Walter F. Block

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.

Highly constrained backprojection for time‐resolved MRI

Recent work in k‐t BLAST and undersampled projection angiography has emphasized the value of using training data sets obtained during the acquisition of a series of images. These techniques have used iterative algorithms guided by the training set information to reconstruct time frames sampled at well below the Nyquist limit. We present here a simple non‐iterative unfiltered backprojection algorithm that incorporates the idea of a composite image consisting of portions or all of the acquired data to constrain the backprojection process. This significantly reduces streak artifacts and increases the overall SNR, permitting decreased numbers of projections to be used when acquiring each image in the image time series. For undersampled 2D projection imaging applications, such as cine phase contrast (PC) angiography, our results suggest that the angular undersampling factor, relative to Nyquist requirements, can be increased from the present factor of 4 to about 100 while increasing SNR per individual time frame. Results are presented for a contrast‐enhanced PR HYPR TRICKS acquisition in a volunteer using an angular undersampling factor of 75 and a TRICKS temporal undersampling factor of 3 for an overall undersampling factor of 225. Magn Reson Med, 2006.

Time-resolved contrast-enhanced imaging with isotropic resolution and broad coverage using an undersampled 3D projection trajectory

Time‐resolved contrast‐enhanced 3D MR angiography (MRA) methods have gained in popularity but are still limited by the tradeoff between spatial and temporal resolution. A method is presented that greatly reduces this tradeoff by employing undersampled 3D projection reconstruction trajectories. The variable density k‐space sampling intrinsic to this sequence is combined with temporal k‐space interpolation to provide time frames as short as 4 s. This time resolution reduces the need for exact contrast timing while also providing dynamic information. Spatial resolution is determined primarily by the projection readout resolution and is thus isotropic across the FOV, which is also isotropic. Although undersampling the outer regions of k‐space introduces aliased energy into the image, which may compromise resolution, this is not a limiting factor in high‐contrast applications such as MRA. Results from phantom and volunteer studies are presented demonstrating isotropic resolution, broad coverage with an isotropic field of view (FOV), minimal projection reconstruction artifacts, and temporal information. In one application, a single breath‐hold exam covering the entire pulmonary vasculature generates high‐resolution, isotropic imaging volumes depicting the bolus passage. Magn Reson Med 48:297–305, 2002.

Improved 3D phase contrast MRI with off-resonance corrected dual echo VIPR

Phase contrast (PC) magnetic resonance imaging with a three‐dimensional, radially undersampled acquisition allows for the acquisition of high resolution angiograms and velocimetry in dramatically reduced scan times. However, such an acquisition is sensitive to blurring and artifacts from off‐resonance and trajectory errors. A dual‐echo trajectory is proposed with a novel trajectory calibration from prescan data coupled with a multi‐frequency reconstruction to correct for these errors. Comparisons of phantom data and in vivo results from volunteer, and patients with arteriovenous malformations patients are presented with and without these corrections and show significant improvement of image quality when both corrections are applied. The results demonstrate significantly improved visualization of vessels, allowing for highly accelerated PC acquisitions without sacrifice in image quality. Magn Reson Med 60:1329–1336, 2008.

Undersampled projection-reconstruction imaging for time-resolved contrast-enhanced imaging

In time‐resolved contrast‐enhanced 3D MR angiography, spatial resolution is traded for high temporal resolution. A hybrid method is presented that attempts to reduce this tradeoff in two of the spatial dimensions. It combines an undersampled projection acquisition in two dimensions with variable rate k‐space sampling in the third. Spatial resolution in the projection plane is determined by readout resolution and is limited primarily by signal‐to‐noise ratio. Oversampling the center of k‐space combined with temporal k‐space interpolation provides time frames with minimal venous contamination. Results demonstrating improved resolution in phantoms and volunteers are presented using angular undersampling factors up to eight with acceptable projection reconstruction artifacts. Magn Reson Med 43:170–176, 2000.

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.

Vastly Undersampled Isotropic Projection Steady-State Free Precession Imaging of the Knee: Diagnostic Performance Compared with Conventional MR

PURPOSE To compare a vastly undersampled isotropic projection steady-state free precession (VIPR-SSFP) sequence and routine magnetic resonance (MR) imaging for evaluating the cartilage, ligaments, menisci, and osseous structures of the knee in symptomatic patients. MATERIALS AND METHODS All subjects signed written informed consent prior to participation in this prospective, HIPAA-compliant, institutional review board-approved study. VIPR-SSFP was added to the routine 1.5-T MR imaging performed on 95 symptomatic patients (52 men, 43 women; average age, 41.6 years) who subsequently underwent arthroscopic knee surgery. All MR examinations were independently reviewed twice by two musculoskeletal radiologists to detect cartilage lesions, anterior and posterior cruciate ligament tears, meniscal tears, and bone marrow edema lesions, first by using routine MR and second by using VIPR-SSFP. By using arthroscopy as the reference standard, the sensitivity and specificity of both MR protocols were calculated. The z test was used to compare sensitivity and specificity values. RESULTS VIPR-SSFP had significantly higher specificity (P < .01) for helping detect cartilage lesions (92.2% for VIPR-SSFP and 88.4% for routine MR), while routine MR had significantly higher sensitivity (P = .02) and accuracy (P = .05) for helping detect lateral meniscal tears (73.2% sensitivity and 88.4% accuracy for VIPR-SSFP and 87.5% specificity and 93.2% accuracy for routine MR). There was no significant difference (P = .14 to >.99) between VIPR-SSFP and routine MR in the remaining sensitivity and specificity values. VIPR-SSFP helped detect 69.3% of bone marrow edema lesions identified at routine MR. CONCLUSION VIPR-SSFP can provide important clinical information regarding the cartilage, ligaments, menisci, and osseous structures of the knee, but is less sensitive than conventional MR imaging at helping detect lateral meniscal tears and bone marrow edema lesions.

Magnetic resonance imaging of frozen tissues: Temperature-dependent MR signal characteristics and relevance for MR monitoring of cryosurgery

Previously, the magnetic resonance (MR) imaging appearance of frozen tissues created during cryosurgery has been described as a signal void. In this work, very short echo times (1.2 msec) allowed MR signals from frozen tissues to be measured at temperatures down to −35°C. Ex vivo bovine liver, muscle, adipose tissue, and water were imaged at steady‐state temperatures from −78° to +6°C. Signal intensity, T2*, and T1 were measured using gradient‐echo imaging. Signal intensity and T2* decrease monotonically with temperature. In the future, these MR parameters may be useful for mapping temperatures during cryosurgery. Magn Reson Med 41:627–630, 1999. 

Rapid fat-suppressed isotropic steady-state free precession imaging using true 3D multiple-half-echo projection reconstruction.

Three‐dimensional projection reconstruction (3D PR)‐based techniques are advantageous for steady‐state free precession (SSFP) imaging for several reasons, including the capability to achieve short repetition times (TRs). In this paper, a multi‐half‐echo technique is presented that dramatically improves the data‐sampling efficiency of 3D PR sequences while it retains this short‐TR capability. The k‐space trajectory deviations are measured quickly and corrected on a per‐sample point basis. A two‐pass RF cycling technique is then applied to the dual‐half‐echo implementation to generate fat/water‐separated images. The resultant improvement in the signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) was demonstrated in volunteer studies. Volumetric images with excellent spatial resolution, coverage, and contrast were obtained with high speed. The non‐contrast‐enhanced SSFP studies show that this technique has promising potential for MR angiography (MRA). Magn Reson Med 53:692–699, 2005.

Contrast-enhanced peripheral magnetic resonance angiography using time-resolved vastly undersampled isotropic projection reconstruction

To investigate the application of time‐resolved vastly undersampled isotropic projection reconstruction (VIPR) in contrast‐enhanced magnetic resonance angiography of the distal extremity (single station), and peripheral run‐off vasculature in the abdomen, thigh, and calf (three stations).