Richard Watts

Possible axonal regrowth in late recovery from the minimally conscious state

We used diffusion tensor imaging (DTI) to study 2 patients with traumatic brain injury. The first patient recovered reliable expressive language after 19 years in a minimally conscious state (MCS); the second had remained in MCS for 6 years. Comparison of white matter integrity in the patients and 20 normal subjects using histograms of apparent diffusion constants and diffusion anisotropy identified widespread altered diffusivity and decreased anisotropy in the damaged white matter. These findings remained unchanged over an 18-month interval between 2 studies in the first patient. In addition, in this patient, we identified large, bilateral regions of posterior white matter with significantly increased anisotropy that reduced over 18 months. In contrast, notable increases in anisotropy within the midline cerebellar white matter in the second study correlated with marked clinical improvements in motor functions. This finding was further correlated with an increase in resting metabolism measured by PET in this subregion. Aberrant white matter structures were evident in the second patients DTI images but were not clinically correlated. We propose that axonal regrowth may underlie these findings and provide a biological mechanism for late recovery. Our results are discussed in the context of recent experimental studies that support this inference.

Quantitative evaluation of susceptibility and shielding effects of nitinol, platinum, cobalt-alloy, and stainless steel stents.

The purpose of this study is to quantitatively estimate the shielding and susceptibility effects of commonly used metallic stents on MR signal. Two experiments were performed using a 3D gradient echo sequence with short TE to image a stent phantom: 1) short TR and high flip angle (contrast enhanced MRA parameters), and 2) long TR (TR ≫ T1) and low flip angle. The factor characterizing susceptibility effects was estimated from the signal phase of the first experiment, and then the factor characterizing the shielding effects was derived from the second experiment. Susceptibility induced signal loss was negligible (<1%) for nonstainless‐steel (nitinol, platinum, and cobalt‐alloy) stents and totally destructive (100%) for the stainless steel stent. Signal loss due to RF shielding was 31–62% for nitinol stents, 14–50% for platinum stents, 50–77% for the cobalt‐alloy stents (undetermined for the stainless steel stent), varied with stent orientation, diameter, and wall geometry. In summary, stents made of nitinol, platinum, and cobalt‐alloy have negligible susceptibility effects but stents made of stainless steel may have complete dephasing. All stents have substantial shielding effects, which vary with composition, geometry, and orientation. Large platinum stents may have the smallest artifacts and are the best suited for postinterventional MR imaging. Magn Reson Med 49:972–976, 2003.

Imaging therapeutic response in human bone marrow using rapid whole-body MRI†

Whole‐body imaging of therapeutic response in human bone marrow was achieved without introduced contrast agents using diffusion‐weighted echo‐planar magnetic resonance imaging of physiologic water. Bone marrow disease was identified relative to the strong overlying signals from water and lipids in other anatomy through selective excitation of the water resonance and generation of image contrast that was dependent upon differential nuclear relaxation times and self‐diffusion coefficients. Three‐dimensional displays were generated to aid image interpretation. The geometric distortion inherent in echo‐planar imaging techniques was minimized through the acquisition of multiple axial slices at up to 12 anatomic stations over the entire body. Examples presented include the evaluation of therapeutic response in bone marrow during cytotoxic therapy for leukemia and metastatic prostate cancer and during cytokine administration for marrow mobilization prior to stem cell harvest. Magn Reson Med 52:1234–1238, 2004.

Diffusion Tensor Tractography of the Motor White Matter Tracts in Man: Current Controversies and Future Directions

The anatomy of the brain is extremely complex, and certain, even large structures, such as the corticospinal tract (CST), remain poorly understood. Diffusion tractography provides an opportunity to explore the white matter tracts in a fundamentally new way. In the current paper, we show how this technique has already added to our understanding of the anatomy of the CST. We also explore the future projects involving diffusion tractography of the motor white matter tracts that will advance this method and further our understanding of brain anatomy.

Differential cingulate and caudate activation following unexpected nonrewarding stimuli.

This study examined the effects of varying the predictability of nonrewarding events on behavior and neural activation using a rapid mixed-trial functional magnetic resonance imagery (fMRI) design. Twelve adult subjects were scanned with echo planar imaging during performance of a visual detection task where the probability of events (target and nontarget) varied. This task included expected and unexpected nonrewarding events (expected target, unexpected nontarget, and omission of target) in a design that closely parallels studies of dopamine function and reward processing in the alert monkey. We predicted that activation in dopamine-rich areas of the forebrain would behave like the animal literature shows that dopamine neurons in the midbrain behave. Specifically, we predicted increased activity in these regions when an unexpected event occurred and decreased activity when an expected event was omitted. Two main regions, the anterior cingulate and dorsal striatum, showed this pattern. The response in these regions was distinguished by enhanced anterior cingulate activity following the occurrence of an unexpected event and greater suppression of caudate activity following the omission of an expected event. These results suggest that neural activity within specific dopamine-rich brain regions can be modulated by violations in the expectation of nonrewarding events and that the direction of the modulation depends on the nature of the violations.

Atherosclerotic disease distribution in carotid and vertebrobasilar arteries: Clinical experience in 100 patients undergoing fluoro‐triggered 3D Gd‐MRA

To determine how often three‐dimensional gadolinium‐enhanced magnetic resonance angiography (3D Gd‐MRA) shows disease outside of the standard two‐dimensional time‐of‐flight magnetic resonance angiography (2D TOF‐MRA) imaging volume.

Recessed elliptical-centric view-ordering for contrast-enhanced 3D MR angiography of the carotid arteries

Fast arterial‐venous transit in the carotid arteries requires accurate, reliable timing of the acquisition to the bolus transit to maximize arterial signal and minimize venous artifacts. The rising edge of the bolus is not utilized in conventional elliptical‐centric view‐ordering because the critical k‐space center must be acquired with full arterial enhancement. In this study, a recessed elliptical‐centric view‐ordering scheme is introduced in which the k‐space center is acquired a few seconds following scan initiation. The recessed view‐ordering is shown to be more robust to timing errors than the conventional scheme in a study of 37 patients. Magn Reson Med 48:419–424, 2002.

Soft tissue enhancement on time-resolved peripheral magnetic resonance angiography

To evaluate the incidence and locations of soft tissue enhancement on time‐resolved two‐dimensional projection magnetic resonance angiography (MRA) of the calf and foot.

k-Space interpretation of the Rose Model: Noise limitation on the detectable resolution in MRI

Noise limitation on the detected spatial resolution, described by the Rose Model, is well known in X‐ray imaging and routinely used in designing X‐ray imaging protocols. The purpose of this article is to revisit the Rose Model in the context of MRI where image data are acquired in the spatial frequency domain. A k‐space signal‐to‐noise ratio (kSNR) is introduced to measure the relative signal and noise powers in a circular annulus in k‐space. It is found that the kSNR diminishes rapidly with k‐space radius. The Rose criterion that the voxel SNR ∼4 is translated to kSNR cutoff values was tested using theoretical derivation and experimental histogram analysis. Experiments demonstrate that data acquisition beyond this cutoff k‐space radius adds little or no information to the image. In order to reduce the noise limit on spatial resolution, the signal strength must be improved through means such as increasing the coil sensitivity, contrast enhancement, and signal averaging. This finding implies that the optimal k‐space volume to be sampled or the optimal scan time in MRI should be matched to the relative SNR level. Magn Reson Med 48:550–554, 2002.

Three‐dimensional MR angiography in imaging platinum alloy stents

To evaluate visualization inside platinum stents with three‐dimensional contrast‐enhanced magnetic resonance angiography (CE‐MRA).