Quantitative T2 and T1ρ mapping are sensitive to ischemic injury to the epiphyseal cartilage in an in vivo piglet model of Legg-Calvé-Perthes disease.

OBJECTIVE: To determine if the quantitative MRI techniques T2 and T1ρ mapping are sensitive to ischemic injury to epiphyseal cartilage in vivo in a piglet model of Legg-Calvé-Perthes disease using a clinical 3T MRI scanner. We hypothesized that T2 and T1ρ relaxation times would be increased in the epiphyseal cartilage of operated vs contralateral-control femoral heads 1 week following onset of ischemia. DESIGN: Unilateral femoral head ischemia was surgically induced in eight piglets. Piglets were imaged 1 week post-operatively in vivo at 3T MRI using a magnetization-prepared 3D fast spin echo sequence for T2 and T1ρ mapping and a 3D gradient echo sequence for cartilage segmentation. Ischemia was confirmed in all piglets using gadolinium contrast-enhanced MRI. Median T2 and T1ρ relaxation times were measured in the epiphyseal cartilage of the ischemic and control femoral heads and compared using paired t-tests. Histological assessment was performed on a subset of five piglets. RESULTS: T2 and T1ρ relaxation times were significantly increased in the epiphyseal cartilage of the operated vs control femoral heads (ΔT2 = 11.9 ± 3.7 ms, 95% CI = [8.8, 15.0] ms, P < 0.0001; ΔT1ρ = 12.8 ± 4.1 ms, 95% CI = [9.4, 16.2] ms, P < 0.0001). Histological assessment identified chondronecrosis in the hypertrophic and deep proliferative zones within ischemic epiphyseal cartilage. CONCLUSIONS: T2 and T1ρ mapping are sensitive to ischemic injury to the epiphyseal cartilage in vivo at clinical 3T MRI. These techniques may be clinically useful to assess injury and repair to the epiphyseal cartilage to better stage the extent of ischemic damage in Legg-Calvé-Perthes disease.

MRI evaluation of articular cartilage in patients with juvenile osteochondritis dissecans (JOCD) using T2∗ mapping at 3T.

OBJECTIVE: Evaluate articular cartilage by magnetic resonance imaging (MRI) T2∗ mapping within the distal femur and proximal tibia in adolescents with juvenile osteochondritis dissecans (JOCD). DESIGN: JOCD imaging studies acquired between August 2011 and February 2019 with clinical and T2∗ mapping MRI knee images were retrospectively collected and analyzed for 31 participants (9F/22M, 15.0 ± 3.8 years old) with JOCD lesions in the medial femoral condyle (MFC). In total, N = 32 knees with JOCD lesions and N = 14 control knees were assessed. Mean T2∗ values in four articular cartilage regions-of-interest (MFC, lateral femoral condyle (LFC), medial tibia (MT), and lateral tibia (LT)) and lesion volume were measured and analyzed using Wilcoxon-rank-sum tests and Spearman correlation coefficients (R). RESULTS: Mean ± standard error T2∗ differences observed between the lesion-sided MFC and the LFC in JOCD-affected knees (28.5 ± 0.9 95% confidence interval [26.8, 30.3] vs 26.3 ± 0.7 [24.8, 27.7] ms, P = 0.088) and between the affected- and control-knee MFC (28.5 ± 0.9 [26.8, 30.3] vs 28.5 ± 0.6 [27.1, 29.9] ms, P = 0.719) were nonsignificant. T2∗ was significantly increased in the lesion-sided MT vs the LT for the JOCD-affected knees (21.5 ± 0.7 [20.1, 22.9] vs 18.0 ± 0.7 [16.5, 19.5] ms, P = 0.002), but this same difference was also observed between the MT and LT in control knees (21.0 ± 0.6 [19.7, 22.3] vs 18.1 ± 1.1 [15.8, 20.4] ms, P = 0.037). There was no significant T2∗ difference between the affected- and control-knee MT (21.5 ± 0.7 [20.1, 22.9] vs 21.0 ± 0.6 [19.7, 22.3] ms, P = 0.905). T2∗ within the lesion-sided MFC was not correlated with patient age (R = 0.20, P = 0.28) or lesion volume (R = 0.06, P = 0.75). T2∗ values were slightly increased near lesions in later-stage JOCD subjects but without statistical significance. CONCLUSIONS: T2∗ relaxations times were not significantly different from control sites in the articular cartilage overlying JOCD lesions in the MFC or adjacent MT cartilage in early-stage JOCD.

Capturing fast relaxing spins with SWIFT adiabatic rotating frame spin-lattice relaxation (T1ρ) mapping.

Rotating frame spin-lattice relaxation, with the characteristic time constant T1ρ, provides a means to access motion-restricted (slow) spin dynamics in MRI. As a result of their restricted motion, these spins are sometimes characterized by a short transverse relaxation time constant T2 and thus can be difficult to detect directly with conventional image acquisition techniques. Here, we introduce an approach for three-dimensional adiabatic T1ρ mapping based on a magnetization-prepared sweep imaging with Fourier transformation (MP-SWIFT) sequence, which captures signal from almost all water spin populations, including the extremely fast relaxing pool. A semi-analytical procedure for T1ρ mapping is described. Experiments on phantoms and musculoskeletal tissue specimens (tendon, articular and epiphyseal cartilages) were performed at 9.4 T for both the MP-SWIFT and fast spin echo (FSE) read outs. In the phantom with liquids having fast molecular tumbling and a single-valued T1ρ time constant, the measured T1ρ values obtained with MP-SWIFT and FSE were similar. Conversely, in normal musculoskeletal tissues, T1ρ values measured with MP-SWIFT were much shorter than the values obtained with FSE. Studies of biological tissue specimens demonstrated that T1ρ-weighted SWIFT provides higher contrast between normal and diseased tissues relative to conventional acquisitions. Adiabatic T1ρ mapping with SWIFT readout captures contributions from the otherwise undetected fast relaxing spins, allowing more informative T1ρ measurements of normal and diseased states.

Surgical induction, histological evaluation, and MRI identification of cartilage necrosis in the distal femur in goats to model early lesions of osteochondrosis.

OBJECTIVE: Identify and interrupt the vascular supply to portions of the distal femoral articular-epiphyseal cartilage complex (AECC) in goat kids to induce cartilage necrosis, characteristic of early lesions of osteochondrosis (OC); then utilize magnetic resonance imaging (MRI) to identify necrotic areas of cartilage. DESIGN: Distal femora were perfused and cleared in goat kids of various ages to visualize the vascular supply to the distal femoral AECC. Vessels located on the axial aspect of the medial femoral condyle (MFC) and on the abaxial side of the lateral trochlear ridge were transected in eight 4- to 5-day-old goats to induce cartilage necrosis. Goats were euthanized 1, 2, 3, 4, 5, 6, 9, and 10 weeks post operatively and operated stifles were harvested. Adiabatic T1ρ relaxation time maps of the harvested distal femora were generated using a 9.4 T MR scanner, after which samples were evaluated histologically. RESULTS: Interruption of the vascular supply to the MFC caused lesions of cartilage necrosis in 6/8 goat kids that were demonstrated histologically. Adiabatic T1ρ relaxation time mapping identified these areas of cartilage necrosis in 5/6 cases. No significant findings were detected after transection of perichondrial vessels supplying the lateral trochlear ridge. CONCLUSIONS: Cartilage necrosis, characteristic of early OC, can be induced by interrupting the vascular supply to the distal femoral AECC in goat kids. The ability of high field MRI to identify these areas of cartilage necrosis in the AECC using the adiabatic T1ρ sequence suggests that this technique may be useful in the future for the early diagnosis of OC.

MR Imaging with T2*- mapping for improved acetabular cartilage assessment in FAI-a case report with arthroscopic correlation.

UNLABELLED: Articular cartilage assessment in femoroacetabular impingement (FAI) is challenging. Recent studies on T2* relaxation time mapping suggest the technique may be useful in diagnosing cartilage damage. The purpose of this case report is to describe how quantitative T2*-mapping may improve cartilage assessment of the acetabulum in patients with FAI. MR arthrography was performed at 3 Tesla (T) using intra-articular Gadolinium and a T2* mapping protocol. Data from the acetabular cartilage was separated from femoral head cartilage data and then superimposed on a flattened, map projection representation of the patient's acetabulum. The areas of unhealthy cartilage observed at the time of arthroscopy - including debonding and delamination - were seen preoperatively at the same anatomic locations as areas of decreased T2* values. T2* mapping values provided a non-invasive assessment of the acetabular articular cartilage. A flattened acetabular map projection allowed for anatomic visualization of areas of unhealthy cartilage. LEVEL OF EVIDENCE: Level IV.

Biochemical cartilage alteration and unexpected signal recovery in T2* mapping observed in ankle joints with mobile MRI during a transcontinental multistage footrace over 4486 km.

OBJECTIVE: The effect of ultra-long distance running on the ankle cartilage with regard to biochemical changes, thickness and lesions is examined in the progress of a transcontinental ultramarathon over 4486 km. METHOD: In an observational field study, repeated follow-up scanning of 22 participants of the TransEurope FootRace (TEFR) with a 1.5 T MRI mounted on a mobile unit was performed. For quantitative biochemical and structural evaluation of cartilage a fast low angle shot (FLASH) T2* weighted gradient-echo (GRE)-, a turbo-inversion-recovery-magnitude (TIRM)- and a fat-saturated proton density (PD)-weighted sequence were utilized. Statistical analysis of cartilage T2* and thickness changes was obtained on the 13 finishers (12 male, mean age 45.4 years, BMI 23.5 kg/m²). None of the nine non-finisher (eight male, mean age 53.8 years, BMI 23.4 kg/m²) stopped the race due to ankle problems. RESULTS: From a mean of 17.0 ms for tibial plafond and 18.0 ms for talar dome articular cartilage at baseline, nearly all observed regions of interest (ROIs) of the ankle joint cartilage showed a significant T2*-signal increase (25.6% in mean), with standard error ranging from 19% to 33% within the first 2500 km of the ultra-marathon. This initial signal behavior was followed by a signal decrease. This signal recovery (30.6% of initial increase) showed a large effect size. No significant morphological or cartilage thickness changes (at baseline 2.9 mm) were observed. CONCLUSION: After initial T2*-increase during the first 2000-2500 km, a subsequent T2*-decrease indicates the ability of the normal cartilage matrix to partially regenerate under ongoing multistage ultramarathon burden in the ankle joints.

Articular osteochondrosis: a comparison of naturally-occurring human and animal disease.

BACKGROUND: Osteochondrosis (OC) is a common developmental orthopedic disease affecting both humans and animals. Despite increasing recognition of this disease among children and adolescents, its pathogenesis is incompletely understood because clinical signs are often not apparent until lesions have progressed to end-stage, and examination of cadaveric early lesions is not feasible. In contrast, both naturally-occurring and surgically-induced animal models of disease have been extensively studied, most notably in horses and swine, species in which OC is recognized to have profound health and economic implications. The potential for a translational model of human OC has not been recognized in the existing human literature. OBJECTIVE: The purpose of this review is to highlight the similarities in signalment, predilection sites and clinical presentation of naturally-occurring OC in humans and animals and to propose a common pathogenesis for this condition across species. STUDY DESIGN: Review. METHODS: The published human and veterinary literature for the various manifestations of OC was reviewed. Peer-reviewed original scientific articles and species-specific review articles accessible in PubMed (US National Library of Medicine) were eligible for inclusion. RESULTS: A broad range of similarities exists between OC affecting humans and animals, including predilection sites, clinical presentation, radiographic/MRI changes, and histological appearance of the end-stage lesion, suggesting a shared pathogenesis across species. CONCLUSION: This proposed shared pathogenesis for OC between species implies that naturally-occurring and surgically-induced models of OC in animals may be useful in determining risk factors and for testing new diagnostic and therapeutic interventions that can be used in humans.

Simultaneous bilateral hip joint imaging at 7 Tesla using fast transmit B1 shimming methods and multichannel transmission - a feasibility study.

The objective of this study was to demonstrate the feasibility of simultaneous bilateral hip imaging at 7 Tesla. Hip joint MRI becomes clinically critical since recent advances have made hip arthroscopy an efficacious approach to treat a variety of early hip diseases. The success of these treatments requires a reliable and accurate diagnosis of intraarticular abnormalities at an early stage. Articular cartilage assessment is especially important to guide surgical decisions but is difficult to achieve with current MR methods. Because of gains in tissue contrast and spatial resolution reported at ultra high magnetic fields, there are strong expectations that imaging the hip joint at 7 Tesla will improve diagnostic accuracy. Furthermore, there is growing evidence that the majority of these hip abnormalities occur bilaterally, emphasizing the need for bilateral imaging. However, obtaining high quality images in the human torso, in particular of both hips simultaneously, must overcome a major challenge arising from the damped traveling wave behaviour of RF waves at 7 Tesla that leads to severe inhomogeneities in transmit B1 (B(1) (+) ) phase and magnitude, typically resulting in areas of low signal and contrast, and consequently impairing use for clinical applications. To overcome this problem, a 16-channel stripline transceiver RF coil was used, together with a B1 shimming algorithm aiming at maximizing B(1) (+) in six regions of interest over the hips that were identified on axial scout images. Our successful results demonstrate that this approach effectively reduces inhomogeneities observed before B1 shimming and provides high joint tissue contrast in both hips while reducing the required RF power. Critical to this success was a fast small flip angle B(1) (+) calibration scan that permitted the computation of subject-specific B1 shimming solutions, a necessary step to account for large spatial variations in B(1) (+) phase observed in different subjects.

Reduced cellular toxicity of a new silver-containing antimicrobial dressing and clinical performance in non-healing wounds.

Bacterial colonisation of wounds may delay wound healing. Modern silver-containing dressings are antimicrobial, yet cellular toxicity is a serious side-effect. We provide data for a newly formulated silver-containing ointment dressing, Atrauman Ag, for antimicrobial activity and cytotoxicity. Atrauman Ag effectively killed a panel of commensal skin as well as pathogenic bacterial strains while cytotoxicity for HaCaT keratinocytes was only around 10%. With these favourable in vitro tests, Atrauman Ag was analysed in 86 patients with traumatic and non-healing wounds of different aetiologies. The wound state was evaluated for 3 subsequent dressing changes. The slough score was reduced from 59.2 to 35.8%, granulation tissue increased from 27 to 40% and epithelialisation went up from 12.1 to 24%. We conclude that Atrauman Ag has a superior profile of antimicrobial activity over cellular toxicity and the low silver ion release rate may prevent interference with wound-healing mechanisms.

Activation of visuomotor systems during visually guided movements: a functional MRI study.

The dorsal stream is a dominant visuomotor pathway that connects the striate and extrastriate cortices to posterior parietal areas. In turn, the posterior parietal areas send projections to the frontal primary motor and premotor areas. This cortical pathway is hypothesized to be involved in the transformation of a visual input into the appropriate motor output. In this study we used functional magnetic resonance imaging (fMRI) of the entire brain to determine the patterns of activation that occurred while subjects performed a visually guided motor task. In nine human subjects, fMRI data were acquired on a 4-T whole-body MR system equipped with a head gradient coil and a birdcage RF coil using a T2*-weighted EPI sequence. Functional activation was determined for three different tasks: (1) a visuomotor task consisting of moving a cursor on a screen with a joystick in relation to various targets, (2) a hand movement task consisting of moving the joystick without visual input, and (3) a eye movement task consisting of moving the eyes alone without visual input. Blood oxygenation level-dependent (BOLD) contrast-based activation maps of each subject were generated using period cross-correlation statistics. Subsequently, each subject's brain was normalized to Talairach coordinates, and the individual maps were compared on a pixel by pixel basis. Significantly activated pixels common to at least four out of six subjects were retained to construct the final functional image. The pattern of activation during visually guided movements was consistent with the flow of information from striate and extrastriate visual areas, to the posterior parietal complex, and then to frontal motor areas. The extensive activation of this network and the reproducibility among subjects is consistent with a role for the dorsal stream in transforming visual information into motor behavior. Also extensively activated were the medial and lateral cerebellar structures, implicating the cortico-pontocerebellar pathway in visually guided movements. Thalamic activation, particularly of the pulvinar, suggests that this nucleus is an important subcortical target of the dorsal stream.

Detection of 13C-labeled metabolites in the in vivo canine heart by B1 insensitive heteronuclear coherent polarization transfer and comparison of signal enhancement with NOE.

A recently developed adiabatic coherent polarization transfer enhancement technique [H. Merkle, H. Wei, M. Garwood, K. Ugurbil. J. Magn. Reson, 99, 480-494 (1992)] was employed to perform 13C spectroscopy in the intact canine heart in vivo during [2-13C]-acetate infusion into the left descending coronary artery, the results were compared with 13C spectra obtained with conventionally employed nuclear Overhauser enhancement. The results demonstrate that both methods can be performed by using surface coils to obtain in vivo 13C spectra and that coherent polarization transfer provides better enhancement than NOE for [2-13C]-acetate but not for short T2 compounds.

Functional magnetic resonance imaging of cerebellar activation during the learning of a visuomotor dissociation task.

We have used functional magnetic resonance imaging (fMRI) to study the changes in cerebellar activation that occur during the acquisition of motor skill in human subjects presented with a new task. The standard paradigm consisted of a center-out movement in which subjects used a joystick to superimposed a cursor onto viusual targets. Two variations of this paradigm were introduced: (1) a learning paradigm, where the relationship between movement of the joystick and cursor was reversed, requiring the learning of a visuomotor transformation to optimize performance and (2) a random paradigm, where the joystick/cursor relationship was changed randomly for each trial. Activation in the cerebellum was highest during the random paradigm and during the early stages of the learning paradigm. In the early stages of learning and during the random paradigm performance was poor with a decrease in the number of completed movements, and an increase in the time and length of movements. With repeated practice at the learning paradigm performance improbed and reached the same level of proficiency as in the standard task. Commensurate with the improbement in performance was a decrease in cerebellar activation, that is, activation in the cerebellum changed in a parallel, but inverse relationship with performance. Linear regression analysis demonstarated that the inverse correlation between cerebellar activation and motor performance was significant. Repeated practice at the random paradigm did not produce improvements in performance and cerebellar activity remained high. The data support the hypothesis that the cerebellum is strongly activated when motor performance is inaccurate, consistent with a role for the cerebellum in the detection of, and correction for visuomotor errors.

[Quantitative evaluation of myocardial perfusion with ultrafast magnetic resonance tomography]. / Quantitative Beurteilung der Myokardperfusion mit der ultraschnellen Magnetresonanztomographie.

With the advent of ultrafast Magnetic Resonance Imaging (MRI), it is now possible to produce images with high temporal resolution. This gives the opportunity to record the passage of the paramagnetic contrast material Gadolinium-DTPA through the tissue of the heart muscle, yielding information on regional myocardial perfusion. We assessed the accuracy of MRI to detect and quantify reductions in coronary flow secondary to stenosis in dogs and patients. Regional blood flow was measured in dogs by left atrial injection of microspheres labeled with different radioactive isotopes. Signal intensity (SI) curves were generated in regions of interest over the myocardium and the cavum of the left ventricle. A newly developed two-compartment model based on the indicator-dilution method was used for interpretation of the SI-curves. In an optimization process the free parameters of the model equation were fitted to the measured SI-curves. The following flow parameters were determined: model parameter Q*, time to peak intensity (T), maximum signal intensity (SImax) and mean transit time (MTT) as calculated from a gamma variate fit. Absolute blood flow values were calculated for the parameters MTT and Q* assuming that the intravascular volume represents 10% of the total myocardial tissue volume. Measurements were performed on a 1.5 T Magnetom SP (Siemens AG, Erlangen) using a Turbo Flash sequence (TR = 6.5 ms, TE = 3 ms, TI = 100 ms, Flip Winkel = 9 degrees). Endsystolic images (voxel size = 1.8, 2.7, 15 mm3) were taken with an 18-cm Helmholtz surface coil in the short-axis view. A Gd-DTPA bolus (0.05 mmol/kg) was injected into the left atrium of 3 anesthetized closed-chest dogs. From the myocardial SI-curves the different parameters of myocardial perfusion were compared with flow assessed by microsphere injection over a wide range of myocardial blood flows (from 0.04 ml/min/g to 7.6 ml/min/g). A third-order polynominal fit showed a good correlation for the parameter Q* and MTT, whereas T and SImax were found to have a poor correlation. The linear regression analysis for a limited range of < 2 ml/min/g showed a superior estimation of myocardial perfusion for the parameter Q* than MTT. Blood flow > 2 ml/min/g was significantly underestimated by the MRT-measurements, but the parameter Q* showed the smallest amount of the divergent changes.(ABSTRACT TRUNCATED AT 400 WORDS)

Imaging at high magnetic fields: initial experiences at 4 T.

This article reviews the preliminary experiences and the results obtained on the human brain at 4 T at the University of Minnesota. Anatomical and functional images are presented. Contrary to initial expectations and the early results, it is possible to obtain high-resolution images of the human brain with exquisite T1 contrast, delineating structures especially in the basal ganglia and thalamus, which were not observed clearly in 1.5-T images until now. These 4-T images are possible using a new approach that achieves maximal contrast for different T1 values at approximately the same repetition time and has built-in tolerance to variations in B1 magnitude. For functional images, the high field provides increased contribution from the venuoles and the capillary bed because the susceptibility-induced alterations in 1/T2* from these small-diameter vessels increase quadratically with the magnitude of the main field. Images obtained with short echo times at 4 T, and by implication at lower fields with correspondingly longer echo times, are expected to be dominated by contributions from large venous vessel or in-flow effects from the large arteries; such images are undesirable because of their poor spatial correspondence with actual sites of neuronal activity.

Functional magnetic resonance imaging of motor cortex: hemispheric asymmetry and handedness.

A hemispheric asymmetry in the functional activation of the human motor cortex during contralateral (C) and ipsilateral (I) finger movements, especially in right-handed subjects, was documented with nuclear magnetic resonance imaging at high field strength (4 tesla). Whereas the right motor cortex was activated mostly during contralateral finger movements in both right-handed (C/I mean area of activation = 36.8) and left-handed (C/I = 29.9) subjects, the left motor cortex was activated substantially during ipsilateral movements in left-handed subjects (C/I = 5.4) and even more so in right-handed subjects (C/I = 1.3).

Localized detection of glioma glycolysis using edited 1H MRS.

In vivo 1H MRS can be used to detect and quantify the lactate resonance at 1.3 ppm provided that overlapping lipid resonances are eliminated. A homonuclear spectral editing method was developed to acquire uncontaminated 1H spectra of lactate with adiabatic pulses. An advantage of the adiabatic pulse sequence is the ability to induce uniform flip angles and to maximize sensitivity in applications employing surface coil transmitters which produce highly inhomogeneous B1. Glycolytic activity in an intracerebral C6 glioma in rats was monitored by using adiabatic editing sequences to observe [3-13C]lactate produced from infused [1-13C]glucose. Acute hyperglycemia (serum glucose > 22 mM, n = 10) had no significant effect (P = 0.08) on the total ([12C] + [13C]) tumor lactate signal intensity.

Contrast-enhanced first pass myocardial perfusion imaging: correlation between myocardial blood flow in dogs at rest and during hyperemia.

The sensitivity of contrast-enhanced MR first pass perfusion imaging in detection and quantification of hypoperfused myocardium was evaluated using an instrumented, closed-chest dog model where graded regional hypoperfusion was induced by applying predetermined levels of stenosis to the left anterior descending artery (LAD). All measurements were performed at rest and under stress induced by dipyridamole (DIP). Myocardial perfusion was assessed both with MR and radiolabeled microspheres injected immediately before the administration of the MR contrast agent. Ultrafast MR imaging was performed using a Turbo FLASH sequence with a 180 degrees inversion prepulse. A Gd-DTPA bolus was injected into the left atrium and T1-weighted images were acquired with every heart beat. Signal intensity measured from the images in regions of the LAD and left circumflex (LCx) perfusion beds was plotted against time to generate signal intensity versus time curves (SI time curve). Various flow indices were derived according to the indicator dilution theory, and compared with and without volume correction due to vasodilation to the myocardial blood flow (MBF) calculated from radiolabeled microspheres. Correlation of the MR and MBF data demonstrated that different transmural and regional myocardial perfusion levels can be easily visualized in the perfusion images and accurately monitored by the SI time curves. Detection of the impairment of myocardial perfusion improved significantly after administration of DIP. The inverse mean transit time calculated from the SI time curve was found to yield a linear correlation to absolute MBF derived from the microsphere data. These results suggest that with intracardiac injections of exogenous contrast agent, myocardial perfusion can be assessed parametrically with first pass contrast enhanced ultrafast MRI.

Functional brain mapping by blood oxygenation level-dependent contrast magnetic resonance imaging. A comparison of signal characteristics with a biophysical model.

It recently has been demonstrated that magnetic resonance imaging can be used to map changes in brain hemodynamics produced by human mental operations. One method under development relies on blood oxygenation level-dependent (BOLD) contrast: a change in the signal strength of brain water protons produced by the paramagnetic effects of venous blood deoxyhemoglobin. Here we discuss the basic quantitative features of the observed BOLD-based signal changes, including the signal amplitude and its magnetic field dependence and dynamic effects such as a pronounced oscillatory pattern that is induced in the signal from primary visual cortex during photic stimulation experiments. The observed features are compared with the results of Monte Carlo simulations of water proton intravoxel phase dispersion produced by local field gradients generated by paramagnetic deoxyhemoglobin in nearby venous blood vessels. The simulations suggest that the effect of water molecule diffusion is strong for the case of blood capillaries, but, for larger venous blood vessels, water diffusion is not an important determinant of deoxyhemoglobin-induced signal dephasing. We provide an expression for the apparent in-plane relaxation rate constant (R2*) in terms of the main magnetic field strength, the degree of the oxygenation of the venous blood, the venous blood volume fraction in the tissue, and the size of the blood vessel.

Functional imaging of human motor cortex at high magnetic field.

1. We used conventional gradient echo magnetic resonance imaging (MRI) at high field strength (4 Tesla) to functionally image the right motor cortex in six normal human subjects during the performance of a sequence of self-paced thumb to digit oppositions with the left hand (contralateral task), the right hand (ipsilateral task), and both hands (bilateral task). 2. A localized increase in activity in the lateral motor cortex was observed in all subjects during the task. The area of activation was similar in the contralateral and bilateral tasks but 20 times smaller in the ipsilateral task. The intensity of activation was 2.3 times greater in the contralateral than the ipsilateral task.

Functional brain mapping using magnetic resonance imaging. Signal changes accompanying visual stimulation.

Easily detectable (5%-20%) transient increases in the intensity of water proton magnetic resonance (MR) signals in human primary visual cortex were observed during visual stimulation in gradient echo images at 4-T field strength. The signal intensity increases were predominantly restricted to areas containing gray matter and were used to produce high-spatial-resolution human functional brain maps. Time dependence of the functional brain maps also was monitored during visual stimulation using images acquired every approximately 5 seconds; these images with high spatial and temporal resolution demonstrated that photic stimulation first resulted in signal increases in a large area of the visual cortex followed by a reduction in the size of the area, and that signal intensity increases in the gray matter were time dependent. Reducing the image acquisition echo times reduced the amplitude of the fractional signal change, suggesting that it is produced by a change in T2 or T2*. The amplitude, sign, and echo time dependence of these intrinsic signal changes are consistent with the idea that neural activation increases regional cerebral with the idea that neural activation increases regional cerebral blood flow (rCBF) with a concomitant increase in venous blood oxygenation.