Biomedical subjects

Yueh Z Lee

Publications and source records attributed to Yueh Z Lee.

6 recordsLinked to original sources

PubMed2005

Hemodynamic and permeability changes in posterior reversible encephalopathy syndrome measured by dynamic susceptibility perfusion-weighted MR imaging.

BACKGROUND AND PURPOSE: The physiological mechanism that gives rise to posterior reversible encephalopathy syndrome (PRES) is currently unknown. We sought to better understand the mechanism of the edema formation in PRES and specifically hypothesized that this edema is caused by increased vascular permeability. METHODS: Eight consecutive patients with PRES who had been studied by using perfusion MR imaging were retrospectively identified. Perfusion images were obtained using a gradient-echo echo planar sequence with contrast enhancement. Measures of apparent diffusion coefficient (ADC), cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time (MTT) and vascular permeability (K2) were calculated in the affected posterior brain areas and normalized to values obtained in unaffected anterior brain. These values were compared with those found in healthy subjects. RESULTS: Regions of interest within the posterior affected brain indicated a significant increase in ADC values in all but one patient, a significant decrease in CBV and CBF values in all patients with PRES (average 30% of control), variable changes in MTT, and no measurable change in K2. CONCLUSION: The decrease in both CBV and CBF in PRES supports the theory of autoregulatory vasoconstriction; changes in K2 were not detected.

Adolescent↗

PubMed2004

Heart to Heart: a computerized decision aid for assessment of coronary heart disease risk and the impact of risk-reduction interventions for primary prevention.

Heart to Heart is a computer-based decision aid for patients and providers that provides personalized, evidence-based information about coronary heart disease (CHD) risk and potential risk-reducing interventions. To develop Heart to Heart, the authors used Framing-ham risk equations and systematic reviews of risk-reducing interventions. The Web version was programmed using PHP: Hypertext Processor, a Web-based programming language, and has separate interfaces for providers and patients. The authors subsequently developed a modified version for personal digital assistants. Heart to Heart uses information about a patient's CHD risk factors (age, gender, total and high-density lipoprotein cholesterol levels, diabetes, smoking, systolic blood pressure, and left ventricular hypertrophy) to calculate risk of total CHD events over 5 or 10 years. Patients and providers can then examine the effect of introducing one or more risk-reducing interventions (aspirin, lipid-lowering drug therapy, antihypertensive medication, or smoking cessation) on the patient's CHD risk. Future research will be directed to determining whether Heart to Heart can improve utilization of effective CHD risk-reducing interventions.

Adult↗

PubMed2003

Rapid perfusion abnormality estimation in acute stroke with temporal correlation analysis.

BACKGROUND AND PURPOSE: Determination of the presence or absence of ischemic penumbra through diffusion-perfusion mismatch requires rapid delineation of both abnormalities. Although singular value decomposition-based perfusion parameter estimation has offered valuable insights into the underlying cerebral hemodynamics, the associated postprocessing procedures have limited its widespread use. We explored the utility of a simple technique to define perfusion abnormality in acute stroke patients. METHODS: Twenty acute stroke patients were studied. The MR dynamic contrast approach was used to obtain cerebral blood flow, cerebral blood volume, and mean transit time (MTT). Temporal correlation was used to correlate 4 reference functions-an arterial input function (AIF), a normal tissue function, a lesion function, and a venous output function-with dynamic contrast MR images, and correlation coefficients (CCs) were calculated pixel by pixel. In addition, chronic-state T2-weighted images were coregistered onto the images acquired acutely for assessing the sensitivity and specificity of CC-defined lesion. RESULTS: Statistically significant differences in cerebral blood flow and MTT were found between CC-defined normal and abnormal tissues with all 4 reference functions used. When the final infarct volume was used as the gold standard, a similar sensitivity between MTT (78%) and AIF (76%) CC-defined lesion was obtained, whereas the specificity was higher for AIF (61%) than that obtained with MTT-defined lesion (52%). CONCLUSIONS: We explored CC maps as a simpler alternative of estimating perfusion abnormality, and results demonstrated the potential clinical utility of a correlation-based technique for estimating brain perfusion status.

Acute Disease↗

PubMed2003

Statistical accuracy of a moving equivalent dipole method to identify sites of origin of cardiac electrical activation.

While radio frequency (RF) catheter ablation (RCA) procedures for treating ventricular arrhythmias have evolved significantly over the past several years, the use of RCA has been limited to treating slow ventricular tachycardias (VTs). In this paper, we present preliminary results from computer and animal studies to evaluate the accuracy of an algorithm that uses the single equivalent moving dipole (SEMD) model in an infinite homogeneous volume conductor to guide the RF catheter to the site of origin of the arrhythmia. Our method involves measuring body surface electrocardiographic (ECG) signals generated by arrhythmic activity and by bipolar current pulses emanating from a catheter tip, and representing each of them by a SEMD model source at each instant of the cardiac cycle, thus enabling rapid repositioning of the catheter tip requiring only a few cycles of the arrhythmia. We found that the SEMD model accurately reproduced body surface ECG signals with a correlation coefficients > 0.95. We used a variety of methods to estimate the uncertainty of the SEMD parameters due to measurement noise and found that at the time when the arrhythmia is mostly localized during the cardiac cycle, the estimates of the uncertainty of the spatial SEMD parameters (from ECG signals) are between 1 and 3 mm. We used pacing data from spatially separated epicardial sites in a swine model as surrogates for focal ventricular arrhythmic sources and found that the spatial SEMD estimates of the two pacing sites agreed with both their physical separation and orientation with respect to each other. In conclusion, our algorithm to estimate the SEMD parameters from body surface ECG can potentially be a useful method for rapidly positioning the catheter tip to the arrhythmic focus during an RCA procedure.

Algorithms↗

PubMed2003

Temporal relationship between apparent diffusion coefficient and absolute measurements of cerebral blood flow in acute stroke patients.

BACKGROUND AND PURPOSE: Diffusion-weighted imaging (DWI) has been established as a marker of acute ischemic brain injury. We sought to determine the relationship between cerebral blood flow (CBF) and apparent diffusion coefficient (ADC) and to explore whether the elapsed time between MRI acquisition and symptom onset alters this relation in acute stroke patients. METHODS: Sixteen acute stroke patients were studied with DWI and perfusion-weighted imaging, from which ADC and CBF were calculated. ADC values were normalized (nADC) to the contralateral, nonischemic hemisphere and then correlated pixel by pixel with CBF within a region of interest defined by abnormal transit time. To explore potential temporal effects on the relationship between CBF and nADC, patients were divided into 2 groups based on the duration between symptom onset and MR imaging for data analysis: group A, 2 to 4 hours (n=8), and group B, 4.5 to 6.5 hours (n=8). RESULTS: nADC was plotted against CBF for each pixel in all 16 subjects, and a composite relationship was derived. After a gradual decline, an abrupt drop in nADC occurred below a CBF threshold value of 21 mL x min(-1) x 100 g(-1). When subjects were divided into early and late imaging groups, the group of patients imaged earlier (group A) had a lower threshold (15 mL x min(-1) x 100 g(-1)) than the group imaged later (group B, 24 mL x min(-1) x 100 g(-1)). CONCLUSIONS: Our results demonstrate that a relationship between nADC and CBF exists in the ischemic brain and that ADC values alone may provide useful information in predicting perfusion status. However, this relationship may change with elapsing time between stroke onset and imaging.

Adult↗

PubMed2003

Apparent diffusion coefficient measurements in the hippocampi in patients with temporal lobe seizures.

BACKGROUND AND PURPOSE: Loss of neurons results in a relative increase in extracellular space that may lead to altered apparent diffusion coefficient (ADC) values in the hippocampi of patients with seizures. Our purpose was to determine if ADC values along the long axis of hippocampi are useful in evaluating patients with partial complex seizures. METHODS: Hippocampi of 23 patients with partial complex seizures and 25 healthy volunteers were evaluated with MR imaging and ADC maps. MR images were evaluated for loss of volume and/or high signal intensity on T2-weighted images and compared with ADC maps. ADCs were compared between patients and controls, as were ADCs along the length of each hippocampus. Mean and SDs were obtained for each measurement, and level of significance was determined (P <.05). The relationship between clinical lateralization and MR imaging and ADCs was studied. RESULTS: No significant variations were found in the ADCs in controls (side to side and along hippocampi). In patients, abnormalities were seen with MR imaging alone in 16, with ADC in 14, and with both in 21. Of 23 hippocampi with an abnormal MR appearance, 14 had abnormal ADCs. Nine hippocampi with a normal MR appearance had abnormal ADCs. Normal MR appearance and ADCs were seen in 13 hippocampi. Most abnormal ADCs were seen in the anterior aspect of the hippocampi. All differences were statistically significant. Of 19 patients who underwent clinical testing, unequivocal lateralization was established in 10. Concordance between clinical tests and MR imaging, ADC, and MR imaging plus ADC was found in five, five, and seven patients, respectively. CONCLUSION: Visual assessment was better than ADCs alone for detection of abnormal hippocampi. MR imaging plus ADCs was better than either technique alone. ADCs may be abnormal when MR images are unremarkable. Concordance with clinical lateralization was better when MR imaging and ADC were jointly evaluated than when either technique was evaluated separately.

Adolescent↗