximal AP difference, -1.97 mm; mean absolute AP difference, 1.63 +/- 25 mm; maximal vertical difference, -0.73 mm; mean absolute vertical difference, 0.34 +/- 0.21 mm). This suggested that improper fiducial rod identification and the subsequent transformation to stereotactic coordinate space were the greatest sources of spatial uncertainty. CONCLUSION With computed tomographic data as the standard, these differences resulted in maximal and minimal composite uncertainties of 2.06 and 1.17 mm, respectively. The measured uncertainties exceed recommended standards for radiosurgery but allow the possible use of MRI-based stereotactic treatment planning for certain intracranial lesions, if the errors are corrected using appropriate software. Clinicians must recognize that error magnitudes vary for different systems, and they should perform systematic, scheduled, institutional error analyses as part of their ongoing quality assurance processes. This phantom provides one tool for measuring such variances.

The effect of injection rate on time‐resolved contrast‐enhanced peripheral MRA

In contrast‐enhanced (CE) magnetic resonance (MR) angiography (MRA), lower injection rates of a fixed contrast agent dose provide longer contrast agent bolus at the expense of lower intravascular signal. This study evaluated the effect of different injection rates in imaging of the vasculature of the lower extremities with time‐resolved, CE MRA. In three volunteers, injection rates of 0.5, 1.5 and 3.0 mL/second were administered in a randomized order and imaged in two separate sessions. Contrast agent bolus dynamics measured in volunteers were used in computer simulations to confirm variations in contrast agent concentration as a source of vessel ringing and blurring artifacts. To validate the effect of injection rate in pathologic vessels, 37 patients with peripheral vascular disease were imaged with a time‐resolved technique using an injection rate of 0.5 mL/second or 1.5 mL/second and retrospectively divided into two groups. In volunteers, higher injection rates caused a stronger modulation of k‐space and resulted in increased ringing artifacts in time‐resolved CE MRA. These results were reproduced with computer simulations. In the qualitative patient study, significantly less vessel blurring was observed using a lower injection‐rate, without a significant loss of vessel contrast. J. Magn. Reson. Imaging 2001;14:401–410.

Phase-contrast with interleaved undersampled projections.

MR phase‐contrast techniques provide velocity‐sensitive angiograms and quantitative flow measurements but require long scan times. Recently it has been shown that undersampled projection reconstruction can acquire higher resolution per unit time than Fourier techniques with acceptable artifacts when used in contrast‐enhanced MR angiography. Undersampled projection reconstruction has similar potential for phase‐contrast acquisitions. Flow sensitization gradients are used with projection trajectories to acquire velocity‐dependent phase information. An acquisition scheme that acquires three flow encoding directions on three sets of angular‐interleaved projections is introduced. Depending on the resolution, acquisition times for 3D datasets can decrease by factors of two to four. Magn Reson Med 43:503–509, 2000.

3D MR DSA: Effects of injection protocol and image masking

The purpose of this study was to investigate the effect on three‐dimensional (3D) magnetic resonance digital subtraction angiography (MR DSA) images of various injection protocol parameters (ie, injection order, volume, and rate), as well as image masking. The pelves of 10 normal volunteers were scanned using seven different contrast agent volume/injection rate combinations. Subtraction of a precontrast mask image resulted in vascular image contrast improvements of between 4.0 and 7.7 times. Image quality and smaller vessel image contrast in the masked data decreased with increasing injection number. Data acquired with a high (0.150 mmol kg−1) volume yielded the highest quality images, although only small nonsignificant differences in image quality and large vessel conspicuity were found between images obtained using the high and medium (0.075 mmol kg−1) volumes. Images acquired with a low (0.038 mmol kg−1) volume, while of lower image contrast, were judged to be of reasonable quality, especially when acquired as the first or second injection. Injection rate (1 ml s−1, 2 ml s−1, and 4 ml s−1) was not found to affect the images significantly, although selection of an injection rate that gave an injection duration of ∼ 10 seconds tended to give better vascular image contrast. Based on these data, a series of escalating volumes for multi‐injection examination is proposed. J. Magn. Reson. Imaging 2000;12:476–487.

Contrast-enhanced MR angiography of the carotid bifurcation

With contrast‐enhanced MR angiographic techniques, a T1‐shortening contrast agent is injected into the blood stream. Imaging during the first pass of the contrast agent permits acquisition of a high‐contrast MR angiogram. Scan parameters such as flip angle, repetition time, echo time, and scan duration, and injection parameters, such as dose and rate, must be carefully chosen to achieve maximum contrast between blood vessels and stationary tissues. A critical parameter affecting image quality is the timing of the acquisition relative to the injection. If the collection of the center of k‐space does not coincide with peak arterial concentration, artifacts, reduced SNR, and venous enhancement may result. Several techniques have been developed to address the timing issue. Post‐processing techniques such as subtracting a pre‐contrast image from a post‐contrast angiogram can be used to improve image quality. Intravascular contrast agents that may also lead to improved image quality are currently being developed. J. Magn. Reson. Imaging 1999;10:317–325.