Biomedical subjects

J R MacFall

Publications and source records attributed to J R MacFall.

At least 37 records · Page 2Linked to original sources

PubMed1998

MR microscopy of lung airways with hyperpolarized 3He.

A technique using hyperpolarized (HP) 3He to image the small airways of the lung by using moderate flip angles and a short scanning period during early inspiration is demonstrated. Flip angles (alpha) ranging from 10-90 degrees were used in guinea pig experiments with scanning during the entire inspiration period. A second series acquired data throughout a short window of the ventilatory cycle with alpha = 45 degrees. The success of the animal studies has motivated implementation of similar imaging techniques in the clinical arena. Human studies involved imaging over the total inspiration period with alpha approximately 10 degrees. The first series of guinea pig experiments demonstrated that larger flip angles (50-90 degrees) destroy the magnetization before it reaches the smaller airways. At moderate flip angles (20-40 degrees), airway branching down to the fourth generation was apparent. Fifth-order branchings were seen in the images of the second series. The trachea down to fourth generation pulmonary airway branching, along with some distal air spaces, was seen in the human lung images.

Animals↗

PubMed1998

Simultaneous multislice acquisition with arterial-flow tagging (SMART) using echo planar imaging (EPI).

Arterial spin tagging techniques have been used to image tissue perfusion in MR without contrast injection or ionizing radiation. Currently, spin tagging studies are performed primarily using single-slice imaging sequences, which are time consuming. This note reports a multislice echo-planar arterial spin tagging technique (Simultaneous Multislice Acquisition with aRterial-flow Tagging, or "SMART"). Multiband RF encoding (Hadamard) is used to provide simultaneous multislice acquisition capability for spin tagging techniques (such as echo planar imaging signal targeting with alternating radio frequency and flow-sensitive alternative inversion recovery). The method is illustrated with a two-slice pulse sequence that was implemented using the FAIR technique to generate two perfusion weighted images simultaneously. Compared with single-slice sequences, this two-slice sequence provided similar image quality, signal-to-noise ratio, and twice the spatial coverage compared with the single-slice technique within the same scan time.

Blood Flow Velocity↗

PubMed1998

Quantification of myocardial perfusion by MRI after coronary occlusion.

The objectives of this study were to define the relationship between the first order constant of Gd-DTPA transfer (K1) and the myocardial blood flow (MBF) at rest and to compare it with an equivalent relationship obtained for positron emission tomography (PET). In a canine model of permanent coronary occlusion (n = 4), myocardial and blood time concentration curves obtained by 13N-ammonia PET and Gd-DTPA-enhanced MRI were fitted by a one-compartment model to determine K1. A linear relationship was observed between MRI-derived K1 and MBF measured by microspheres (K1 = 0.88 x flow -0.015, R = 0.95), which compares favorably with the equivalent relationship derived from PET (K1 = 0.74 x flow +0.16, R = 0.88). The results of this preliminary study suggest that, at rest and distal to a permanently occluded coronary artery, myocardial perfusion quantification by MRI is possible and can challenge PET.

Animals↗

PubMed1998

Single-shot, variable flip-angle slice-selective excitation with four gradient-modulated adiabatic half-passage segments.

Adiabatic pulses, although useful in generating uniform spin nutation in the presence of inhomogeneous B1 fields, are limited for NMR imaging applications due to the lack of slice-selective excitation capability. Selective excitation techniques using gradient modulation have been introduced; however, present methods require either a minimum of two excitations or eight adiabatic segments. Here, a scheme is presented that allows single-shot, arbitrary flip-angle, and slice-selective excitation with only four adiabatic half-passage segments. The technique is demonstrated via computer simulation and experimental tests on a phantom. Furthermore, issues associated with the implementation of these gradient-modulated adiabatic pulses are discussed.

Computer Simulation↗

PubMed1998

Functional MRI of the rat somatosensory cortex: effects of hyperventilation.

Functional mapping of the rat somatosensory cortex was performed with T2*-sensitized MRI using a forepaw electrical stimulation model in alpha-chloralose-anesthetized rats at 7 T under both normocapnia and mild hyperventilation-induced hypocapnia. A highly localized activation area, consistent with the known somatosensory cortical region, was detected in all seven animals studied during hypocapnia and in five of the same animals during normocapnia. Quantitatively, hypocapnia was found to significantly increase both the size of the fMRI activation area (3.4 +/- 0.6 mm2 versus 1.5 +/- 0.6 mm2 in normocapnia, mean +/- standard error, n = 7, P < 0.03) and the average fMRI signal intensity increase (3.4 +/- 0.6% versus 2.7 +/- 0.4%, n = 5, P < 0.05). The increased sensitivity of fMRI to functional activation may reflect a widened arterial-venous oxygenation difference resulting from an increased effective oxygen extraction during hyperventilation. The dependence of the fMRI response on the ventilation state underscores the need to control for physiological parameters in animal fMRI studies.

Afferent Pathways↗

PubMed1997

MRI quantitative myocardial perfusion with compartmental analysis: a rest and stress study.

K1 (first-order transfer constant from arterial plasma to myocardium for Gd-DTPA) and Vd (distribution volume of Gd-DTPA in myocardium) were measured in vivo in a canine model (n = 5) using MRI-derived myocardial perfusion curves and a compartmental model. Perfusion curves were obtained after a bolus injection of Gd-DTPA (0.04 mM/kg) with an inversion-prepared fast gradient echo sequence. Myocardium and blood signal intensity were converted to a concentration of Gd-DTPA, according to a model appropriate for short (<1 s) interimage intervals characteristic of cardiac-triggered acquisitions. Before dipyridamole-induced stress, K1 and Vd, obtained from the fit of the MRI-derived perfusion curves, were 6.2 +/- 1.4 (mHz) and 17.5 +/- 4.2%, respectively. After dipyridamole infusion, a K1 increase of a factor of 2.82 +/- 0.72 was measured (P = 0.003). No change was observed in Vd (P = 0.98). These results suggest that the K1 increase after dipyridamole reflects a flow-related effect that can be useful to quantify the MRI-derived perfusion curves.

Animals↗

PubMed1997

Age and sex effects on brain morphology.

1. Brain morphology can be assessed readily in vivo using magnetic resonance imaging (MRI). 2. In this study, the effects of age and sex on whole-brain morphology were examined using an operator-controlled computer-segmentation protocol. 3. Results indicated that age was associated with gray-matter volume reduction. 4. Brain-size differences between males and females were primarily attributable to white-matter volume. 5. This study confirms the importance of controlling for age and sex in brain-morphology studies.

Adult↗

PubMed1997

Flow quantification using fast cine phase-contrast MR imaging, conventional cine phase-contrast MR imaging, and Doppler sonography: in vitro and in vivo validation.

OBJECTIVE: Our purpose was to assess the accuracy of measurements of flow velocity and volume flow rate in an in vitro phantom and in healthy human volunteers using a cardiac-gated, segmented K-space, fast cine phase-contrast (PC) MR imaging technique with view sharing (fast PC). We compared this method with conventional cine PC MR imaging and Doppler sonography. SUBJECTS AND METHODS: Pulsatile flow was generated in a flow phantom that consisted of a cylindric tube having various degrees of tapered stenosis. Phase-encoded velocity maps were obtained using cine PC and fast PC MR imaging. Doppler sonography was also performed. Measurements of aortic and pulmonary artery peak systolic and minimum diastolic velocity and volume flow rate were then compared in eight healthy volunteers using the three imaging techniques. RESULTS: We found excellent agreement between fast PC and cine PC measurements of peak systolic velocity when regions of interest were drawn to exclude vessel margins (r > .99 for phantom studies, and r = .80 for human studies). Correlation between minimum diastolic velocity measurements by MR imaging was limited by noise that resulted from high encoding velocity settings. However, such correlation improved with signal averaging. When compared with predicted values of volume flow rates, both cine PC (r > .99) and fast PC (r = .97) MR imaging were more accurate than Doppler sonography (r = .78) in vitro. Measurements of cardiac output were adversely affected by low signal to noise, especially during diastole; estimates based on systolic forward flow resulted in better agreement between the two MR imaging methods. CONCLUSION: Fast PC MR flow quantification may prove to be a useful adjunct to routine MR studies for measurements of peak flow velocity. However, estimates of volume flow rate using fast PC MR imaging are limited because of increased noise during low diastolic flow as well as edge artifacts.

Adult↗

PubMed1996

Accuracy and reproducibility of brain and tissue volumes using a magnetic resonance segmentation method.

Magnetic resonance (MR) imaging now allows the qualitative and quantitative assessment of the human brain in vivo. As MR imaging resolution has improved, precise measurement of small brain structures has become possible. Methods of measuring brain regions from MR images include both manual and semiautomated methods. Despite the development of numerous volumetric methods, there have been only limited attempts so far to evaluate the accuracy and reproducibility of these methods. In this study we used phantoms to assess the accuracy of the segmentation process. Our results with simple and complex phantoms indicate an error of 3-5% using either manual or semiautomated techniques. We subsequently used manual and semiautomated volumetric methodologies to study human brain structures in vivo in five normal subjects. Supervised segmentation is a semiautomated method that accomplishes the division of MR images into several tissue types based on differences in signal intensity. This technique requires the operator to manually identify points on the MR images that characterize each tissue type, a process known as seeding. However, the use of supervised segmentation to assess the volumes of gray and white matter is subject to pitfalls. Inhomogeneities of the radiofrequency or magnetic fields can result in misclassification of tissue points during the tissue seeding process, limiting the accuracy and reliability of the segmentation process. We used a structured seeding protocol that allowed for field inhomogeneity that produced reduced variation in measured tissue volumes. We used repeated segmentations to assess intra- and inter-rater reliability, and were able to measure small and large regions of interest with a small degree of variation. In addition, we demonstrated that measurements are reproducible with repeat MR acquisitions, with minimal interscan variability. Segmentation methods can accurately and reliably measure subtle morphometric changes, and will prove a boon to the study of neuropsychiatric disorders.

Adult↗

PubMed1996

1H MRI phase thermometry in vivo in canine brain, muscle, and tumor tissue.

The temperature sensitivity of the chemical shift of water (approximately 0.01 ppm/degree C) provides a potential method to monitor temperature changes in vivo or in vitro through the changes in phase of a gradient-echo magnetic resonance (MR) image. This relation was studied at 1.5 T in gel materials and in vivo in canine brain and muscle tissue, heated with a radio frequency (rf) annular phased array hyperthermia antenna. The rf fields associated with heating (130 MHz) and imaging (64 MHz) were decoupled using bandpass filters providing isolation in excess of 100 dB, thus allowing simultaneous imaging and rf heating without deterioration of the MR image signal-to-noise ratio. In a gel, temperature sensitivity of the MR image phase was observed to be (4.41 +/- 0.02) phase degrees/degree C for Te = 20 ms, which allowed temperature changes of 0.22 degree C to be resolved for a 50-mm3 region in less than 10 s of data acquisition. In vivo, for Te = 20 ms, the temperature sensitivity was (3.2 +/- 0.1) phase degrees/degree C for brain tissue, (3.1 +/- 0.1) phase degrees/degree C for muscle, and (3.0 +/- 0.2) phase degrees/degree C for a muscle tumor (sarcoma), allowing temperature changes of 0.6 degree C to be resolved in a 16-mm3 volume in less than 10 s of data acquisition. We conclude that, while the technique is very sensitive to magnetic field inhomogeneity, stability, and subject motion, it appears to be useful for in vivo temperature change measurement.

Animals↗

PubMed1996

Human lung air spaces: potential for MR imaging with hyperpolarized He-3.

Two healthy volunteers who had inhaled approximately 0.75 L of laser-polarized helium-3 gas underwent magnetic resonance imaging at 1.5 T with fast gradient-echo pulse sequences and small flip angles ( < 10 degrees). Thick-section (20 mm) coronal images, time-course data (30 images collected every 1.8 seconds), and thin-section (6 mm) images were acquired. Subjects were able to breathe the gas (12% polarization) without difficulty. Thick-section images were of good quality and had a signal-to-noise ratio (S/N) of 32:1 near the surface coil and 16:1 farther away. The time images showed regional differences, which indicated potential value for quantitation. High-resolution images showed greater detail and a S/N of approximately 6:1.

Adult↗

PubMed1995

Detection of pulmonary embolism: comparison of contrast-enhanced spiral CT and time-of-flight MR techniques.

We compared the conspicuity of acute pulmonary emboli with contrast-enhanced spiral computed tomography (CT) and two- and three-dimensional time-of-flight magnetic resonance (MR) techniques. Seven dogs who received experimental pulmonary emboli and one control were imaged with spiral CT and with 2-D (FMPVAS and FASTCARD) and 3-D time-of-flight MR. Blinded, independent, prospective evaluations of the CT and MR images by two MR radiologists and two chest radiologists were then compared to the location of the emboli as determined by subsequent pathologic evaluation of the excised lungs. Embolus/blood contrast-to-noise ratios (CNRs) were calculated on both MR and CT images for pulmonary emboli that could be identified. Fifty emboli ranging from 1.0 to 5.5 mm (mean, 2.7, +/- 0.14 SEM) in diameter and from 3.0 to 60 mm (mean, 28.1 +/- 1.9 SEM) in length were found in the seven embolized dogs on pathologic examination. Three of the four radiologists identified more thrombi on CT images than they did on their best MR pulse sequence (FASTCARD) and with greater confidence. The fourth radiologist identified an equal percentage of clot on CT and FASTCARD images with confidence slightly greater on FASTCARD MR than on spiral CT. Mean CNR for the best MR technique was 43.4 (+/- 3.9 SEM) and for CT was 20.7 (+/- 1.3 SEM). In general, pulmonary emboli were detected more accurately on contrast-enhanced spiral CT than on MR. This occurred although the embolus/blood CNR was higher on MR than on CT. Better pulmonary embolus conspicuity on CT images was attributed to better spatial resolution and fewer artifacts on CT than on MR. One MR radiologist performed equally well with both spiral CT and FASTCARD techniques, suggesting that experience may be a factor in performance.

Animals↗

PubMed1994

Relative accuracy and reproducibility of regional MRI brain volumes for point-counting methods.

Volumetric magnetic resonance imaging (MRI) methods for the measurement of various neuroanatomical regions are of great interest in studies of neuropsychiatric disorders. Both manual and semiautomated methods have been developed. Manual methods include tracing and point counting. Point counting methods are widely used in post-mortem and microscopy studies. Point counting has been well validated for these purposes. In this article, we report in a series of separate studies the accuracy and reproducibility of point-counting methods. Absolute accuracy was evaluated with a spherical phantom. Accuracy and time efficiency were subsequently assessed with an anatomically realistic phantom and various size grids. The point-counting method was also compared to a tracing method. Finally, the reproducibility of the point-counting method for the caudate and putamen was evaluated on four subjects in a test-retest experiment. These studies provide an estimate of the accuracy and time efficiency of point-counting methods. The test-retest reliability was also high for both caudate and putamen. Findings suggest that point counting is a reliable and efficient method for estimating volumes.

Brain↗

PubMed1993

MR microscopy of the rat lung using projection reconstruction.

Projection reconstruction has been implemented with self-refocused selection pulses on a small bore, 2.0 T MR microscope, to allow imaging of lung parenchyma. Scan synchronous ventilation and cardiac gating have been integrated with the sequence to minimize motion artifacts. A systematic survey of the pulse sequence parameters has been undertaken in conjunction with the biological gating parameters to optimize resolution and signal-to-noise (SNR). The resulting projection images with effective echo time of < 300 microseconds allow definition of lung parenchyma with an SNR improvement of approximately 15 x over a more conventional 2DFT short echo gradient sequence.

Animals↗

PubMed1993

Normal brain F-18 FDG-PET and MRI anatomy.

Image registration techniques will become increasingly important in correlative multimodality imaging. In the case of PET, a structural imaging study can be invaluable in correlating structure metabolism relationships. A registered brain atlas of PET and MRI has been developed by the authors that allows clinicians, residents, fellows, and others to refer to a structural abnormality on MRI or metabolic abnormality on PET and correlate it neuro-anatomically.

Brain↗

PubMed1993

Clot-blood contrast in fast gradient-echo magnetic resonance imaging.

RATIONALE AND OBJECTIVES: Contrast between clot and blood in magnetic resonance imaging (MRI) at 1.5T using fast gradient-echo pulse sequences (fast GRE), with 8 ms < TR < 20 mseconds was studied both in vitro and in clinical human deep venous thrombosis (DVT) to assess whether good contrast could be obtained at such short repetition times and at clinically relevant flow rates. METHODS: In vitro studies used an apparatus that contained flowing MnCl2[aq] (water adjusted with manganese chloride to have T1, T2 similar to blood) and an immobilized clot (T1, T2 similar to those in DVT) for flow velocities between 0 and 16.5 cm/sec. Seven patients with DVT were imaged with the fast GRE sequences to observe the clot-blood contrast in vivo. RESULTS: Peak contrast-to-noise ratio (CNR) was achieved using flip angles between 20 degrees and 40 degrees (increasing with flow velocity) with or without radiofrequency "spoiling," consistent with a natural spoiling effect of flow. The CNR between MnCI2[aq] and clot decreased less than 10% as TR was reduced 56% from 18 mseconds to 8 mseconds (30 degrees flip angle). In four patients with nonocclusive DVT, fast GRE imaging provided good contrast while in occlusive cases (three patients) the contrast was not as good as conventional GRE sequences with longer TR values (TR = 33 mseconds). CONCLUSION: A fast GRE sequence with TR = 8 mseconds, TE = 3 mseconds, and a flip angle = 40 degrees is a promising approach to speeding up the detection of nonocclusive clinical DVT.

Blood↗

PubMed1993

Pulmonary angiography with MR imaging: preliminary clinical experience.

PURPOSE: To study the accuracy of magnetic resonance (MR) pulmonary angiography in 20 patients in whom pulmonary embolism (PE) was clinically suspected. MATERIALS AND METHODS: Fourteen patients (group 1) were recruited for the MR pulmonary angiography study before they underwent conventional pulmonary angiography (CPA) based on clinical findings. Six patients (group 2) did not undergo CPA but were considered to have PE on the basis of findings in other studies. MR venography was performed at the time of MR pulmonary angiography in 13 patients. RESULTS: MR pulmonary angiography had a sensitivity of 92%-100% and specificity of 62% for detection of PE. Performance of MR pulmonary arteriography and MR venography in a single examination to demonstrate thrombus in both the arterial and deep venous systems was proved feasible. CONCLUSION: This report describes an early clinical implementation of new MR pulmonary angiographic techniques. Further advances to improve specificity by enhancing sensitivity to slow flow and increasing spatial resolution are necessary before routine clinical use of MR pulmonary angiography is justified.

Adult↗

PubMed1993

Accuracy of registration of PET, SPECT and MR images of a brain phantom.

Accuracy of a surface-fitting algorithm for three-dimensional image registration of single photon emission computed tomography (SPECT), positron emission tomography (PET), and magnetic resonance (MR) images was tested using a three-dimensional, water-fillable brain phantom. Multislice or volume image sets were acquired for each modality. Small fiducial markers were attached to assess accuracy of surface fitting and provide an alternate fitting technique. A maximum gradient technique was found to work well for SPECT and PET edge detection. Transformation parameters for translation, rotation and scaling were determined by surface fit to match each SPECT and PET scan with MR images. Using the markers, overall translation errors were found to be < 2 mm in each direction and rotational errors < 2 degrees in every case. Errors for specific internal regions were also determined to be < 2 mm for most regions, with only a few fits resulting in errors > 3 mm for some cortical regions. Results indicate surface fitting to be sufficiently accurate for visual comparison of registered images and for enhanced SPECT and PET region of interest (ROI) determination and image reconstruction.

Algorithms↗