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Biomedical subjects

Graham A Wright

Publications and source records attributed to Graham A Wright.

15 recordsLinked to original sources
PubMed2003

Optimized spiral imaging for measurement of myocardial T2 relaxation.

Microcirculation oxygen levels and blood volumes should be reflected in measurements of myocardial T(2) relaxation. This work describes the optimization of a spiral imaging strategy for robust myocardial T(2) measurement to minimize the standard deviation of T(2) measurement (sigmaT(2)). Theoretical and experimental studies of blurring at muscle/blood interfaces enabled the derivation of parameter sets which reduce sigma T(2) to the level of 5%. T(2) relaxation mapping within healthy volunteers provided estimation of residual sigmaT(2) within the optimized technique. The standard deviation in T(2) measurement across regions of interest (ROIs) in different locations is about 9%. The standard deviation in T(2) measurement in an ROI across different time points is about 5%.

Humans↗
PubMed2003

Factors affecting the correlation coefficient template matching algorithm with application to real-time 2-D coronary artery MR imaging.

This paper characterizes factors affecting the accuracy of the correlation coefficient (CC) template matching algorithm, as applied to motion tracking from two-dimensional real-time coronary artery magnetic resonance images. The performance of this algorithm is analyzed in the presence of both random and systematic error. In the presence of random error, it is shown that a necessary and sufficient condition for accurate motion tracking is a large CC difference-to-noise ratio (CCDNR). The CCDNR itself is in turn affected by five factors: image and template size, image and template structure, and the magnitude of the noise. Techniques are introduced for manipulating some of these factors in order to increase the CCDNR for greater motion tracking accuracy. In the presence of superimposed systematic error it is shown that, while large CCDNR is necessary, it alone is not sufficient to ensure accurate motion tracking. Techniques are developed for improving motion tracking accuracy that minimize the effects of systematic error, while maintaining an adequate CCDNR level. The ability of these techniques to improve motion tracking accuracy is demonstrated both in phantoms and in coronary artery images.

Algorithms↗
PubMed2003

Intracranial venous system: gadolinium-enhanced three-dimensional MR venography with auto-triggered elliptic centric-ordered sequence--initial experience.

PURPOSE: To evaluate gadolinium-enhanced three-dimensional auto-triggered elliptic centric-ordered (ATECO) magnetic resonance (MR) venography for imaging of the intracranial venous system. MATERIALS AND METHODS: ATECO MR venography was performed in 23 patients, eight of whom also underwent two-dimensional time-of-flight (TOF) MR venography for imaging of the intracranial venous system. Seventeen predefined venous structures were evaluated on all venograms by two neuroradiologists. Visualization of venous structures was defined as completely visible (including clearly pathologic), partially visible, or not visible. Readers were also asked to compare the visibility of these predefined structures on ATECO and TOF MR venograms, where available. RESULTS: Of the 23 patients, six had dural venous sinus disease. Of the remaining 17 healthy patients, five underwent both ATECO and TOF MR venography and 12 underwent ATECO MR venography alone. On ATECO MR venograms obtained in the healthy patients, visibility of the 17 predefined venous structures was complete in 92% (531 of 578) of evaluations. For the five normal TOF MR venograms, the rate of complete visibility of the same venous structures was 61% (104 of 170). The rate of complete visibility of the large dural venous sinuses was 99% for ATECO MR venograms and 75% for TOF MR venograms. CONCLUSION: ATECO MR venography provides high-quality images of the intracranial venous anatomy and was superior to TOF MR venography for consistent complete visibility of venous structures.

Adult↗
PubMed2003

Left ventricular ejection fraction and volumes from gated blood-pool SPECT: comparison with planar gated blood-pool imaging and assessment of repeatability in patients with heart failure.

UNLABELLED: Gated blood-pool SPECT (GBPS) has several potential advantages over planar radionuclide ventriculography (PRNV), including the possibility of greater repeatability of left ventricular ejection fraction (LVEF) and the noninvasive calculation of left ventricular end-systolic volume and left ventricular end-diastolic volume (LVEDV). The aim of this study was to assess the repeatability of LVEF and LVEDV from GBPS and to compare LVEF with those from PRNV. METHODS: Fifty patients underwent PRNV and GBPS, 23 of whom also had repeated studies in the same session. GPBS studies were processed using the Cedars Sinai Quantitative Blood-Pool SPECT (QBS) software that automatically calculates LVEF and LVEDV. Automatic processing with QBS was successful in 70% of the GBPS studies, with the remaining studies processed using the manual option in QBS. All PRNV studies were processed using a manual processing technique. RESULTS: Comparison of LVEF from PRNV and GBPS yielded a correlation coefficient of 0.80. Bland-Altman analysis demonstrated a mean difference of 0.74% +/- 7.62% (mean +/- SD) between LVEF from the 2 techniques. The 95% limits of agreement are therefore -14.50% to +15.98%. The correlation between repeated measurements was 0.87 for GBPS and 0.95 for PRNV. Bland-Altman analysis revealed poorer repeatability for GBPS (95% limits of agreement, -9.63% to +14.97% vs. -4.66% to +5.92%; P = 0.003). The mean LVEDV was 198 +/- 94 mL, with a mean difference of 9 +/- 47 mL between repeated measurements. The 95% limits of agreement are therefore -85 to +103 mL. CONCLUSION: GBPS provides a less repeatable measurement of LVEF than PRNV. Repeatability of LVEDV measurements from GBPS is poor.

Cardiac Output↗
PubMed2002

Vasodilator response assessment in porcine myocardium with magnetic resonance relaxometry.

BACKGROUND: This research describes an early preclinical study of the biophysical mechanisms governing changes in myocardial T2 during vasodilation in normal myocardium. METHODS AND RESULTS: Theoretical modeling and experimental studies in an instrumented pig model (n=7) provided measures of changes in myocardial T2, relative blood volume (BV), and microcirculation oxygen levels (%O2) during intracoronary adenosine infusion. Intracoronary adenosine increases perfusion without increasing blood volume or cardiac metabolic rate; thus, T2 elevations should reflect elevated microcirculation oxygen levels. Robust strategies were used for magnetic resonance imaging (MRI) data collection. Measures of myocardial and vascular T1 before and after Clariscan (Amersham Health) injection provided blood volume assessment. Changes in microcirculation oxygen levels were estimated via direct blood sampling from the left anterior descending (LAD) coronary vein. Perfusion changes were monitored using a Doppler flow wire within the left main coronary artery. Myocardial T2 elevations (DeltaT2=17+/-8%) within the LAD arterial perfusion bed were related to elevations in perfusion (coronary velocity reserve=3.2+/-0.4) and coronary venous %O2 [Delta(LAD CV%O2) =56+/-11%], whereas blood volume (DeltaBV=0 +/-2%) and cardiac metabolic rate [Delta(heart rate x blood pressure) = -4+/-11%] remained constant. CONCLUSIONS: Myocardial T2 elevation during intracoronary adenosine infusion was significant and repeatable, caused by increases in microcirculation oxygen levels. Changes in microcirculation oxygen levels of approximately 40%O2 should be detectable by this technique. This sensitivity should suffice for differentiating normal from abnormal myocardium via measurement of myocardial perfusion reserve.

Adenosine↗
PubMed2002

Quantitative gated SPECT: the effect of reconstruction filter on calculated left ventricular ejection fractions and volumes.

Gated SPECT (GSPECT) offers the possibility of obtaining additional functional information from perfusion studies, including calculation of left ventricular ejection fraction (LVEF). The calculation of LVEF relies upon the identification of the endocardial surface, which will be affected by the spatial resolution and statistical noise in the reconstructed images. The aim of this study was to compare LVEFs and ventricular volumes calculated from GSPECT using six reconstruction filters. GSPECT and radionuclide ventriculography (RNVG) were performed on 40 patients; filtered back projection was used to reconstruct the datasets with each filter. LVEFs and volumes were calculated using the Cedars-Sinai QGS package. The correlation coefficient between RNVG and GSPECT ranged from 0.81 to 0.86 with higher correlations for smoother filters. The narrowest prediction interval was 111 +/- 2%. There was a trend towards higher LVEF values with smoother filters, the ramp filter yielding LVEFs 2.55 +/- 3.10% (p < 0.001) lower than the Hann filter. There was an overall fall in ventricular volumes with smoother filters with a mean difference of 13.98 +/- 10.15 ml (p < 0.001) in EDV between the Butterworth-0.5 and Butterworth-0.3 filters. In conclusion, smoother reconstruction filters lead to lower volumes and higher ejection fractions with the QGS algorithm, with the Butterworth-0.4 filter giving the highest correlation with LVEFs from RNVG. Even if the optimal filter is chosen the uncertainty in the measured ejection fractions is still too great to be clinically acceptable.

Algorithms↗
PubMed2002

MRI-based blood oxygen saturation measurements in infants and children with congenital heart disease.

BACKGROUND: Vessel oxygen saturation can be determined with MR oximetry using an in vivo measurement of signal decay (T2) and the calibration curve relating T2 to blood oxygen saturation (%HbO(2)), where: 1/T2=1/T2O+K (1-%HbO(2)/100)(2) and K is a constant parameter which correlates with measured fibrinogen levels. The ability to noninvasively measure %HbO(2) in cardiac chambers and vessels has enormous potential in children with congenital heart disease (CHD). OBJECTIVE: The purpose of the study was to prospectively characterize the T2-%HbO(2) relationship in infants where T2-%HbO(2) is the relationship between T2 and %HbO(2) (blood oxygen saturation) expressed by the equation given above, and to determine whether adult values for K and T2O (where T2O is the T2 of fully oxygenated blood) can be extrapolated to pediatric patients with CHD. A second objective was to apply this method to calculate the %HbO(2) in vivo using MR imaging in infants with CHD. MATERIALS AND METHODS: Fifteen patients with CHD undergoing cardiac catheterization (9 male; 6 female), median age 8 months, were recruited for the calibration study. T2O and K were measured directly from blood samples, compared with the values estimated from adult population statistics, and plotted against hematocrit and fibrinogen, respectively. In four studies of infants with CHD, T2 measurements were converted to %HbO(2) using the calibration curve. RESULTS: The T2-%HbO(2) relationship in infants correlated with the adult calibration statistics (1/T2O vs. hematocrit, r=0.77; K vs. fibrinogen, r=0.61). Our initial in vivo studies demonstrated that the MR oximetry reflected the expected oxygen saturations. CONCLUSION: Adult values for T2-%HbO(2) calibration can be used to measure blood oxygen saturation in vivo in children with CHD.

Blood Gas Analysis↗
PubMed2002

Analysis of subtraction methods in three-dimensional contrast-enhanced peripheral MR angiography.

PURPOSE: To compare the effectiveness of three image subtraction algorithms designed to improve arterial conspicuity in first-pass contrast-enhanced magnetic resonance (MR) angiography. MATERIALS AND METHODS: Three subtraction methods were analyzed through computer simulations, phantom studies, and clinical studies. These algorithms were: complex subtraction, magnitude subtraction, and maximum intensity projection subtraction. RESULTS: In high resolution three-dimensional imaging, maximum intensity projection subtraction generally yields the best background suppression. Complex subtraction is effective in reducing partial volume effects in low resolution imaging. Magnitude subtraction works better in high resolution, low contrast concentration protocols. CONCLUSION: Choosing the appropriate subtraction method according to the protocol is helpful in optimizing image quality.

Algorithms↗
PubMed2002

On the parameters affecting the sensitivity of MR measures of pressure with microbubbles.

Recently, it has been suggested that gas encapsulated distensible microbubbles may serve as pressure probes in the MR field through the relationship between bubble size and 1/T(2) or 1/T(*)(2). Currently, in vivo application of this technique is hindered by the ability of T(2) or T(*)(2) to detect pressure changes that are clinically relevant. This work identifies and characterizes, through numerical simulations, the set of parameters which optimize the ability of this technique to detect small pressure changes. Results show that when the bubbles do not interact magnetically, the T(2)- and T(*)(2)-based measurements of pressure are strongly influenced by the bubble size at atmospheric pressure, static magnetic field strength, magnitude of the susceptibility difference between the encapsulated gas and plasma, bubble volume fraction, and the refocusing interval. In particular, to detect clinically relevant pressure changes, microbubbles need to be approximately 2-3 microm in radius, distributed at a volume fraction of 0.15%, and have a volumetric magnetic susceptibility difference of at least 34 ppm.

Blood Pressure↗
PubMed2002

Variable-density adaptive imaging for high-resolution coronary artery MRI.

Variable-density (VD) spiral k-space acquisitions are used to acquire high-resolution (0.78 mm), motion-compensated images of the coronary arteries. Unlike conventional methods, information for motion compensation is obtained directly from the coronary anatomy itself. Specifically, periods of minimal coronary distortion are identified by applying the correlation coefficient template matching algorithm to real-time images generated from the inner, high-density portions of the VD spirals. Combining the data associated with these images together, high-resolution, motion-compensated coronary images are generated. Because coronary motion is visualized directly, the need for cardiac-triggering, breath-holding, and navigator echoes is eliminated. The motion compensation capability of the technique is determined by the inner-spiral spatial and temporal resolution. Results indicate that the best performance is achieved using inner-spiral images with high spatial resolution (1.6-2.9 mm), even though temporal resolution (four to six independent frames per second) suffers as a result. Image quality within the template region in healthy volunteers was found to be comparable to that achieved with cardiac-triggered breath-hold scans, although extended acquisition times of around 5 min were needed to overcome reduced SNR efficiency.

Algorithms↗
PubMed2002

High-order multiband encoding in the heart.

Spatial encoding with multiband selective excitation (e.g., Hadamard encoding) has been restricted to a small number of slices because the RF pulse becomes unacceptably long when more than about eight slices are encoded. In this work, techniques to shorten multiband RF pulses, and thus allow larger numbers of slices, are investigated. A method for applying the techniques while retaining the capability of adaptive slice thickness is outlined. A tradeoff between slice thickness and pulse duration is shown. Simulations and experiments with the shortened pulses confirmed that motion-induced excitation profile blurring and phase accrual were reduced. The connection between gradient hardware limitations, slice thickness, and flow sensitivity is shown. Excitation profiles for encoding 32 contiguous slices of 1-mm thickness were measured experimentally, and the artifact resulting from errors in timing of RF pulse relative to gradient was investigated. A multiband technique for imaging 32 contiguous 2-mm slices, with adaptive slice thickness, was developed and demonstrated for coronary artery imaging in healthy subjects. With the ability to image high numbers of contiguous slices, using relatively short (1-2 ms) RF pulses, multiband encoding has been advanced further toward practical application.

Arteries↗
PubMed2002

A widely available method for the assessment of aerosol delivery in cystic fibrosis.

Whilst nebulisers are commonly used in the treatment of cystic fibrosis (CF), nebulised aerosol lung deposition in individual patients is not routinely assessed in clinical practice. The present study was designed to evaluate whether a comparative measurement of aerosol lung deposition from nebulisers using a widely available scintigraphic method could be employed to assist the selection of the best system for individual patients. Lung deposition of the radiolabelled aerosol from the Pari LC Plus (Pari Medical Ltd) nebuliser and the HaloLite Adaptive Aerosol Delivery (AAD) system (Profile Therapeutics Ltd) was measured using planar scintigraphy in 10 healthy volunteers and 6 CF patients. The HaloLite AAD delivered on average 2.1 times (P=0.003) as much aerosol to the lungs compared with Pari LC Plus. Only two subjects had higher lung deposition from Pari LC Plus than HaloLite AAD system. There was marked inter-individual variation in the deposition pattern in CF patients. The aerosol deposition from HaloLite AAD had higher central distribution than that obtained with the Pari LC Plus. The overall intersubject variability of the delivered dose was 56% with Pari LC Plus and 24% with HaloLite AAD (P<0.05). The measurement of aerosol deposition from nebulisers can be performed using a simple and widely available methodology, and may improve nebuliser selection in CF patients.

Adult↗
PubMed2002

Spinal dural arteriovenous fistula localization with a technique of first-pass gadolinium-enhanced MR angiography: initial experience.

Nine patients with initial magnetic resonance (MR) imaging and clinical findings suggestive of spinal dural arteriovenous fistula (AVF) underwent spinal MR angiography with an autotriggered elliptic centric ordered three-dimensional gadolinium-enhanced technique (hereafter, this MR angiographic technique) before conventional intraarterial angiography. In all nine patients, findings with this MR angiographic technique correctly and precisely localized the spinal dural AVF. Observer error resulted in one case in which the site of the fistula was not prospectively reported but was easily identified retrospectively on the spinal MR angiogram.

Adult↗
PubMed2002

Measurement of gastric emptying in gastroesophageal reflux-related chronic cough.

STUDY OBJECTIVES: Gastroesophageal reflux (GER) is one of the most common causes of chronic cough, but the mechanisms of GER-related cough are not well-understood. We tested the hypothesis that gastric emptying is delayed in patients with GER-related chronic cough. DESIGN: We studied 12 patients (7 women; mean age, 53 years; age range, 37 to 68 years) with GER-related chronic cough and a control group of 27 asymptomatic healthy volunteers (16 women; mean age, 37 years; age range, 18 to 62 years). Gastric emptying scintigraphy was performed, and the time at which 50% of the radiolabeled material had left the stomach (T(1/2)) was calculated. RESULTS: There was no statistically significant difference in T(1/2) values between healthy volunteers and subjects with GER-related cough (99 +/- 26 min vs 86 +/- 20 min, respectively; difference between the means, 13 min [95% confidence interval, -4 to 30 min]; p = 0.13). CONCLUSIONS: Gastric emptying was not delayed in patients with GER-related chronic cough. The measurement of gastric emptying did not therefore provide further insights into the mechanisms of GER-related cough or clinically relevant information that would assist in patient management.

Adolescent↗