Biomedical subjects

David J Larkman

Publications and source records attributed to David J Larkman.

11 recordsLinked to original sources

PubMed2006

Beyond the g-factor limit in sensitivity encoding using joint histogram entropy.

The maximum practical speed-up that can be achieved using parallel imaging methods is widely accepted to be limited by g-factor noise. An approximate expression for the g-factor noise as a function of the principal eigenvector of the inverse sensitivity matrix is derived. This formulation allows g-factor enhanced noise to be reduced by a constrained optimization procedure with joint image histogram entropy between a reference image and a SENSE image as an image quality metric. The reference image does not need to have identical resolution or contrast. The reference image may also be used for coil calibration. The limits of the method are explored using simulated and real array coil data with high g-factor using a variety of contrast and resolution combinations. The method preserves image structure, contrast, and lesions even when these were not observable in the reference data. In all cases g-factor was dramatically reduced.

Brain Mapping↗

PubMed2006

Parallel imaging.

Explore the source record for details and available documents.

Image Enhancement↗

PubMed2006

Axial and radial diffusivity in preterm infants who have diffuse white matter changes on magnetic resonance imaging at term-equivalent age.

OBJECTIVE: Diffuse excessive high signal intensity (DEHSI) is observed in the majority of preterm infants at term-equivalent age on conventional MRI, and diffusion-weighted imaging has shown that apparent diffusion coefficient values are elevated in the white matter (WM) in DEHSI. Our aim was to obtain diffusion tensor imaging on preterm infants at term-equivalent age and term control infants to test the hypothesis that radial diffusivity was significantly different in the WM in preterm infants with DEHSI compared with both preterm infants with normal-appearing WM on conventional MRI and term control infants. METHODS: Diffusion tensor imaging was obtained on 38 preterm infants at term-equivalent age and 8 term control infants. Values for axial (lambda1) and radial [(lambda2 + lambda3)/2] diffusivity were calculated in regions of interest positioned in the central WM at the level of the centrum semiovale, frontal WM, posterior periventricular WM, occipital WM, anterior and posterior portions of the posterior limb of the internal capsule, and the genu and splenium of the corpus callosum. RESULTS: Radial diffusivity was elevated significantly in the posterior portion of the posterior limb of the internal capsule and the splenium of the corpus callosum, and both axial and radial diffusivity were elevated significantly in the WM at the level of the centrum semiovale, the frontal WM, the periventricular WM, and the occipital WM in preterm infants with DEHSI compared with preterm infants with normal-appearing WM and term control infants. There was no significant difference between term control infants and preterm infants with normal-appearing WM in any region studied. CONCLUSIONS: These findings suggest that DEHSI represents an oligodendrocyte and/or axonal abnormality that is widespread throughout the cerebral WM.

Brain↗

PubMed2005

Padé methods for reconstruction and feature extraction in magnetic resonance imaging.

Methods utilizing Padé approximants are investigated for implementation with magnetic resonance imaging data and are presented both for direct image reconstruction and for feature extraction. Padé approximants are a numerical tool that can be used to accelerate the convergence of a slowly converging sequence by estimating the fully converged sequence values from early data points. Padé approximants can be calculated directly from k-space data by solving a set of linear matrix equations to produce signal values for any desired location in the image domain. This gives an estimate of the fully converged signal intensity at each pixel location in the image, raising the possibility of reconstructing a better estimate of the object from a reduced data set. These methods have been tested on phantom and human data both for image reconstruction and for feature extraction. In image reconstruction, considerable convergence acceleration can be achieved, with steep intensity boundaries reproduced in keeping with higher resolution reconstructions and oscillatory truncation artifact characteristic of Fourier reconstruction removed. The convergence acceleration is variable and there is the possibility of fine detail suppression when insufficient data are included. The use of Padé methods as a tool for feature extraction has shown good agreement with extraction from high-resolution reference data. In this approach the edge information comes intrinsically from Padé reconstruction.

Algorithms↗

PubMed2004

Coil-based artifact reduction.

Multiple MRI receiver coils provide extra information and can enable the reconstruction of multiple images using data from different combinations of coils. Comparison of these images shows that artifacts due to motion or flowing blood appear with different intensities due to the differing coil sensitivities. Typically, the artifact appears amplified in regions of low coil sensitivity. An optimization routine was developed to correct for the artifact by comparing reconstructions from various coil combinations and favoring a self-consistent solution. It is demonstrated that images artifacted by blood flowing in the aorta, or translational motion of the head, can be improved.

Algorithms↗

PubMed2004

Correction of high-order eddy current induced geometric distortion in diffusion-weighted echo-planar images.

Diffusion-weighted images acquired with the echo-planar imaging technique are highly sensitive to eddy current induced geometric distortions that vary with the magnitude and direction of the diffusion sensitizing gradients. Such distortions cause misalignment of images acquired with different diffusion strengths and orientations. This in turn can result in errors when calculating maps of the apparent diffusion coefficient and diffusion tensor. Previous correction methods either require separate calibration data or only deal with low-order errors. In this study, we demonstrate a method that can correct for higher-order errors. The method relies on collecting pairs of images with diffusion sensitizing gradients reversed. This paired data are first corrected for shifts and linear distortion and then combined to cancel higher-order errors. All acquired data contribute to the final results. The method has been tested by simulation, on phantoms, on adult volunteers, and on neonatal brain examinations.

Adult↗

PubMed2004

Artifact reduction using parallel imaging methods.

Multiple receiver coils produce images with different but complementary views of a patient. This can be used to shorten scans times but there often remain image artifacts caused by patient motion or physiological processes such as flowing blood. This paper reviews how the extra information from the multiple coils can be used to reduce image artifacts. In one method, affected portions of data can be identified and discarded but enough information is still available to reconstruct an improved image. In other methods, the motion itself is determined and the corrupted data is then corrected, leading to an image with reduced artifacts. Results are presented from images corrupted by motion or by flowing blood.

Aortic Diseases↗

PubMed2003

Esophageal cancer staging with endoscopic MR imaging: pilot study.

The authors defined esophageal anatomy and evaluated esophageal cancer staging in a pilot group by comparing endoscopic magnetic resonance (MR) imaging results with pathologic and endoscopic ultrasonographic (US) results when available. A porcine esophagus, one volunteer, and 23 patients suspected of having esophageal cancer were imaged at 0.5 T. MR imaging was successful in 21 patients. Eight of these patients underwent esophagectomy (one after chemotherapy, which invalidated comparison with MR imaging; another did not undergo lymphadenectomy) and one underwent laparoscopy and nodal staging only; eight underwent US. When verified with pathologic staging, endoscopic MR imaging was accurate in six of seven patients (T stage) and five of six patients (N stage; nodal areas too obscured by artifact for comparison in one case). MR imaging and US results concurred in seven of eight (T stage) and five of eight (N stage) patients. No complications were observed. Endoscopic MR imaging is safe and probably comparable to endoscopic US, but with a tendency to overstage the disease.

Adult↗

PubMed2003

Diffusion-weighted imaging of the brain in preterm infants with focal and diffuse white matter abnormality.

OBJECTIVE: The most common finding on magnetic resonance imaging (MRI) of the brain in preterm infants at term-equivalent age is diffuse excessive high signal intensity (DEHSI) in the white matter. It is unclear whether DEHSI represents a biological abnormality. This study used diffusion-weighted imaging (DWI) to compare apparent diffusion coefficient (ADC) values in DEHSI with infants with normal imaging and those with overt brain damage to determine whether DEHSI shows the diffusion characteristics of normal or abnormal tissue. METHODS: MRI, using conventional and diffusion-weighted imaging (DWI), was performed in 50 preterm infants at term-equivalent age using a 1.5 Tesla MR scanner. The infants were divided into 3 groups on the basis of their MRI results: 1) normal white matter, 2) DEHSI, or 3) overt white matter pathology. ADC values were measured in the frontal, central, and posterior white matter at the level of the centrum semiovale. ADC values in the 3 groups of preterm infants were compared using a 1-way analysis of variance with a Bonferroni test for multiple comparisons. RESULTS: ADC values were significantly higher in infants with DEHSI and infants with overt white matter pathology than in infants with normal white matter. There was no significant difference between ADC values in infants with DEHSI and those with overt white matter pathology. CONCLUSIONS: This study provides objective evidence that DEHSI represents diffuse white matter abnormality.

Body Water↗

PubMed2002

Generalized SMASH imaging.

A generalized parallel imaging method has been developed that uses coil profiles to generate missing k-space lines. The proposed method is an extension of SMASH, which uses linear combinations of coil sensitivity profiles to synthesize spatial harmonics. In the generalized SMASH approach described here, coil sensitivity profiles are represented directly in the Fourier domain to provide a general description of the spatial properties of the coils. This removes restrictions imposed by conventional SMASH, so that the choice and position of the receiver coils can be made on the basis of sensitivity to the volume of interest rather than suitability for constructing spatial harmonics. Generalized SMASH also intrinsically allows the freedom to accommodate acquisitions with uniform or nonuniform k-space sampling. The proposed method places SMASH on an equal footing with other parallel imaging techniques (SENSE and SPACE-RIP), while combining strengths from each. The method was tested on phantom and human data and provides a robust method of data recovery.

Fourier Analysis↗

PubMed2002

Contrast-enhanced magic-angle MR imaging of the Achilles tendon.

OBJECTIVE: The objective of this study was to image the Achilles tendon with MR imaging at the magic angle (the long axis of the tendon at 55 degrees relative to the direction of the main static magnetic field [B(0)]) to detect signal from the tendon, to measure the T1 of the tendon, and to determine patterns of contrast enhancement in control subjects and patients. CONCLUSION: Mean T1 values of 311 +/- 30 msec (at 1.0 T) were found in six volunteers. In six control volunteers, slow uptake of contrast material that dispersed over 40 min-1.5 hr was shown without focal change, with elimination in most cases occurring within 18-24 hr. Small rapidly enhancing focal areas of enhancement were seen next to the insertion of the tendon and centrally within 5-10 min in two control volunteers. The focal areas were located at the sites of the blood supply. A patient with chronic tendinopathy showed early local contrast enhancement that extended widely within the tendon over several hours. Two patients with a partially ruptured or repaired tendon showed marked rapid contrast enhancement. The enhancement was obvious with the tendon at the magic angle but was not evident with the tendon in the usual orientation for MR examinations parallel to B(0).

Achilles Tendon↗