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Biomedical subjects

E M Haacke

Publications and source records attributed to E M Haacke.

At least 55 records · Page 3Linked to original sources
PubMed1996

Commutator filter: a novel technique for the identification of structures producing significant susceptibility inhomogeneities and its application to functional MRI.

A robust postprocessing method to detect regions of magnetic field change is presented. High resolution gradient echo data is acquired to create a complex baseline image. From this image, two low resolution images are obtained by applying a low pass filter on the magnitude image and the complex image, respectively. Subtraction of the magnitude of the filtered complex image from the filtered magnitude image gives a new image called a commutator image in which regions of susceptibility-induced field variation are greatly enhanced. Results on phantoms and applications to visualize paramagnetic venous blood vessels in the human brain are presented. The use of this technique on a 3D gradient echo data set represents a new approach to obtain an MR venogram. The application of this method to blood oxygenation level dependent (BOLD) functional brain MR imaging is demonstrated and the important role susceptibility plays in this model is verified.

Algorithms↗
PubMed1996

In vivo regional cerebral blood volume: quantitative assessment with 3D T1-weighted pre- and postcontrast MR imaging.

PURPOSE: To assess the use of pre-and postcontrast three-dimensional (3D) T1-weighted images to obtain quantitative regional cerebral blood volume (rCBV) maps and to delineate the vascular system of the brain. MATERIALS AND METHODS: Sagittal 3D pre-and postcontrast T1-weighted images were acquired in 10 patients and two volunteers. The images were processed with a knowledge-based segmentation algorithm to obtain independent blood volume maps of gray matter (GM), white matter (WM), and blood vessels and to quantitate rCBV in various cortical and deep cortical structures. Localized maximum intensity projection (MIP)-rCBV images were used to reveal the brain's venous system and vascularity in and around any lesions present. RESULTS: Group-averaged (n = 8) rCBV maps from both GM (4.80% +/- 0.37) and WM (2.02% +/- 0.14) yielded a ratio of GM-to-WM rCBV of 2.38 +/- 0.20. The ratio of rCBV between cortical and deep cortical GM was 1.15. rCBV maps showed better vascular contrast than did the postcontrast images in delineation of blood vessels and in the lesions present in two patients. A localized MIP-rCBV image had more than five times higher vascular contrast-to-noise ratio than that of the postcontrast MIP image. CONCLUSION: rCBV maps can be obtained in a standard clinical setting and can be used to reveal information on local blood volume.

Adult↗
PubMed1996

Coronary arteries: three-dimensional MR imaging with retrospective respiratory gating.

Thin-section, three-dimensional (3D) gradient-echo magnetic resonance imaging of the coronary arteries was performed without and with retrospective respiratory gating in 12 healthy volunteers and one patient. In all examinations, results were improved with gating. In five of seven volunteer examinations, coronary artery delineation on images reconstructed by using the least-squares method for motion detection with navigator echoes was found to be equal to that obtained by using edge detection. Images in five other volunteers covered the entire heart with multiple overlapping 3D slabs. The arteries were segmented from the background and could be viewed from any orientation. The lengths of contiguously visible vessels were as follows: left main coronary artery, 11.5 mm +/- 0.4 (mean +/- standard deviation); left anterior descending branch, 115.9 mm +/- 19.7; left circumflex branch, 97.2 mm +/- 12.5; and right coronary artery, 125.9 mm +/- 18.8. This respiratory gating technique clearly improved depiction of the coronary arteries.

Adult↗
PubMed1995

Tissue characterization of arteries with 4 MHz ultrasound.

Reflections of 4 MHz ultrasound pulses from 60 sites on 15 human aorta specimens were analysed using the plane-wave Born approximation (PWBA) inverse scattering method. The recovered high-resolution acoustic impedance profile for each site was correlated with an analysis (by microscope) of sections taken from that site. The shape of the impedance profile was found to be sensitive to the pathology of the tissue at each site. The severity of the various conditions could be gauged from the recovered impedance profiles. Some of the insonifications were done on aorta specimens that were shielded from the ultrasound transducer by a human tissue specimen about 1.25 cm thick in order to study the effects of intervening tissue. We found that it was possible to compensate for the distortion due to the frequency dependent attenuation of the intervening tissue. We conclude that ultrasound imaging, with the reflections analysed by means of PWBA inverse scattering provides a promising non-invasive tissue characterization modality.

Aorta↗
PubMed1995

Cerebral arteriovenous malformations: flow quantitation by means of two-dimensional cardiac-gated phase-contrast MR imaging.

PURPOSE: To test two-dimensional cardiac-gated phase-contrast (2DGPC) magnetic resonance (MR) imaging in the evaluation of cerebral arteriovenous malformations (AVMs) and the alterations in flow that accompany embolization therapy. MATERIALS AND METHODS: A 2DGPC sequence was used to measure flow velocity in the feeding vessels and corresponding contralateral vessels (CCVs) of seven patients (six men and one woman, 21-55 years of age; mean, 37 years) with cerebral AVMs and in the vessels of four healthy volunteers (all men; mean age, 22 years). RESULTS: In the patients, the flow in the feeding vessels exceeded the flow in the CCVs by a margin far greater than the asymmetry in flow in the volunteers. The mean reduction in flow after embolization was 54.9% +/- 11.1 in embolized feeding vessels (n = 6) and 5.3% +/- 2.8 in nonembolized vessels (n = 3). Postembolization flow enhancement was observed in the CCV (n = 2), which suggests a degree of autoregulatory dysfunction. CONCLUSION: This technique can be used to evaluate the flow patterns of AVMs and the hemodynamic changes that occur with embolization.

Adult↗
PubMed1995

Renal arteries: clinical comparison of three-dimensional time-of-flight MR angiographic sequences and radiographic angiography.

PURPOSE: To prospectively compare three magnetic resonance (MR) angiographic techniques in the renal arteries. MATERIALS AND METHODS: Twenty-five adult patients underwent three-dimensional time-of-flight MR angiography with three different sequences: conventional, tilted optimized nonsaturating excitation (TONE), and selective inversion-recovery rapid gradient-echo (SIR-RAGE). Fifteen also underwent radiographic angiography. Stenosis grade measured with each MR angiographic technique was compared with that measured with radiographic angiography by using correlation coefficients. Visible artery lengths with each MR angiographic technique were compared by using the Turkey method. RESULTS: Correlation between stenosis grades with each MR angiographic technique and with radiographic angiography was good (P < .01). Stenosis was correctly excluded with SIR-RAGE findings in six patients. Mean visible artery length was greatest with SIR-RAGE (P < .01). CONCLUSION: SIR-RAGE depicts a greater length of renal artery than does conventional MR angiography. TONE also improved depiction of distal arteries. Normal SIR-RAGE findings were highly predictive of normal arteries.

Adult↗
PubMed1995

Cardiac MR imaging: principles and techniques.

Magnetic resonance (MR) imaging is becoming an important tool in diagnosing cardiovascular disease. Many technical advances have contributed to the improved capability of MR imaging in this arena. In this article, we review the technical concepts and recent advances for a variety of cardiovascular applications, including evaluation of congenital heart and great vessel disease, global cardiac function using cine acquisitions and automated ventricular volume segmentation, valvular heart disease, myocardial wall motion detection with magnetization tagging, two-dimensional and three-dimensional coronary artery imaging, and myocardial perfusion. Sequence diagrams are shown and important technical issues are discussed. Example images are shown to demonstrate the current status of research and future potential. Most results shown here are high-resolution scans acquired with new gradient coil systems, including coronary artery imaging using an in-plane resolution of 350 microns x 600 microns. This review serves as an introduction to the discussion of various clinical applications of cardiovascular MR imaging provided in this issue.

Blood Volume↗
PubMed1994

Three-dimensional time-of-flight MR angiography using selective inversion recovery RAGE with fat saturation and ECG-triggering: application to renal arteries.

A three-dimensional (3D), ECG-triggered, selective inversion recovery (SIR) rapid gradient-echo (RAGE) technique is proposed to obtain MR angiograms of the main renal arteries. By using the selective inversion recovery and fat saturation, the background is significantly suppressed while blood maintains a high signal intensity as compared with conventional 3D time-of-flight (TOF) MR angiography. The sequence is ECG-triggered so that blood in-flow is maximized during systole, and intravoxel dephasing and pulsatile flow artifacts are minimized by collecting data during diastole. As a result, vessel boundary blurring and ghosting artifacts due to background motion are dramatically reduced, and the conspicuity and lumen definition of the arteries are significantly improved. High-quality MR angiograms of the main renal arteries with excellent blood/tissue contrast and suppression of motion artifacts have been consistently obtained for normal volunteers, with the length of visualization being 51 +/- 07 mm for the left, and 57 +/- 06 mm for the right renal arteries, significantly greater than using conventional 3D TOF pulse sequences. Statistical analysis was performed by using a one-sided Student's t test.

Angiography↗
PubMed1994

Three-dimensional time-of-flight MR angiography with variable TE (VARIETE) for fat signal reduction.

Uniform fat saturation over a large region of interest remains a problem in time-of-flight (TOF) magnetic resonance angiography applications. We demonstrate that a variable echo time with an opposed phase value at low spatial slice select frequencies can effectively reduce most of the fat signal in an otherwise standard three-dimensional TOF acquisition. We evaluated this method at 1.5 T using a short TE = 5.3 ms and a long TE = 6.75 ms for different values of the slice encoding gradient (i.e., different kz values). Shorter echo time (TE = 5.3 msec) was used at higher spatial slice select frequencies, but all echoes have the same gradient structures. By keeping the number of slice encoding steps with longer echoes to a minimum, field inhomogeneity effects on flow compensation remained small. A magnetization transfer saturation pulse was used to suppress signal of brain parenchyma. Overall, highly uniform and selective fat signal reduction was obtained while maintaining superior flow compensation in all volunteer studies.

Adipose Tissue↗
PubMed1994

Theory of NMR signal behavior in magnetically inhomogeneous tissues: the static dephasing regime.

This paper is devoted to a theory of the NMR signal behavior in biological tissues in the presence of static magnetic field inhomogeneities. We have developed an approach that analytically describes the NMR signal in the static dephasing regime where diffusion phenomena may be ignored. This approach has been applied to evaluate the NMR signal in the presence of a blood vessel network (with an application to functional imaging), bone marrow (for two specific trabecular structures, asymmetrical and columnar) and a ferrite contrast agent. All investigated systems have some common behavior. If the echo time TE is less than a known characteristic time tc for a given system, then the signal decays exponentially with an argument which depends quadratically on TE. This is equivalent to an R2* relaxation rate which is a linear function of TE. In the opposite case, when TE is greater than tc, the NMR signal follows a simple exponential decay and the relaxation rate does not depend on the echo time. For this time interval, R2* is a linear function of a) volume fraction sigma occupied by the field-creating objects, b) magnetic field Bo or just the objects' magnetic moment for ferrite particles, and c) susceptibility difference delta chi between the objects and the medium.

Blood Vessels↗
PubMed1994

2D and 3D high resolution gradient echo functional imaging of the brain: venous contributions to signal in motor cortex studies.

Recent emphasis on high resolution gradient echo studies in functional imaging has led to the conclusion that there are likely three domains of response to the blood circulation in the brain when considering field inhomogeneity effects of the venous blood pre- and during activation. The first is a coherent effect due to large or macroscopic vessels on the order of the size of the voxel (ca 200-500 microns in most studies). These can lead to very large signal changes (up to 100%). The second is at the venule level (ca 50-200 microns) and is associated with smaller parenchymal changes (usually ca 10% or less). The third is at the capillary level and is associated with much smaller signal changes at 1.5 T and even up to 4 T. The actual signal changes depend on field strength and sequence design. In this paper, we present our experience in detecting the first two domains with 2D and 3D gradient echo imaging at 1.5 T. We find that high resolution enables visualization of the larger small veins in motor cortex studies and that, on occasion, it is possible to see the venule effects as well. We suggest a simple model to explain the large signal changes based on susceptibility changes and partial volume effects. Comparisons of the functional imaging data to this model and to MR angiographic studies are also shown as evidence of the venous sources of the susceptibility changes.

Brain↗
PubMed1994

Optimizing three-dimensional time-of-flight MR angiography with variable repetition time.

PURPOSE: To implement and evaluate a variable repetition time (TR) modification of three-dimensional (3D) time-of-flight (TOF) magnetic resonance (MR) angiography. MATERIALS AND METHODS: Variable- and constant-TR 3D TOF MR angiography of the intracranial vasculature was performed in 20 healthy volunteers and 10 patients with known or suspected cerebrovascular disease. Total acquisition time was short and held constant for all studies. Frequency-selective fat saturation (FS) and magnetization transfer saturation (MTS) pulses were applied. The associated maximum-intensity projection (MIP) images were evaluated quantitatively for contrast-to-noise (C/N) and qualitatively for depiction of vessels that exhibit slow flow. RESULTS: Variable-TR MIP images exhibited improved C/N and depiction of small peripheral vessels and venous structures when compared with constant-TR MIP images. These observations were consistent for all studies. CONCLUSIONS: The variable-TR modification improved the depiction of intracranial vessels that exhibit slow flow.

Adult↗
PubMed1994

Abnormal connectivity of the visual pathways in human albinos demonstrated by susceptibility-sensitized MRI.

We studied activation of the human visual cortex (VC) using susceptibility-sensitized MRI at 1.5 Tesla. Three albinos and six healthy controls underwent a series of monocular and binocular photic flash stimulation. Monocular stimulation in albinos caused predominant contralateral activation with a small, well-delineated area in the anterior part of the VC in the ipsilateral hemisphere. This finding was consistent with a chiasmal crossing anomaly in albinism. All controls had symmetric patterns of activation during monocular stimulation. Functional MRI represents a promising method for evaluation of the visual pathways in humans.

Adolescent↗
PubMed1994

Short versus long echo time for cranial MR angiography in children and adults.

PURPOSE: To evaluate the ability of short-echo-time (TE) versus long-TE three-dimensional time-of-flight MR angiography sequences to decrease phase-related signal loss and refocus signal from blood in intracranial MR angiography of adults and children. METHODS: We evaluated 3-D time-of-flight cranial MR angiography in 33 cases (18 children and 15 adults) using two sequences. The longer-echo reference sequence had a TE of 8.0 milliseconds and a field echo of 6.5 milliseconds; the shorter-echo sequence had a TE of 5.1 and a field echo of 4.2 milliseconds. Repetition time, flip angle, and matrix were constant. The bandwidth for the longer-echo sequence was 130 Hz, 195 Hz for the shorter-echo sequence. RESULTS: The greatest improvement in diagnostic images was for children; significant and mildly improved signal recovery was demonstrated in 15 and 2 cases, respectively, of a total of 18 studies. This allowed improved diagnostic assessment. However, in the adult group significantly and mildly improved signal recovery were present in only 2 and 6 cases, respectively, of a total of 15 studies. In the group of children and adults combined, decreased lumen definition and peripheral vessel visibility were present in 24 and 30 of 33 cases, respectively, because of higher signal from background tissue when the shorter-TE field-echo sequence was used and, hence, reduced vascular contrast. CONCLUSION: The use of a short-field-echo/TE sequence is therefore recommended as the initial study in children but as a secondary examination in areas of abnormality in adults. This study illustrates the improved signal recovery from phase-related sources and improved visibility of intracranial stenosis in children with the use of a short-echo sequence. In adults, the short-echo sequence should not be used for the initial screening but reserved for secondary evaluation.

Adolescent↗
PubMed1993

Predicting RF field penetration in heterogeneous bodies using a 3-D finite element approach: preliminary results.

We present preliminary results for a 3D finite element calculation to evaluate RF penetration in conducting dielectric materials at high field strengths. A tetrahedral mesh is used along with a Coulomb gauge constraint in a finite element method that yields excellent numerical stability at high frequencies. Accuracy is verified by comparisons with analytic solutions for single-layer and multiple-layer heterogeneous systems and for a 3D spherical model. We have also compared the finite element model with experimental results presented by Foo et al., Magn. Reson. Med. 23, (1992). Agreement is very good and argues for the usefulness of the method in the calculation of RF penetration and RF power deposition effects in heterogeneous objects.

Animals↗
PubMed1993

Identification of vascular structures as a major source of signal contrast in high resolution 2D and 3D functional activation imaging of the motor cortex at 1.5T: preliminary results.

We have measured the T2* signal response associated with cortical activation due to finger motion at 1.5 Tesla. Both thin slice 2D and 3D images show signal intensity changes which vary from 2% to 32% depending on volunteer, echo time, slice thickness, and in-plane resolution. The largest signal change occurred for the thinnest slices and highest resolution (2 mm3). This is consistent with reducing partial volume effects and a simple difference in phase between the intravascular signal and surrounding parenchyma. No inflow enhancement was seen on the 2D or 3D scans, confirming the nature of the signal difference for this approach was due to local field inhomogeneity effects. Using 3D imaging, multiple effects can be seen simultaneously. With a 3D MRA method, it was possible to locate the vessel that was the source of the T2* behavior; it was in each case a vein on the surface of the cortical parenchyma.

Brain↗
PubMed1993

Intracranial MR angiography: application of magnetization transfer contrast and fat saturation to short gradient-echo, velocity-compensated sequences.

To circumvent artifacts related to saturation, turbulence, and data processing in three-dimensional (3D) time-of-flight (TOF) magnetic resonance (MR) angiography, the authors used a high-resolution (256 x 512), short, asymmetric velocity-compensated gradient-echo sequence, magnetization transfer contrast (MTC) between all excitation pulses to suppress brain parenchyma signal, and frequency-offset fat saturation (FS) to suppress fat signal. Signal intensity and blood-background contrast was measured in several regions of interest on conventional 3D TOF MR images, MTC images, and those with MTC and FS in seven volunteers. One patient each with an arteriovenous malformation, an aneurysm, and stenosis in the intracranial vascular system underwent the protocols with an echo time (TE) of 5 msec and then a TE of 8 msec. Use of the 5-msec-TE sequence with MTC and FS led to reduced problems associated with signal void in areas of fast flow in patients. In volunteers and patients, vascular visualization was superior to that of current 3D TOF techniques.

Artifacts↗