Biomedical subjects

T K Foo

Publications and source records attributed to T K Foo.

61 records · Page 4Linked to original sources

PubMedDate not supplied

Fast three-dimensional time-of-flight MR angiography of the intra-cranial vasculature.

Magnetic resonance angiography is most commonly performed with the three-dimensional (3D) time-of-flight (TOF) technique. As currently practiced, this requires long image acquisition times (5-10 minutes). The authors show that the acquisition time of 3D TOF images can be reduced to less than 1 minute by using a very short TR (< 10 msec). Under normal flow conditions, the major vessels of the circle of Willis were consistently well demonstrated on these fast 3D TOF images. Signal saturation was observed in studies of patients with abnormal blood flow. In those cases, it was demonstrated that serial acquisition of fast 3D TOF data during and after contrast agent administration could be used to overcome the saturation effects. Time-resolved fast 3D TOF imaging during and after contrast agent administration can also provide qualitative assessment of flow and may depict other features that cannot be observed in TOF studies with long imaging times.

Adult↗

PubMedDate not supplied

Improved reproducibility in measuring LV volumes and mass using multicoil breath-hold cine MR imaging.

There is a generally recognized need for improvement in quality of fast cardiac MR images. Consequently, breath-hold cine MR images were obtained with multiple surface coils connected to phased array receivers, and C/N, intra-observer and inter-observer variabilities for LV volumes and mass were evaluated. Two sets of short-axis images of the LV, one with multiple surface coils and another with a body coil, were acquired in eight subjects with a fast cine MR sequence using k-space segmentation (TR/TE = 7/2.2 msec, temporal resolution = 56 msec). C/N with multicoil imaging was 32.2 +/- 7.6 (mean +/- SD), significantly higher than that with a body coil (11.0 +/- 3.3, P < .01). The mean percentage differences in intra-observer and inter-observer measurements with multicoil imaging were significantly better than those with a body coil. In conclusion, multicoil imaging provides significant gain in C/N on breath-hold cine MRI of the heart. In addition, intra-observer and inter-observer reproducibilities are improved with multicoil imaging.

Adult↗

PubMedDate not supplied

Breath-hold MR measurements of blood flow velocity in internal mammary arteries and coronary artery bypass grafts.

Breath-hold velocity-encoded cine MR (VENC-MR) imaging is a feasible method for measuring phasic blood flow velocity in small vessels that move during respiration. The purposes of the current study are to compare breathhold VENC-MR measurements of flow velocities in the internal mammary arteries (IMA) with nonbreath-hold measurements and to characterize the systolic and diastolic flow velocity curves in a cardiac cycle in native IMA and IMA grafts. Flow velocity in 30 native IMA and 8 IMA grafts were evaluated with a breath-hold VENC-MR sequence with K-space segmentation and view-sharing reconstruction (TR/TE = 16/9 msec, VENC = 100 cm/s). In 10 native IMA, nonbreath-hold VENC-MR images were acquired as well for comparison. Breath-hold VENC-MR imaging showed significantly higher systolic and diastolic peak velocities in native IMA (43.1 cm/second +/- 15.0 and 10.0 cm/second +/- 4.8), in comparison to those of nonbreath-hold VENC-MR imaging (27.6 cm/second +/- 10.2 and 7.3 cm/second +/- 3.9, P < .05). The diastolic/systolic peak velocity ratio in the IMA grafts (.88 +/- .41) was significantly higher than that in native IMA (.24 +/- .08, P < .01). Interobserver variability in the flow velocity measurement was less than 4%. Breath-hold VENC-MR imaging demonstrated higher peak flow velocity in the IMA than nonbreath-hold VENC-MR imaging. This technique is a rapid and effective method for the noninvasive assessment of blood flow velocity in IMA grafts.

Adult↗

PubMedDate not supplied

Prospective comparison of helical CT and MR imaging in clinically suspected acute pulmonary embolism.

The purpose of this study is to compare sensitivity and specificity of helical CT and MR imaging for detecting acute pulmonary embolism(PE). Patients who were suspected clinically of having PE were randomly assigned to undergo either helical contrast-enhanced CT or gradient-echo MR (if one modality was contraindicated, the patient was assigned to the other.) Patients were considered to have PE if they had: 1) high-probability V-Q scan and low clinical probability of PE; 2) pulmonary angiogram positive for PE. Patients were considered not to have PE if they had either:1)normal V-Q scan; 2) low probability V-Q scan and low clinical probability of PE; or 3) pulmonary angiogram negative for PE. The CT and MR images were read randomly and independently by five radiologists with varying levels of CT and MR experience. Twenty eight patients underwent CT and 25 MR. A total of 21 patients underwent pulmonary angiography (6 had PE, 15 did not have PE). Of the other 32 patients, 15 had high probability scan/high clinical probability and 17 had low probability scan/low clinical probability. For the five observers, the average sensitivity of CT was 75% and of MR 46%; the average specificity of CT was 89% and of MR 90%. Experience with vascular MR and enhanced CT influenced diagnostic accuracy. For the two vascular MR experts, average sensitivity and specificity of MR were 71% and 97%, and of CT 73% and 97%. In this pilot study, when CT and MR were interpreted with comparable expertise, they had similar accuracy for detecting pulmonary embolism.

Adult↗

PubMedDate not supplied

Using cardiac phase to order reconstruction (CAPTOR): a method to improve diastolic images.

A method is proposed to reconstruct multiphase images that accurately depicts the entire cardiac cycle. A segmented, gradient-recalled-echo sequence (FASTCARD) was modified to acquire data continuously. Images were reconstructed retrospectively by selecting views from each heartbeat based on cardiac phase rather than the time elapsed from the QRS complex. Cardiac phase was calculated using a model that compensates for beat-to-beat heart rate changes. Images collected using cardiac phase to order reconstruction (CAPTOR) depict the entire cardiac cycle and lack the temporal gap that is characteristic of prospectively reconstructed sequences. Time-volume curves of the left ventricle capture the contribution of atrial contraction to end-diastolic volume (EDV). Transmitral phase-contrast flow measurements show a second peak inflow (alpha wave) that is absent in the standard sequence. Because atrial contraction contributes to ventricular EDV, images using CAPTOR potentially may provide a more reliable measure of EDV, stroke volume, and ejection fraction than standard techniques.

Cardiac Volume↗

PubMedDate not supplied

Evaluation of thoracic aortic dissection using breath-holding cine MRI.

OBJECTIVE: Our goal was to determine if breath-hold cine MRI in transaxial planes can be used for the evaluation of thoracic aortic dissection instead of conventional cine MRI since rapid imaging is required in this clinical setting. MATERIALS AND METHODS: Twelve patients with thoracic aortic dissection were imaged using a 1.5 T imager. Breath-hold images were acquired with fast cine MR sequence (TR/TE = 9/2.8, 20 degrees flip angle) using segmented k-space data acquisition. Conventional non-breath-hold cine MR images (TR/TE = 22/7.5, 35 degrees flip angle, 2 averages) were taken with flow and respiratory compensation. RESULTS: Sharpness of edges of the vessels on fast cine MR images was better than that on conventional cine MR images in 34 (57%) of 60 images. Inhomogeneous blood signal in aortic lumen due to motion artifacts was found in 2 (3%) of fast cine MR images and in 15 (25%) of conventional cine MR images. The contrast-to-noise ratios of fast cine MR images were significantly better than those of conventional cine MR images (26.4 +/- 9.1 vs. 18.5 +/- 10.1; p < 0.05) when the region of interest for noise was placed to include ghosting artifacts. CONCLUSION: Breath-hold cine MRI is a rapid technique that gives high quality images of thoracic aortic dissection and can provide a diagnosis in < 10 min of imaging time.

Aged↗

PubMedDate not supplied

Measurement of coronary blood flow and flow reserve using magnetic resonance imaging.

PURPOSE: It was the purpose of this study to demonstrate the feasibility of performing coronary artery flow and coronary flow reserve (CFR) measurements in normal human volunteers using a magnetic resonance (MR) phase contrast technique. MATERIALS AND METHODS: Coronary flow rate, flow velocity, peak flow and CFR were determined at rest and during pharmacologically induced hyperemia in 10 healthy volunteers. The flow measurements were obtained during a single breath-hold by using a fast, prospectively gated, segmented k-space gradient-echo phase contrast acquisition with view sharing (FASTCARD PC) that was modified to improve sampling of the diastolic flow. Data were processed using the standard phase difference (PD) processing techniques as well as a new complex difference (CD) flow measurement method intended to improve the accuracy of flow measurements in small vessels. RESULTS: Mean hyperemic flow velocity (40 +/- 16 cm/s) and blood flow (3.9 +/- 1.5 ml/s) rates differed significantly from resting velocity (13 +/- 6.6 cm/s) and flow (1.1 +/- 0.4 ml/s) measurements (p < 0.0001). PD methods consistently measured larger flow rates at rest (24% larger, p < 0.0005) and stress (29% larger, p < 0.0001). CFR, calculated as the ratio of the mean PD flows (4.7 +/- 2.8), was higher than CFR calculated as the ratio of mean CD flows (4.2 +/- 1.8); however, the differences did not reach statistical significance (p = 0.07). Flow measurements performed in adjacent slices of the same vessel correlated well (r = 0.88). CONCLUSIONS: Coronary flow and CFR measurements using the MR techniques are feasible and are similar to those reported in the literature for healthy volunteers.

Adult↗