Biomedical subjects

Jennifer Keegan

Publications and source records attributed to Jennifer Keegan.

17 recordsLinked to original sources

PubMed2006

Improved turbo spin-echo imaging of the heart with motion-tracking.

PURPOSE: To improve dark-blood and short tau inversion recovery (STIR) prepared turbo spin-echo (TSE) imaging of the heart, particularly in the basal short-axis plane where cardiac misregistration between the preparation and imaging phases is high. MATERIALS AND METHODS: In the first approach (tracked), the basal short-axis plane was labeled and tracked over the cardiac cycle. The slice-selective 180 degrees dark-blood and STIR preparation pulses were then independently positioned on the appropriately timed labeled images. In the second approach (offset), the preparation pulses were output in the same orientation as the imaging plane, but with a user-defined slice offset that was derived from the labeled data. Both approaches were compared with the standard untracked dark-blood STIR TSE sequence (7-mm slice thickness) in 10 healthy volunteers. RESULTS: For typical preparation slice thicknesses, tracked and offset TSE images were superior to the untracked images (both P < 0.01). For the more mobile right ventricle (RV), the image quality of the tracked images was superior to that of the offset images (P < 0.05). CONCLUSION: Tracking the through-plane motion of the heart between preparation and imaging phases improves the quality of thin-slice basal short-axis TSE images, particularly for the more mobile RV.

Algorithms↗

PubMed2006

Development of a novel optimized breathhold technique for myocardial T2 measurement in thalassemia.

PURPOSE: To develop a reproducible fast spin-echo (FSE) technique for accurate myocardial T2 measurement with application to iron overload assessment in thalassemia. MATERIALS AND METHODS: An FSE sequence was developed to permit acquisition of multiple TE images in one breathhold (BH-FSE). A dynamic black-blood scheme was introduced to better cancel blood signal. A nonselective refocusing train was also adopted to suppress stimulated echoes. The optimized technique was tested on phantoms and then applied to 10 normal volunteers and 10 thalassemia patients. Interstudy reproducibility was measured on all the 20 subjects. RESULTS: The mean difference in T2 values was 1.7% from phantom experiments between BH-FSE and the conventional spin-echo (SE) technique. High contrast BH-FSE images were acquired from human subjects, with minimal stimulated echoes and effective blood suppression (P = 0.0005). The coefficient of variation for interstudy reproducibility was 4.3%. T2 values from thalassemia patients were substantially lower than those from the normal subjects (45.2 +/- 26.1 msec vs. 56.9 +/- 8.4 ms, P = 0.02). CONCLUSION: The dynamic black-blood T2 sequence is a fast reproducible acquisition that compares favorably with conventional techniques, is robust to motion artifacts, and yields high blood-myocardium contrast. This technique may provide a useful tool in thalassemia and other scenarios requiring myocardial T2 quantification.

Adolescent↗

PubMed2005

Applications of phase-contrast flow and velocity imaging in cardiovascular MRI.

A review of cardiovascular clinical and research applications of MRI phase-contrast velocity imaging, also known as velocity mapping or flow imaging. Phase-contrast basic principles, advantages, limitations, common pitfalls and artefacts are described. It can measure many different aspects of the complicated blood flow in the heart and vessels: volume flow (cardiac output, shunt, valve regurgitation), peak blood velocity (for stenosis), patterns and timings of velocity waveforms and flow distributions within heart chambers (abnormal ventricular function) and vessels (pulse-wave velocity, vessel wall disease). The review includes phase-contrast applications in cardiac function, heart valves, congenital heart diseases, major blood vessels, coronary arteries and myocardial wall velocity.

Blood Flow Velocity↗

PubMed2005

Comparison between three-dimensional volume-selective turbo spin-echo imaging and two-dimensional ultrasound for assessing carotid artery structure and function.

PURPOSE: To compare a volume-selective three-dimensional turbo spin echo (TSE) technique with ultrasound (US) for assessing carotid artery wall structure and function. MATERIALS AND METHODS: A three-dimensional volume-selective TSE technique was used to image the carotid artery in 10 healthy subjects and five hypertensive subjects (each of whom were scanned three times while they received different hypertension treatments). Lumen and wall area were measured on MR images. Two-dimensional US measurements of the intima-media thickness (IMT) and lumen diameter were taken in three orientations through a single cross section. The lumen area change over the cardiac cycle was used to determine distension. For validation, a Bland-Altman analysis was used to compare the vessel wall and lumen areas measured by three-dimensional MRI volumes with those obtained by US scans. RESULTS: Agreement between the two methods was found. The mean difference in distension between US and MRI was 1.2% (+/-5.1%). For the wall area measurements, good agreement was shown, but there was a systematic difference due to the visualization of the adventitia by MRI. Both techniques offer an easy way to objectively measure lumen indices. MRI can provide the complete circumference over the length of a vessel, while US is flexible and relatively inexpensive. The application of US is limited, however, when subjects are poorly echogenic. A difference between hypertensive and healthy subjects was found. CONCLUSION: There was a good agreement between MRI and the clinically established two-dimensional US method. The MRI method has the advantage of providing increased vessel coverage, which permits one to assess localized abnormalities without assuming vessel uniformity.

Adult↗

PubMed2005

3D volume-selective turbo spin echo for carotid artery wall imaging with navigator detection of swallowing.

PURPOSE: To improve 3D volume-selective turbo spin echo (TSE) carotid artery wall imaging by incorporating navigators to reduce artifacts caused by swallowing. MATERIALS AND METHODS: Images were acquired on a Siemens Magnetom Sonata 1.5T scanner. 3D volume-selective TSE scans of the carotid arteries were acquired in six healthy volunteers. A cross-pair navigator placed on the back of the tongue was used to detect swallowing and movement. Two swallowing patterns were tested: 1) a single swallow approximately halfway through the scan time, at the center of k(z), and 2) repeated swallowing as often as possible throughout the scan period. Images were acquired with and without navigators for comparison. Signal intensity in the lumen was quantified for the quality of blood suppression, and the clarity of the vessel wall in the common carotid was ranked by four independent blinded observers. RESULTS: In general, lower signal intensity was recorded in the lumen, and decreased blurring and ghosting were observed on scans with navigator control. This reduction in lumen signal intensity signifies an improvement in the black-blood imaging technique. The differences likely reflect the improved double inversion/blood suppression efficiency due to cycles being rejected when the heart rate changed at the point of swallowing, or decreased motional blurring/ghosting of tissue when the navigator is used, or a combination of these two effects. A statistical analysis of image quality showed a significant difference between navigated and non-navigated scans as scored by four independent, blinded observers. For both swallowing patterns, the mean score for the navigator images was on average 0.6 greater than that of non-navigator images (on a scoring scale of 0-5, where 0 = no vessel visible, and 5 = good delineation and blood suppression) and P-values for all observers were less than 0.01. Overall, the central swallow scans were scored higher than the repeated swallow scans. One reason for this may be the fact that the heart rate increased on swallowing, and this often lasted for one or two cardiac cycles after the navigator returned to the normal acceptance position. The effect of the increased heart rate after swallowing is likely to have an effect on double inversion blood suppression efficiency. Therefore, the increased amount of heart rate changes with repeated swallowing may have a greater adverse effect, even if the navigator rejects data views during the swallowing motion. CONCLUSION: The clarity of vessel wall delineation and the apparent efficiency of blood suppression are reduced by swallowing during acquisition. Both motion blurring and quality of blood suppression are factors that can be improved with the use of a navigator accept/reject method.

Artifacts↗

PubMed2005

Cardiovascular magnetic resonance of anomalous coronary arteries.

Cardiovascular magnetic resonance of anomalous coronary arteries is a class I indication. The term anomalous coronary artery encompasses those with an abnormal origin (from the incorrect sinus, too-high or too-low from the correct sinus, or from the pulmonary artery) and/or number of ostia. Their clinical significance results from the increased risk of myocardial infarction and sudden cardiac death associated with those traversing an interarterial course between the aorta and main pulmonary artery/right ventricular outflow tract. In this article, we review the role and practice of cardiovascular magnetic resonance in this field.

Cardiovascular System↗

PubMed2004

Comparison of spiral and FLASH phase velocity mapping, with and without breath-holding, for the assessment of left and right coronary artery blood flow velocity.

PURPOSE: To develop high temporal resolution coronary artery spiral phase velocity mapping sequences and to compare the results obtained with those from FLASH sequences. MATERIALS AND METHODS: Velocity curves were obtained in eight left and eight right coronary arteries using breath-hold interleaved spiral (BH_SP), free-breathing interleaved spiral (FB_SP), breath-hold segmented FLASH (BH_FL), and free-breathing FLASH (FB_FL) sequences. Spatial resolution, temporal resolution, and acquisition durations (cardiac cycles) were as follows-BH_SP: 0.9 mm x 0.9 mm, 30 msec, 20 cycles; FB_SP: 0.9 mm x 0.9 mm, 42 msec, 100 cycles; BH_FL: 0.9 mm x 1.8 mm, 70 msec (effective), 20 cycles; FB_FL: 0.9 mm x 1.8 mm, 30 msec, 480 cycles. Peak systolic, peak diastolic, and mean velocities were compared between sequences. RESULTS: For left and right arteries, the FB_SP velocity profiles closely followed those from the FB_FL sequence. By comparison, the BH_FL sequence failed to resolve the sharp peaks in the temporal velocity profiles of the right coronary artery, significantly underestimating the peak systolic (88 mm/second vs. 252 mm/second, P < 0.001), peak diastolic (114 mm/second vs. 153 mm/second, P < 0.01), and mean (56 mm/second vs. 93 mm/second, P < 0.001) velocities. For the less mobile left artery, the peak systolic, peak diastolic, and mean velocities were also underestimated by the BH_FL sequence, although this only reached statistical significance for the systolic peak (80 mm/second vs. 135 mm/second, P < 0.01), 142 mm/second vs. 168 mm/second, (P = ns), and 87 mm/second vs. 101 mm/second, (P = ns) respectively. CONCLUSION: We have shown that the FB_SP sequence developed agrees well with the FB_FL sequence, while the study duration is reduced by a factor of 10 for the same spatial resolution. By comparison, the BH_FL sequence underestimates flow velocities, particularly in the more mobile right coronary artery.

Adult↗

PubMed2004

Intra- and interstudy reproducibility of coronary artery diameter measurements in magnetic resonance coronary angiography.

PURPOSE: To determine the intra- and interstudy reproducibility of right coronary artery diameter assessment using serial magnetic resonance (MR) coronary angiography. MATERIALS AND METHODS: Two-dimensional (2D) navigator-gated segmented fast low angle shot (FLASH) images of the proximal right coronary artery were acquired three times in 11 healthy volunteers, the first two times in the same study session and the third time after repositioning the subject in the scanner. Coronary artery diameters were determined using automated segmentation software and intra- and interstudy reproducibility calculated as the standard deviation (SD) of the signed differences between measurements within and between study sessions, respectively. The reproducibility of the segmentation software was determined by repeated analysis of each individual scan. RESULTS: One subject was excluded from the study due to poor-quality images. In the remaining 10 subjects, the mean (+/- SD) intrastudy difference in coronary artery diameters was -0.05 +/- 0.12 mm, a value that is very similar to between-frame (same-film) differences reported in quantitative coronary angiography (QCA). The mean (+/- SD) interstudy difference in coronary artery diameters was 0.16 +/- 0.43 mm, although this was greatly skewed by one subject with poor image plane repositioning. Excluding that subject resulted in a mean (+/- SD) interstudy difference of 0.04 +/- 0.20 mm. The reproducibility of the segmentation software was excellent, with the mean difference between repeat analyses of the images being 0.00 +/- 0.03 mm. CONCLUSION: The intrastudy variability of coronary artery diameter measurements is low, potentially allowing MR coronary angiography to be used as a tool for the noninvasive assessment of serial changes following pharmacological intervention. A major contributing factor to this is the high reproducibility of the segmentation software. Interstudy variability is approximately three times the intrastudy variability.

Adult↗

PubMed2004

Spiral phase velocity mapping of left and right coronary artery blood flow: correction for through-plane motion using selective fat-only excitation.

PURPOSE: To develop a method of correcting both right and left coronary artery flow velocities for the through-plane motion of the vessel, in order to allow details in the temporal flow profiles to be viewed. MATERIALS AND METHODS: The methods developed use selective excitation and velocity mapping of the epicardial fat surrounding the artery, either in a separate acquisition (temporal resolution = 22 msec) or interleaved with the water-excitation acquisition (temporal resolution = 44 msec) used to determine coronary blood flow velocities. The two methods were compared in 10 right and 13 left coronary arteries in healthy volunteers. RESULTS: For the right coronary arteries, correction for through-plane motion significantly reduces the mean systolic flow velocity (75.3 mm/second vs. 90.0 mm/second, P < 0.01), while the mean diastolic flow velocity is unchanged (96.8 mm/second vs. 94.5 mm/second, P = ns). The resulting profiles are biphasic, with approximately equal flow in systole and diastole. For the left arteries, correction for through-plane motion reduces the mean systolic flow velocity (25.0 mm/second vs. 72.8 mm/second, P < 0.001), resulting in the expected diastolic predominant flow profiles. For the right arteries, there were no significant differences in the mean systolic and mean diastolic velocities after correction with the separate fat-excitation acquisition, and after correction the poorer temporal resolution combined water excitation/fat excitation acquisition. However, for the left coronary arteries, the combined water excitation/fat excitation acquisition resulted in a slight reduction in the mean diastolic velocity (121.5 mm/second vs. 130.9 mm/second, P < 0.05). CONCLUSION: Selective excitation of the surrounding epicardial fat enables through-plane correction of both left and right coronary flow velocities, enabling the temporal details of flow velocity to be viewed. With a combined WE/FE acquisition, this can be performed without extending the study duration; however, the reduced temporal resolution and temporal mismatch of the excitations results in a blunting of rapidly changing flow profiles. As such, it may be less suitable for the left coronary artery, which has a greater range of through-plane motion than the right, and correction using separate WE and FE acquisitions, or the adjacent myocardium, may be preferable.

Adult↗

PubMed2004

Combined coronary and perfusion cardiovascular magnetic resonance for the assessment of coronary artery stenosis.

The purpose of this study was to evaluate the feasibility and accuracy of combined coronary and perfusion cardiovascular magnetic resonance (CMR) in the assessment of coronary artery stenosis. Thirty-five consecutive patients (27 men, eight women, age range 34-81 years), undergoing cardiac catheterization, were assessed with 3D coronary CMR and rest-stress perfusion CMR. Significant coronary stenosis was determined by vessel narrowing or signal loss with coronary CMR, and by abnormal contrast enhancement with perfusion CMR. Coronary artery diameter stenosis greater than 50% was considered significant with conventional cardiac catheterization. Seventeen patients had significant coronary artery disease, and in these there were 35 significant stenoses on cardiac catheterization. All left main stem arteries were normal on both cardiac catheterization and coronary CMR. For the diagnosis of coronary artery stenosis, coronary CMR had a sensitivity of 92% for the left anterior descending artery (LAD), 79% for the right coronary artery (RCA), but only 13% for the circumflex coronary artery (LCX). Perfusion CMR had corresponding sensitivities of 69%, 86%, and 63%, respectively. For all arteries the accuracies for coronary and perfusion CMR were 67% and 72%, respectively. Combining coronary and perfusion CMR improved the accuracy to 77%. These data demonstrate that in patients with suspected coronary artery disease, combined coronary and perfusion CMR is feasible, increases the accuracy of detection of significant coronary stenosis, and offers the possibility of combined anatomical and hemodynamic assessment of coronary artery stenosis.

Adult↗

PubMed2004

Predictive cardiac motion modeling and correction with partial least squares regression.

Respiratory-induced cardiac deformation is a major problem for high-resolution cardiac imaging. This paper presents a new technique for predictive cardiac motion modeling and correction, which uses partial least squares regression to extract intrinsic relationships between three-dimensional (3-D) cardiac deformation due to respiration and multiple one-dimensional real-time measurable surface intensity traces at chest or abdomen. Despite the fact that these surface intensity traces can be strongly coupled with each other but poorly correlated with respiratory-induced cardiac deformation, we demonstrate how they can be used to accurately predict cardiac motion through the extraction of latent variables of both the input and output of the model. The proposed method allows cross-modality reconstruction of patient specific models for dense motion field prediction, which after initial modeling can be used for real-time prospective motion tracking or correction. Detailed numerical issues related to the technique are discussed and the effectiveness of the motion and deformation modeling is validated with 3-D magnetic resonance data sets acquired from ten asymptomatic subjects covering the entire respiratory range.

Algorithms↗

PubMed2003

Coronary artery anomalies: assessment with free-breathing three-dimensional coronary MR angiography.

PURPOSE: To evaluate a simplified protocol by using free-breathing three-dimensional (3D) coronary magnetic resonance (MR) angiography to determine the anatomy of anomalous coronary arteries, in particular the relationship of the vessels to the aortic root. MATERIALS AND METHODS: Twenty-six patients (18 men, eight women; mean age, 50 years; age range, 18-77 years) who had a history of chest pain, palpitations, or syncope and who were suspected of having coronary artery anomalies were examined with free-breathing MR angiography. Multiple 3D volume slabs were acquired at the level of the sinuses of Valsalva by using diaphragmatic navigators for respiratory artifact suppression. The proximal anatomy of the coronary arteries was determined. RESULTS: Six anomalous circumflex arteries originated from the right sinus of Valsalva and passed behind the aortic root. Six right coronary arteries arose from the left sinus of Valsalva and coursed between the aortic root and the right ventricular outflow tract (RVOT). Nine left coronary arteries arose from the right sinus of Valsalva; seven of nine coursed between the aortic root and the RVOT. Five patients had minor anomalies. Overall, in eight patients with anomalous arteries that coursed between the aortic root and the RVOT, conventional coronary angiography could not be used confidently to identify the proximal course. CONCLUSION: Free-breathing 3D coronary MR angiography can be used to identify the proximal anatomy of anomalous coronary arteries.

Adolescent↗

PubMed2003

Volume-selective 3D turbo spin echo imaging for vascular wall imaging and distensibility measurement.

PURPOSE: To use a volume-selective 3D turbo spin echo (TSE) technique to image the carotid artery wall and measure distensibility. MATERIALS AND METHODS: A high-resolution volume-selective 3D TSE sequence has been developed. Volume selection is accomplished by orthogonal gradients for the 90 degrees and 180 degrees excitations and allows a 3D volume of vessel wall to be imaged in a relatively short time. The technique has been developed to allow imaging at any defined phase of the cardiac cycle so that the vascular function and distensibility can be studied. RESULTS: Scan efficiency is increased by the reduced phase encode field of view (FOV) (k(y) steps) by the use of selective volume excitation. Significantly more slices (k(z) phase encode steps) for vessel coverage can be acquired with the same scan time as that of the conventional sequence while maintaining signal-to-noise ratio (SNR) levels. The practical value of the technique was demonstrated on 10 normal subjects with high-resolution vessel distensibility measurements of the carotid arteries. CONCLUSION: A volume-selective TSE method has been used for carotid artery wall imaging and measurement of distensibility in normal subjects. Larger coverage of the vessel, and therefore more information for clinical diagnostics, was achieved with the same overall scan time with an SNR comparable to that of 2D full FOV images.

Adult↗

PubMed2003

Breath-hold signal-loss sequence for the qualitative assessment of flow disturbances in cardiovascular MR.

PURPOSE: To develop a breath-hold segmented sequence which generates similar patterns of signal loss to a non-breath-hold, relatively long echo time, conventional gradient echo sequence for the qualitative assessment of valvular heart disease. MATERIALS AND METHODS: Both velocity-sensitized and acceleration-sensitized segmented sequences were developed. The sensitivities were empirically adjusted to give similar degrees of signal loss to a conventional sequence. These sequences were compared with a conventional sequence in eight patients with flow disturbances and in four healthy subjects. RESULTS: There was no significant difference in the extent of signal loss observed when using the breath-hold velocity- and acceleration-sensitized sequences developed and the conventional sequence (1862 mm(2), 1831 mm(2), and 1782 mm(2), respectively; P = ns). However, the image quality obtained was significantly better with the breath-hold sequences (both P < 0.01). Furthermore, the image quality achieved with the acceleration-sensitized sequence was significantly better than that achieved with the velocity-sensitized sequence (P < 0.01) where artifacts from beat-to-beat variations in blood-flow velocities were a frequent problem. CONCLUSION: Signal loss in complex flow is best demonstrated using the breath-hold acceleration-sensitized sequence where the signal from both stationary and constant velocity material is rephased at the echo time.

Adult↗

PubMed2003

Reduced k-space encoding for dual-contrast TrueFISP imaging.

Dual-contrast TrueFISP imaging relies on the use of two RF pulses with different RF flip-angles for enhancing image contrast and performing automatic tissue classification based on multispectrum clustering. The original technique, however, involves an extended imaging time, which limits its clinical application. The purpose of this study is to compare the applicability of two reduced k-space sampling techniques, reduced imaging using generalized series reconstruction (RIGR) and key-hole imaging, for minimizing the imaging time required. The performance of both approaches was evaluated theoretically and practically using 3D cine datasets acquired from eight asymptomatic subjects.

Contrast Media↗

PubMed2002

Initial experience with the intravascular contrast agent NC100150-injection (Clariscan) for breath-hold and navigator-gated magnetic resonance coronary artery imaging.

PURPOSE: To examine magnetic resonance coronary artery imaging after NC100150-Injection. MATERIALS AND METHODS: Breath-hold and navigator-gated images were acquired in five patients. RESULTS: Breath-hold image quality, coronary artery-fat SDNR, and coronary artery SNR improved. Respiratory artifacts due to reduced liver signal intensity degraded navigator-gated image quality. CONCLUSION: NC100150-Injection improves breath-hold coronary artery imaging. Navigator-gated acquisitions should use techniques that are insensitive to T2* effects.

Aged↗

PubMed2002

Coronary artery motion with the respiratory cycle during breath-holding and free-breathing: implications for slice-followed coronary artery imaging.

The displacement of the right coronary artery (RCA) origin with respiratory position was determined relative to the dome of the right hemidiaphragm in three orthogonal directions in eight healthy subjects. Both multiple breath-hold and free-breathing acquisitions were used, and motion correction factors for slice-following applications were determined. The correction factors for all three directions showed considerable intersubject variability. The mean superior-inferior factor was slightly less in free-breathing than in breath-holding (0.26 vs. 0.29, P = ns), and much less than the fixed value of 0.6 frequently implemented with slice-following. The anterior-posterior correction factors were uniformly low in free-breathing, and significantly less than those obtained from breath-holding (0.04 vs. 0.14, P <.05), while the mean left-right correction factors were approximately 0.1 for both. It is concluded that subject variability in correction factors, together with within-subject differences between breath-holding and free-breathing, is such that slice-following should be performed with subject-specific factors determined from free-breathing acquisitions.

Adult↗