Biomedical subjects

L R Frank

Publications and source records attributed to L R Frank.

At least 37 records · Page 2Linked to original sources

PubMed1999

Quantitative perfusion imaging using arterial spin labeling.

Arterial spine labeling (ASL) techniques have matured to the point that they can provide robust quantitative multislice measurements of cerebral blood flow (CBF) under most circumstances. These techniques provide better spatial and temporal resolution than positron-emission tomography (PET) and are entirely noninvasive, requiring no injections or radiation. The most obvious clinical application is in the evaluation of acute stroke, in which the primary pathology is a lack of CBF, precisely the quantity that is measured directly by ASL. The one major technical challenge that currently prevents more general application in the brain is the sensitivity to abnormally long transit delays.

Arteries↗

PubMed1998

Probabilistic analysis of functional magnetic resonance imaging data.

Probability theory is applied to the analysis of fMRI data. The posterior distribution of the parameters is shown to incorporate all the information available from the data, the hypotheses, and the prior information. Under appropriate simplifying conditions, the theory reduces to the standard statistical test, including the general linear model. The theory is particularly suited to handle the spatial variations in the noise present in fMRI, allowing the comparison of activated voxels that have different, and unknown, noise. The theory also explicitly includes prior information, which is shown to be critical in the attainment of reliable activation maps.

Humans↗

PubMed1998

Correction of off resonance-related distortion in echo-planar imaging using EPI-based field maps.

We present, here, a simple method for measurement and correction of off-resonance related geometric distortion in echo-planar imaging (EPI). This method uses high signal-to-noise ratio (SNR) EPI-based field maps, rapidly acquired using a series of gradient recalled images collected across a range of TE values. This field map is distorted in the same manner as the EPI images to be unwarped, providing a direct look-up table for the correct location of each pixel of data. This method adds very little scan time and is robust and easy to implement.

Algorithms↗

PubMed1998

Quantitative imaging of perfusion using a single subtraction (QUIPSS and QUIPSS II).

In the pulsed arterial spin labeling (ASL) techniques EPISTAR, PICORE, and FAIR, subtraction of two images in which inflowing blood is first tagged and then not tagged yields a qualitative map of perfusion. An important reason this map is not quantitative is that there is a spatially varying delay in the transit of blood from the tagging region to the imaging slice that cannot be measured from a single subtraction. We introduce here two modifications of pulsed ASL (QUIPSS and QUIPSS II) that avoid this problem by applying additional saturation pulses to control the time duration of the tagged bolus, rendering the technique relatively insensitive to transit delays and improving the quantitation of perfusion.

Brain↗

PubMed1998

Dynamics of blood flow and oxygenation changes during brain activation: the balloon model.

A biomechanical model is presented for the dynamic changes in deoxyhemoglobin content during brain activation. The model incorporates the conflicting effects of dynamic changes in both blood oxygenation and blood volume. Calculations based on the model show pronounced transients in the deoxyhemoglobin content and the blood oxygenation level dependent (BOLD) signal measured with functional MRI, including initial dips and overshoots and a prolonged poststimulus undershoot of the BOLD signal. Furthermore, these transient effects can occur in the presence of tight coupling of cerebral blood flow and oxygen metabolism throughout the activation period. An initial test of the model against experimental measurements of flow and BOLD changes during a finger-tapping task showed good agreement.

Blood Glucose↗

PubMed1998

A theoretical and experimental comparison of continuous and pulsed arterial spin labeling techniques for quantitative perfusion imaging.

Under ideal conditions, continuous arterial spin labeling (ASL) techniques are higher in SNR than pulsed ASL techniques by a factor of e. Presented here is a direct theoretical and experimental comparison of continuous ASL and pulsed ASL, using versions of both that are amenable to multislice imaging and insensitive to variations in transit times (continuous ASL with a delay before imaging, and QUIPSS II (Quantitative Imaging of Perfusion Using a Single Subtraction-second version)). Perfusion image quality for comparable imaging time was nearly identical for both single-slice and multislice imaging. The measured raw signal was approximately 25% higher with continuous ASL, but the SNR per unit time was identical.

Artifacts↗

PubMed1998

A general kinetic model for quantitative perfusion imaging with arterial spin labeling.

Recently, several implementations of arterial spin labeling (ASL) techniques have been developed for producing MRI images sensitive to local tissue perfusion. For quantitation of perfusion, both pulsed and continuous labeling methods potentially suffer from a number of systematic errors. In this study, a general kinetic model for the ASL signal is described that can be used to assess these errors. With appropriate assumptions, the general model reduces to models that have been used previously to analyze ASL data, but the general model also provides a way to analyze the errors that result if these assumptions are not accurate. The model was used for an initial assessment of systematic errors due to the effects of variable transit delays from the tagging band to the imaging voxel, the effects of capillary/tissue exchange of water on the relaxation of the tag, and the effects of incomplete water extraction. In preliminary experiments with a human subject, the model provided a good description of pulsed ASL data during a simple sensorimotor activation task.

Arousal↗

PubMed1998

Dynamic knee-extensor and cycle exercise: functional MRI of muscular activity.

Repeated studies using human dynamic knee-extensor exercise have reported high mass specific blood flows. These studies suggest that the high perfusion-to-muscle mass ratio can approach 400 ml(-1) x min x 100 g(-1) in the human quadriceps. However, in these studies mass specific blood flows were calculated based on the assumption that the quadriceps are the only muscles involved in the knee-extensor exercise, which is difficult to verify in an in vivo human model. Previous validations of this assumption have been performed using electromyography (EMG) and assessments of strain gauge tracings, but neither has been able to completely assess the involvement of all thigh muscles in this exercise. To address this issue four subjects exercised at 90% of their work rate maximum for 2.0-2.5 minutes (45-100 watts) and then a transverse section of the thigh (20 cm proximal to the knee) was studied using proton (1H) transverse relaxation time (T2) weighted magnetic resonance (MR) imaging to distinguish active from non-active muscles by the increased signal intensity (SI). On a separate occasion, measurements following 2.0-2.5 minutes of conventional two legged cycle ergometry at 90% of maximum work rate (150-400 watts) were made in the same subjects to contrast this traditional "whole leg" exercise with the unique muscle recruitment in dynamic knee-extension. Following knee-extensor exercise there was a clearly visible change in SI and a significant increase in T2 only in the four muscles of the quadriceps (P<0.05). After bicycle exercise SI changes and T2 revealed a varied muscle use across all muscles. From these MR data it can be concluded that unlike cycle exercise, in which all muscles are recruited to varying extents, single leg knee-extensor exercise is limited to the four muscles of the quadriceps. Thus, the common practice of normalizing blood flow and metabolic data to the quadriceps muscle mass in human knee-extensor exercise studies appears appropriate.

Adult↗

PubMed1998

Evaluation of patellar cartilage in cadavers with a low-field-strength extremity-only magnet: comparison of MR imaging sequences, with macroscopic findings as the standard.

PURPOSE: To assess a low-field-strength extremity-only magnet in the evaluation of patellar cartilage abnormalities. MATERIALS AND METHODS: Four regions in each of 10 patellae from cadavers were examined in the transaxial plane with a 0.2-T extremity-only magnet and the following sequences: T1-weighted spin echo, proton density- and T2-weighted turbo spin echo, short inversion time inversion recovery, and two- and three-dimensional gradient echo with and without magnetization transfer contrast subtraction. Lesions depicted with MR imaging and seen in anatomic sections of the patellae were classified according to a modified standardized arthroscopic grading system. MR imaging and pathologic correlation was then analyzed. RESULTS: On the basis of macroscopic findings, 14 of 40 cartilage regions were found to be intact, grade 2A lesions were present in eight regions, grade 2B lesions in eight, and grade 3 lesions in 10. For the various MR imaging techniques, sensitivity was 25%-62% for grade 2A lesions, 50%-75% for grade 2B lesions, and 60%-90% for grade 3 lesions. Specificity was 81%-97% for grades 2A and 2B lesions, and 80%-97% for grade 3 lesions. Accuracy was 75%-82% for grade 2A lesions, 75%-92% for grade 2B lesions, and 80%-92% for grade 3 lesions. CONCLUSION: High-grade cartilaginous lesions can be evaluated reliably with low-field-strength MR imaging by using a combination of imaging sequences.

Aged↗

PubMed1998

Putaminal infarct in methanol intoxication: case report and role of brain imaging studies.

Methanol toxicity can cause severe central nervous system insult in which a characteristic pattern of bilateral putaminal injury is noted on brain imaging studies. We present a fatal case of subacute methanol toxicity with associated diffuse brain involvement, including bilateral putaminal necrosis and cerebral edema with ventricular compression. Theoretical basal ganglia toxicologic mechanisms of methanol poisoning are reviewed, and the role of brain imaging studies will regard to diagnosis, prognosis and impact on management is discussed.

Adult↗

PubMed1997

Slice profile effects in adiabatic inversion: application to multislice perfusion imaging.

Imperfections in the slice profile of the adiabatic inversion induced by relaxation effects are shown to cause signal variations in pulsed arterial tagging schemes on the order of magnitude of perfusion changes, and result in gross errors in perfusion quantitation. Significant improvement can be made with minor modifications to the inversion pulse which facilitate the acquisition of quantitative, multislice perfusion images, as demonstrated in both a phantom and a normal human volunteer.

Blood Flow Velocity↗

PubMed1997

A model for the coupling between cerebral blood flow and oxygen metabolism during neural stimulation.

A general mathematical model for the delivery of O2 to the brain is presented, based on the assumptions that all of the brain capillaries are perfused at rest and that all of the oxygen extracted from the capillaries is metabolized. The model predicts that disproportionately large changes in blood flow are required in order to support small changes in the O2 metabolic rate. Interpreted in terms of this model, previous positron emission tomography (PET) studies of the human brain during neural stimulation demonstrating that cerebral blood flow (CBF) increases much more than the oxygen metabolic rate are consistent with tight coupling of flow and oxidative metabolism. The model provides a basis for the quantitative interpretation of functional magnetic resonance imaging (fMRI) studies in terms of changes in local CBF.

Brain↗

PubMed1997

Short echo time projection reconstruction MR imaging of cartilage: comparison with fat-suppressed spoiled GRASS and magnetization transfer contrast MR imaging.

PURPOSE: To evaluate short echo time (TE) projection reconstruction magnetic resonance (MR) imaging in the detection of cartilage lesions. MATERIALS AND METHODS: Twenty-seven cartilage regions of 10 human patellar specimens were examined with the following MR sequences: short TE projection reconstruction (repetition time msec/TE msec, 400/0.15), fat-suppressed three-dimensional spoiled gradient-recalled acquisition in the steady state (Spoiled GRASS) (50/10, 60 degrees flip angle), and magnetization transfer contrast (MTC) subtraction (400/6). MR findings were correlated with histopathologic grading of the cartilage. RESULTS: For detection of cartilage lesions, sensitivity of projection reconstruction imaging (100%) was significantly greater (P = .03) than that of MTC (62%) but not significantly greater (P > .05) than that of Spoiled GRASS (81%) imaging. Accuracy of projection reconstruction was significantly greater than that of MTC (P = .004) and Spoiled GRASS (P = .03) imaging. Unmasking of collagen fibers was most predictive of abnormal signal intensity of the cartilage with all sequences. CONCLUSION: In vitro, short TE projection reconstruction MR imaging provides superior delineation of cartilage lesions when compared with two other sequences. On Spoiled GRASS and MTC images, signal intensity of the superficial layer of cartilage is not a reliable sign for surface integrity.

Aged↗

PubMed1997

MR imaging truncation artifacts can create a false laminar appearance in cartilage.

OBJECTIVE: The purpose of this study was to investigate the laminar appearance of cartilage on MR images. MATERIALS AND METHODS: Theoretical modeling of truncation artifacts was used to predict spatial patterns and associated intensity variations in MR imaging. A numerical simulation of a ring model was used to show truncation artifacts as a function of the angle in the image plane for unequal in-plane resolutions. MR imaging of 10 cadaveric human patellae at several resolutions used an imaging protocol that produced high-contrast images of cartilage. The high-resolution image of each MR imaging set was reduced in resolution by low-pass filtering and compared with the acquired images of equivalent resolution. Variable-resolution images of the patella of a healthy human volunteer were also acquired. RESULTS: Truncation artifacts from opposing cartilage edges can create false laminae and artifactual intensities. The resulting geometric variations can alter the apparent width of the cartilage as well. The intensity variations produced by truncation artifacts can be as much as 22% of the actual intensity. The most pronounced artifactual trilaminar appearance occurs when cartilage thickness exceeds the image resolution by a factor of 4. Truncation artifacts vary as a function of the angle in the imaging plane for unequal resolutions in the two directions. CONCLUSION: Truncation artifacts can produce an artifactual laminar appearance in cartilage and alter the apparent cartilage width.

Artifacts↗

PubMed1995

Theoretical considerations and in vitro results for the development of percutaneous transcatheter balloon embolectomy.

RATIONALE AND OBJECTIVES: To prove the feasibility of performing percutaneous transcatheter embolectomy using a funnel-shaped catheter tip and balloon embolectomy catheters. METHODS: A theoretical model is described to assess the influence of the diameter of an introductory device and coaxially introduced embolectomy catheter. A funnel-shaped catheter tip was attached to 7F, 8F, and 9F introductory sheaths. In vitro embolectomy of 5 g and 12 g thrombi was performed with 3F and 4F embolectomy catheters. RESULTS: The number of extractions required was significantly related to the ratio of the diameters of the sheaths and embolectomy catheter shafts. The combination of a 7F sheath with a 4F embolectomy catheter required the greatest number of extractions (8.4 +/- 1.7). The least number of extractions was needed for the combination of a 9F sheath with 4F and 3F embolectomy catheters (1.6 +/- 0.7 and 1.8 +/- 0.4, respectively). CONCLUSIONS: Our results indicate that percutaneous embolectomy with balloon embolectomy catheters is feasible. However, further research is necessary before the final catheter design is chosen.

Catheterization↗

PubMed1994

Blood flow dynamics in the vertebrobasilar system: correlation of a transparent elastic model and MR angiography.

PURPOSE: To describe the flow patterns in a model of the vertebrobasilar artery and use these observations to explain the appearance of the flow on the MR images. METHODS: We created an anatomically precise, transparent elastic model of the human vertebrobasilar artery containing a basilar tip aneurysm and perfused the model with non-Newtonian fluid which has similar rheologic properties to blood. Flow patterns in the vessels were directly observed. MR angiogram images were obtained with commercially available two-dimensional time-of-flight, three-dimensional time-of-flight, and 3-D phase-contrast MR angiographic pulse sequences, and they were correlated with the directly seen flow patterns. Quantitative flow velocity measurements were performed with 2-D cine phase-contrast MR angiography and correlated with the flow measured with an electromagnetic flow meter. RESULTS: Visualization studies showed the dye stream patterns in the vertebrobasilar arteries to be extremely complex and variable. During the MR experiments we found that often the same segment of a vessel could appear very different depending on the pulse sequence. In some instances, the model experiments helped to explain the MR appearance of the vessels. Flow profiles measured with 2-D cine phase contrast were found to be consistent with those measured directly with an electromagnetic flow meter. CONCLUSION: Clear elastic models can be used to duplicate the flow in human cranial vessels and thus provide a unique means to observe these flow patterns directly. The flow patterns helped to explain the variation in appearance of the vessels and the artifacts with different MR angiography pulse sequences. The artifacts depend on both the geometry of the vessel and the flow pattern within it. Two-dimensional cine phase-contrast MR provides temporal flow field information that is directly related to physiological information about flow volumes and velocity patterns.

Adult↗

PubMed1993

Distortions from curved flow in magnetic resonance imaging.

We present an analysis of how vessel curvature can create distortions in magnetic resonance images of flowing blood. Steady flow in curved vessels produces distortions of the vessel shape and intensity variations in the image due to motion during the interval between phase encoding or slice selection and the echo center. Even with steady flow, vessel curvature produces motion moments higher than velocity (acceleration, etc.), but use of a first order oblique flow compensated phase encoding gradient waveform reduced the distortion in the image. Numerical calculations of image distortions based on simple flow models are in good agreement with experimental results in a phantom.

Magnetic Resonance Imaging↗

PubMed1993

Pulsatile flow artifacts in 3D magnetic resonance imaging.

Some of the important features of how pulsatile flow generates artifacts in three-dimensional magnetic resonance imaging are analyzed and demonstrated. Time variations in the magnetic resonance signal during the heart cycle lead to more complex patterns of artifacts in 3D imaging than in 2D imaging. The appearance and location of these artifacts within the image volume are shown to be describable as displacements along a line in a plane parallel to that defined by the phase and volume encode directions. The angle of the line in the plane depends solely upon the imaging parameters while the ghost displacement along the line is proportional to the signal modulation frequency. Aliasing of these ghosts leads to a variety of artifact patterns which are sensitive to the pulsation period and repetition time of the pulse sequence. Numerical simulations of these effects were found to be in good agreement with experimental images of an elastic model of a human carotid artery under simulated physiological conditions and with images of two human subjects.

Aged↗