Biomedical subjects

Thomas K Foo

Publications and source records attributed to Thomas K Foo.

8 recordsLinked to original sources

PubMed2006

Breath-hold 3D steady-state free precession coronary MRA compared with conventional X-ray coronary angiography.

PURPOSE: To evaluate the use of breath-hold three-dimensional (3D) steady-state free precession (SSFP) coronary magnetic resonance angiography (MRA) in patients with coronary artery disease (CAD) in comparison with conventional coronary x-ray angiography (XRA). MATERIALS AND METHODS: Twenty-eight patients with suspected CAD were examined with the use of a breath-hold 3D-SSFP-MRA sequence and conventional XRA. To assess the accuracy of MRA, two clinicians who were blinded to patient information independently reviewed the MRA and XRA data, which were presented in a randomized order. To identify discrepancies between MRA and XRA, and assess features of coronary lesions on MRA, two additional clinicians examined MRA and XRA data that were presented side by side, divided into proximal, mid, and distal segments, and compared them segment by segment. RESULTS: The sensitivity and specificity for diagnosing significant coronary stenoses (> 50% diameter narrowing) were 64% and 94%, respectively. At sites of coronary lesions identified on XRA, bright signals and enlarged vessel profiles, in addition to the characteristic narrow lumen, were frequently observed on MRA. CONCLUSION: Breath-hold SSFP coronary MRA has good specificity but inconclusive sensitivity in diagnosing significant coronary stenoses, and provides important image features for depicting coronary lesions.

Adult↗

PubMed2005

The apparent inversion time for optimal delayed enhancement magnetic resonance imaging differs between the right and left ventricles.

BACKGROUND: Delayed post-contrast magnetic resonance (MR) imaging involves suppression of signal from myocardium using inversion times (TI) between 150-225 ms, when the myocardium appears dark and fibrotic scar appears bright. We noticed that at a TI optimized for signal suppression of the left ventricle (LV), the right ventricle (RV) appeared brighter. PURPOSE: The purpose of this study was to evaluate the TI for signal suppression in RV compared to LV, and to try and identify the cause of this observation. Methods. We studied 31 patients (ages ranged from 17-79 years, 11 females) who had an MR scan on a 1.5 T GE scanner. Delayed post-contrast short-axis images were obtained 20 minutes after injection of 0.2 mmol/kg of intravenous gadolinium chelate. TI optimization was performed by acquiring a range of TI times within a single breath hold, in increments of 25 msec. The TI time that resulted in lowest signal for the RV arid LV was recorded. RESULTS: With the imaging sequence employed, the TI leading to LV signal suppression ranged from 150-225 ms. At the TI that resulted in LV signal suppression, the corrected signal from the RV was significantly higher as compared to the LV (29 +/- 13 au vs. 15 +/- 8 au, p < 0.001). The findings were similar using only the body coil. The TI required to suppress the RV was usually < or =150 msec. The observation persisted before and after gadolinium infusion. CONCLUSION: The TI for myocardial signal suppression appears to be different between LV and RV. Potential mechanisms include partial volume averaging with fat or blood pool (related to increased trabeculation) in the RV. Alternatively, increased blood pool signal (within Thebesian veins or arterioluminal communications) in RV compared to LV leads to altered TI times due to similar partial volume effects.

Adolescent↗

PubMed2005

A novel method for assessing arterial endothelial function using phase contrast magnetic resonance imaging: vasoconstriction during reduced shear.

We investigated whether endothelial-dependent arterial constriction during reduced shear can be measured using phase contrast magnetic resonance imaging (PCMRI). A cross-section of the femoral artery was acquired during a 5-minute distal occlusion in 33 subjects. Systolic shear rate and radius were measured from the velocity profile via a best-fit parabola. Systolic shear rate decreased immediately after cuff inflation (404 +/- 78 to 233 +/- 75 sec(-1) p < .0001). Radius decreased at 2 min into inflation (3.52 +/- .41 to 3.43 +/- .42 mm, p < .0001). In conclusion, arterial constriction during reduced flow can be measured using PCMRI. This new method may add important information toward a comprehensive evaluation of endothelial function.

Adult↗

PubMed2004

Why is flow-mediated dilation dependent on arterial size? Assessment of the shear stimulus using phase-contrast magnetic resonance imaging.

Flow-mediated dilation (FMD) is strongly dependent on arterial size, but the reasons for this phenomenon are poorly understood. We have previously shown that FMD is greater in small brachial arteries because the shear stress stimulus is greater in small brachial arteries. However, it is unclear why the shear stimulus is greater in small arteries. Furthermore, this relationship has not been investigated in other, differently sized arterial beds. Postischemic systolic shear stress and resulting FMD were evaluated in the brachial and femoral arteries of 24 young, healthy adults using phase-contrast magnetic resonance imaging. Arterial shear and radius were calculated from the velocity profile via a best-fit parabola before and after occlusion. Summing the velocity pixels provided hyperemic systolic flow. FMD was proportional to hyperemic shear in the brachial and femoral arteries (P < 0.0001, r = 0.60). Hyperemic systolic flow was proportional to radius2 (P < 0.0001, r = 0.93). Applying this relationship to the Poiseuille equation (shear is proportional to flow/radius3) shows that hyperemic shear is proportional to radius2/radius3 and, therefore, explains why hyperemic shear is proportional to 1/radius. We conclude that FMD is proportional to hyperemic systolic shear stress in both the brachial and the femoral arteries. The hyperemic shear stimulus for FMD is greater in small arteries due to the dependence of postischemic systolic flow on radius squared. Therefore, greater FMD in small arteries does not necessarily reflect better conduit artery endothelial function. Evaluating the shear stimulus using phase-contrast magnetic resonance imaging enhances the understanding of mechanisms underlying FMD.

Adult↗

PubMed2004

An accurate, robust, and computationally efficient navigator algorithm for measuring diaphragm positions.

PURPOSE: The purpose of this study is to develop an improved algorithm for measuring the position of the diaphragm using navigator echoes. METHODS: This algorithm was applied to navigator echo data acquired from 14 cardiac patients. For each patient, 160 navigator echo profiles were acquired across the right hemi-diaphragm along the superior-inferior direction. RESULTS: The accuracy of the proposed edge-detection algorithm was evaluated together with that of the least-squares and linear phase-shift algorithms. The estimated measurement error of the proposed algorithm was approximately two times smaller than that of the least-squares algorithm (Magn Reson Med, 1996:36: 117-123), and was approximately four times smaller than that of the linear phase-shift algorithm (Magn Reson Med, 1999;42:548-553). The computational efficiency of this algorithm was 7.5 times higher than that of the least-squares algorithm and was comparable with that of the linear phase-shift algorithm. CONCLUSION: The presented algorithm is accurate, robust, and computationally efficient in the measurement of the diaphragm position.

Adult↗

PubMed2003

MRI-guided coronary catheterization and PTCA: A feasibility study on a dog model.

The aim of this work was to demonstrate the feasibility of MRI-guided coronary artery catheterization and intervention in a dog model. Experiments were performed on 10 healthy dogs. A 9F introducer sheath was placed through a right carotid artery cutdown. A prototype 0.014-inch coronary MRI guidewire, a prototype 7 French MRI-guiding catheter, and two flexible surface coils were connected to a GE 1.5 T CV/i scanner for simultaneous visualization of the guidewire, guiding catheter, and chest anatomy. Images were displayed in real time on an in-room monitor. A nongated, single-slice fast gradient-echo sequence was used to obtain real-time images of the catheters and background anatomy during the intervention. Fifteen selective catheterizations were attempted in the coronary arteries, and all were successful. Selective injection of diluted gadolinium into the MRI-guiding catheter provided dynamic 2D projection coronary angiography in all cases, confirming successful catheterization. Percutaneous transluminal coronary angioplasty (PTCA) was attempted after two catheterizations, and all attempts were successful. Inflation of the balloon angioplasty catheter was performed successfully in the left anterior and circumflex arteries. Our results indicate that coronary artery catheterization and intracoronary balloon angioplasty are feasible with MRI guidance only. MRI guidance may be used as an alternative to X-ray guidance in coronary artery interventions in the future.

Angioplasty, Balloon, Coronary↗

PubMed2002

Cardiac MRI: recent progress and continued challenges.

Cardiac MRI continues to develop and advance. MRI accurately depicts cardiac structure, function, perfusion, and myocardial viability with an overall capacity unmatched by any other single imaging modality. MRI is an accepted and widely utilized tool for cardiovascular research. Its clinical use has been limited, but is increasing because of its proven clinical efficacy, the proliferation of cardiac-capable MRI systems, and the development of improved pulse sequences. The following article reviews the landmark developments in this field, with an emphasis on recent progress in the evaluation of ischemic or acquired heart disease.

Heart↗

PubMedDate not supplied

Delayed enhancement MR imaging: utility in myocardial assessment.

Use of magnetic resonance (MR) imaging for diagnosis of cardiac diseases and treatment monitoring is expanding. Delayed myocardial enhancement MR imaging is performed after administration of paramagnetic contrast agents and is used for a growing number of clinical applications. This technique was developed primarily for characterization of myocardial scarring after myocardial infarction. On delayed enhancement MR images, scarring or fibrosis appears as an area of high signal intensity that is typically subendocardial or transmural in a coronary artery distribution. However, delayed myocardial enhancement is not specific for myocardial infarction and can occur in a variety of other disorders, such as inflammatory or infectious diseases of the myocardium, cardiomyopathy, cardiac neoplasms, and congenital or genetic cardiac conditions, as well as after cardiac interventions. In nonischemic myocardial disease, the delayed enhancement usually does not occur in a coronary artery distribution and is often midwall rather than subendocardial or transmural. Therefore, the patient's clinical history is critical in the evaluation of delayed myocardial enhancement MR images.

Cardiomyopathies↗