Automated vessel edge detection in velocity-encoded cine-MR (VEC-MR) flow measurements: a retrospective evaluation in critically ill patients

OBJECTIVE: To assess feasibility of automated edge detection in magnetic resonance (MR) flow calculations in a clinical setting with critically ill patients. MATERIAL AND METHODS: Velocity encoded cine-MR (VEC-MR) flow measurements cross-sectional area (CSA), mean spatial velocity (MSV), instantaneous flow (IF), flow (F), 0.5 T Philips, TR 800-800, TE=8 ms, 30 degrees flip angle, FOV 280 mm, 128 x 256 matrix, temporal resolution 16 time frames/RR, VENC=120 cm/s) were obtained in 20 major thoracic human vessels (ascending aorta, main, right and left pulmonary artery-AAO, MPA, RPA, LPA) of five patients, suffering from severe chronic thromboembolic pulmonary hypertension (CTEPH). Flow maps were evaluated by two independent observers using conventional manual edge detection (INTER m/m). Flow calculations were performed by one observer using both, manual and automated edge detection (INTRA m/a), by a second observer using automated edge detection two times (INTRA a/a) and by two independent observers using automated edge detection (INTER a/a). Evaluation time was measured. Linear regression analysis and Student's t-test were performed. RESULTS: Overall regression coefficients (r2) for INTER m/m, INTRA m/a, INTER a/a and INTRA a/a, respectively, were as follows: CSA, 0.91, 0.91, 0.96, 0.98; MSV, 0.97, 0.99, 0.99, 0.99; IF, 0.98, 0.99, 0.99, 0.99; F, 0.98, 0.99, 0.99, 0.99. Manual CSA values differed significantly from automated data in MPA (P=0.01), RPA (P=0.0008) and LPA (P=0.02). No difference was found for the other assessed parameters of the pulmonary circulation. Average evaluation time per vessel was 20.2+/-2.6 min for manual and 2.1+/-0.7 min for automated edge detection (P<0.00001). CONCLUSION: The software program used provided reproducible data, lead to a 90% reduction in evaluation and calculation time and, therefore, might excel the utilization of VEC-MR flow measurements. Despite variations in the evaluation of the pulmonary circulation CSAs, flow assessment is feasible in critically ill patients.     

Valve and great vessel stenosis: assessment with MR jet velocity mapping

For measurement of poststenotic jet velocities with magnetic resonance (MR) imaging, the authors reduced the echo time (TE) of the field even-echo rephasing (FEER) velocity mapping sequence from 14.0 to 3.6 msec, so minimizing the problem of MR signal loss from turbulent fluid. In vitro use of rotating disk and stenotic flow phantoms confirmed that the 3.6-msec TE sequence enables accurate measurement of jet velocities of up to 6.0 m/sec (r = .996). Peak jet velocity measurements were made with MR imaging in 36 patients with stenosis of native heart valves (n = 9), conduits (n = 19), or Fontan connections (n = 2) or with aortic coarctation (n = 6). Peak velocity measurements made with MR imaging agreed well with measurements made with Doppler ultrasound (US), which were available in 18 cases (standard deviation = 0.2 m/sec). Velocity mapping with fast-echo MR imaging is likely to have considerable importance as a noninvasive means of locating and evaluating stenoses, particularly at sites inaccessible to US, but care must be taken to prevent errors caused by malalignment, signal loss, phase wrap, or partial-volume effects.     

Right and left ventricular stroke volume measurements with velocity-encoded cine MR imaging: in vitro and in vivo validation

The accuracy of measurements of flow velocity determined by using cine MR phase velocity mapping--velocity-encoded cine (VEC) MR--was assessed by comparing VEC MR data with independent measurements in a flow phantom and in human subjects. Constant flow velocities generated in a phantom (range, 20-408 cm/sec) were determined correctly by VEC MR (r = .997, standard error of the estimate [SEE] = 7.9 cm/sec). Peak systolic velocities in the main pulmonary artery determined by VEC MR correlated well with the measurements obtained by using continuous-wave Doppler echocardiography (r = .91). Stroke volumes measured at the aorta by VEC MR and continuous-wave Doppler imaging also correlated well with each other (r = .80). VEC MR measurements of aortic and pulmonary flow provided left and right ventricular stroke volumes that correlated well with left ventricular stroke volumes determined by short-axis cine MR images (r = .98, SEE = 3.7 ml, and r = .95, SEE = 4.8 ml, respectively). Intra- and interobserver variabilities were small for both left and right ventricular stroke volumes as measured with VEC MR. These results indicate that VEC MR accurately and reproducibly measures aortic and pulmonary flow velocities and volumes in the physiologic range of humans, and can be used to measure right and left ventricular stroke volumes under normal flow conditions.     

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, nonbreathhold 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.     

Zero filled partial fourier phase contrast MR imaging: in vitro and in vivo assessment

PURPOSE: To validate partial Fourier phase contrast magnetic resonance (PC MR) with full number of excitation (NEX) PC MR measurements in vitro and in vivo. MATERIALS AND METHODS: MR flow measurements were performed using a partial Fourier and a full NEX PC MR sequence in a flow phantom and in 10 popliteal and renal arteries of 10 different healthy volunteers. Average velocity, peak velocity, and flow results were calculated and compared with regression analysis. RESULTS: Excellent correlations in average velocities (r = 0.99, P < 0.001), peak velocities (r = 0.99, P < 0.001), and flow rates (r = 0.98, P < 0.001) were demonstrated in vitro between the two different acquisitions. For the popliteal arteries there was excellent correlation between peak velocities for both acquisitions (r = 0.98, P < 0.0001); the correlation of average velocity measurements when using all data points in the cardiac cycle for all volunteers was 0.96 (P < 0.001). For the renal arteries the same comparison resulted in a good correlation for average velocity (0.93, P < 0.001) and peak velocity measurements (r = 0.91, P = 0.002), although the correlation coefficient for flow rates was 0.88 (P = 0.004). Blurring of the vessel margins was consistently observed on magnitude images acquired with the partial Fourier method, causing overestimation of the vessel area and some error in the flow measurements. CONCLUSION: Partial Fourier PC MR is able to provide comparable average and peak velocity values when using 1 NEX PC MRI as a reference.     

Closed circuit MR compatible pulsatile pump system using a ventricular assist device and pressure control unit

The aim of this study was to evaluate the performance of a closed circuit MR compatible pneumatically driven pump system using a ventricular assist device as pulsatile flow pump for in vitro 3D flow simulation. Additionally, a pressure control unit was integrated into the flow circuit. The performance of the pump system and its test‐retest reliability was evaluated using a stenosis phantom (60% lumen narrowing). Bland–Altman analysis revealed a good test–retest reliability (mean differences = ?0.016 m/s, limits of agreement = ±0.047 m/s) for in vitro flow measurements. Furthermore, a rapid prototyping in vitro model of a normal thoracic aorta was integrated into the flow circuit for a direct comparison of flow characteristics with in vivo data in the same subject. The pneumatically driven ventricular assist device was attached to the ascending aorta of the in vitro model to simulate the beating left ventricle. In the descending part of the healthy aorta a flexible stenosis was integrated to model an aortic coarctation. In vivo and in vitro comparison showed significant (P = 0.002) correlations (r = 0.9) of mean velocities. The simulation of increasing coarctation grade led to expected changes in the flow patterns such as jet flow in the post‐stenotic region and increased velocities. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Fundamental study of the relation between flow velocity and signal intensity in the TrueFISP sequence

The purpose of this study was to evaluate the effect of inflow phenomenon on TrueFISP. We created a phantom using a vinyl tube and distilled water, and applied a pump-oxygenator to the phantom to obtain stationary flow. First, to evaluate the effect of inflow and the dephase phenomenon on signal intensity, the phantom was measured for the signal intensity of variable flow velocity. Second, the relation of TR/TE with signal intensity was analyzed. The results showed that a flow velocity of less than 15 cm/sec did not participate in signal reduction; however, signal intensity was reduced when flow velocity was more than 30 cm/sec. Moreover, the reduction of signal intensity was remarkable with a flow velocity of 50-100 cm/sec, which corresponds with arterial flow velocity. In the analysis of TR/TE, signal intensity was increased when TR of less than 5 ms was applied to the slow velocity of 15 cm/sec. Signal intensity was decreased when the same TR was applied to the high velocity of 50-100 cm/sec. When TR was 6-9 ms, peak signal intensity was recognized at the high velocity of 50-100 cm/sec. This peak, however, might correspond only to the inflow phenomenon, and steady state might have already collapsed. Based on these results, we concluded that TrueFISP is suitable for the imaging of slow flow velocity. A short TR of less than 5 ms was effective for obtaining high signal intensity. Our next goal will be to apply TrueFISP to MR venography, although further investigation will be necessary.     

Variable-density one-shot Fourier velocity encoding.

In areas of highly pulsatile and turbulent flow, real-time imaging with high temporal, spatial, and velocity resolution is essential. The use of 1D Fourier velocity encoding (FVE) was previously demonstrated for velocity measurement in real time, with fewer effects resulting from off-resonance. The application of variable-density sampling is proposed to improve velocity measurement without a significant increase in readout time or the addition of aliasing artifacts. Two sequence comparisons are presented to improve velocity resolution or increase the velocity field of view (FOV) to unambiguously measure velocities up to 5 m/s without aliasing. The results from a tube flow phantom, a stenosis phantom, and healthy volunteers are presented, along with a comparison of measurements using Doppler ultrasound (US). The studies confirm that variable-density acquisition of kz-kv space improves the velocity resolution and FOV of such data, with the greatest impact on the improvement of FOV to include velocities in stenotic ranges.     

Balanced phase-contrast steady-state free precession (PC-SSFP): a novel technique for velocity encoding by gradient inversion.

A technique for measuring velocity is presented that combines cine phase contrast (PC) MRI and balanced steady-state free precession (SSFP) imaging, and is thus termed PC-SSFP. Flow encoding was performed without the introduction of additional velocity encoding gradients in order to keep the repetition time (TR) as short as in typical SSFP imaging sequences. Sensitivity to through-plane velocities was instead established by inverting (i.e., negating) all gradients along the slice-select direction. Velocity sensitivity (VENC) could be adjusted by altering the first moments of the slice-select gradients. Disturbances of the SSFP steady state were avoided by acquiring different flow echoes in consecutively (i.e., sequentially) executed scans, each over several cardiac cycles, using separate steady-state preparation periods. A comparison of phantom measurements with those from established 2D-cine-PC MRI demonstrated excellent correlation between both modalities. In examinations of volunteers, PC-SSFP exhibited a higher intrinsic signal-to-noise ratio (SNR) and consequently low phase noise in measured velocities compared to conventional PC scans. An additional benefit of PC-SSFP is that it relies less on in-flow-dependent signal enhancement, and thus yields more uniform SNRs and better depictions of vessel geometry throughout the whole cardiac cycle in structures with slow and/or pulsatile flow.     

Estimation of left ventricular performance through temporal pressure variations measured by MR velocity and acceleration mappings

PURPOSE: To describe a method for assessing pressure variation vs. time (dp/dt) using blood flow acceleration measured by MRI, and to demonstrate its applicability in estimating left ventricular (LV) function. MATERIALS AND METHODS: The method was tested in vitro using a pulsatile phantom, and a strong correlation was found between transducer and MRI determinations of dp/dt (r = 0.98). Selected aortic flow parameters were then measured in 10 patients and the results were compared with transducer measurements of the LV dp/dt. RESULTS: The correlation coefficients for the reference estimations of global myocardial function and MRI were 0.59 for aortic velocity, 0.74 for aortic acceleration, and 0.86 for aortic dp/dt. CONCLUSION: MR measurements of velocity and acceleration within the ascending aorta offer a noninvasive method for determining indices, such as the aortic dp/dt, that are closely correlated with the global myocardial contractility function.     

Plow quantitation with echo-planar phase-contrast velocity mapping: In vitro and in vivo evaluation

We present a multishot echo-planar imaging (EPI) phase-contrast implementation for flow quantitation. The measurement accuracy of this technique was evaluated in vitro and in vivo. A gated eight-shot EPI phase-contrast sequence (TR/TE = 16/7.4, 45° flip angle), with a flow-phase interval of 32 msec and an in-plane resolution of 2× 2 mm was initially evaluated in a pulsatile flow phantom. Subsequently, EPI phase-contrast flow measurements of the ascending and descending aorta, obtained in 10 volunteers, were compared with flow volume data acquired with a conventional cine phase-contrast sequence (TR/TE = 24/7, 45° flip angle, 48-msec flow-phase interval, 2 × 1 mm in-plane resolution). Comparisons between flow measurements were made using data obtained with the flow probe and cine phase contrast as the standard of reference for in vitro and in vivo measurements, respectively. EPI phase-contrast sequences reduced data acquisition times tenfold compared with cine phase-contrast sequences. EPI phase-contrast flow measurements correlated were with phantom flow (r = 0.98, slope = 1.1) as well as with aortic cine phase-contrast flow volume determinations (r = 0.98). A 95% confidence interval of measurement differences between echo-planar and cine phase-contrast imaging, ranging from 2.0 to - 1058 mL/min was computed. Ultrafast phase-contrast flow measurements are possible. Multishot EPI phase-contrast imaging provides high measurement accuracy in pulsatile vessels while keeping the image acquisition interval short enough to be accomplished in a comfortable breath-hold.     

Variable velocity encoding in a three‐dimensional,three‐directional phase contrast sequence: Evaluation in phantom and volunteers

Purpose:

To evaluate accuracy and noise properties of a novel time‐resolved, three‐dimensional, three‐directional phase contrast sequence with variable velocity encoding (denoted 4D‐vPC) on a 3 Tesla MR system, and to investigate potential benefits and limitations of variable velocity encoding with respect to depicting blood flow patterns.

Materials and Methods:

A 4D PC‐MRI sequence was modified to allow variable velocity encoding (VENC) over the cardiac cycle in all three velocity directions independently. 4D‐PC sequences with constant and variable VENC were compared in a rotating phantom with respect to measured velocities and noise levels. Additionally, comparison of flow patterns in the ascending aorta was performed in six healthy volunteers.

Results:

Phantom measurements showed a linear relationship between velocity noise and velocity encoding. 4D‐vPC MRI presented lower noise levels than 4D‐PC both in phantom and in volunteer measurements, in agreement with theory. Volunteer comparisons revealed more consistent and detailed flow patterns in early diastole for the variable VENC sequences.

Conclusion:

Variable velocity encoding offers reduced noise levels compared with sequences with constant velocity encoding by optimizing the velocity‐to‐noise ratio (VNR) to the hemodynamic properties of the imaged area. Increased VNR ratios could be beneficial for blood flow visualizations of pathology in the cardiac cycle. J. Magn. Reson. Imaging 2012; 36:1450–1459. © 2012 Wiley Periodicals, Inc.     

Correction of temporal misregistration artifacts in jet flow by conventional phase-contrast MRI

PURPOSE: To show that accuracy of jet flow representation by magnetic resonance (MR) phase-contrast (PC) velocity-encoded (VE) cine imaging is dominated by error terms resulting from the temporal distribution of data, and to present a generally applicable data interpolation-based approach to correct for this phenomenon. MATERIALS AND METHODS: Phase-contrast data were acquired in a stenotic orifice flow phantom using a physiologic pulsatile flow waveform. A temporally registered scan, acquired without data segmentation or interleaving was obtained (17 minutes) and taken as the reference (REF). Conventional PC data sets were acquired using segmentation and data interleaving. An enhanced temporal registration (ETR) algorithm was applied to the acquired data to temporally interpolate component sets and output data at matching time points, thereby reducing temporal dispersion. RESULTS: Compared to the REF data, conventionally processed PC data consistently overestimated peak velocities in laminar jet flow regions (127% +/- 28%) and exhibited relatively weak correlations (r = 0.67 +/- 0.23). The ETR-processed data better represented peak velocities (101% +/- 13%, P < 0.001) and correlated more closely with the REF data (r = 0.94 +/- 0.05, P < 0.001). CONCLUSION: The temporal distribution of PC data impacts the accuracy of velocity representation in pulsatile jet flow. A temporal registration postprocessing algorithm can minimize loss of accuracy.     

MR measurement of blood flow in the true and false channel in chronic aortic dissection

Velocity encoded (VEC) cine MR imaging is a new noninvasive technique for the quantification of blood flow velocity in the cardiovascular system. Six patients with type B aortic dissection underwent VEC cine MR imaging at 1.5 T. This technique provides cine MR magnitude and VEC phase images at approximately 16 equally spaced intervals during an average cardiac cycle. A region of interest encompassing a vascular structure, i.e., false channel, provides a spatially averaged velocity for the time interval at which the image was acquired. Interpretation of velocity values from the 16 intervals during the cardiac cycle provides a temporally average velocity. Velocity mapping across the aortic lumen in these six cases showed average spatial and temporal velocity of 13.4 +/- 1.49 cm/s in the true channel and 3.1 +/- 0.84 cm/s in the false channel (p less than 0.05). The peak systolic velocity (temporal peak) was 43.6 +/- 7.20 cm/s in the true channel and 14.3 +/- 2.30 cm/s in the false channel (p less than 0.05). The flow volume per cardiac cycle was not significantly different between the ture (23.1 +/- 5.04 ml/cycle) and false channel (27.1 +/- 10.14 ml/cycle). There was substantial retrograde flow in the false channel of two patients. The intraobserver and interobserver variability was less than 10% (r = 0.98 to 0.99) for the measurement of flow parameters in both the true and the false channel. We conclude that VEC cine MR imaging demonstrates substantial differences in the hemodynamic pattern in the true and false channel in aortic dissection.     

MRI在心脏瓣膜病中的诊断价值

目的 探讨多序列MR扫描在心脏瓣膜病中的诊断价值。方法 回顾性分析56例心脏瓣膜病患者的影像学资料，患者分别采用超声心动图（UCG）、二维（2D）黑血及亮血序列、K-空间节段真实稳态进动快速扫描序列（True FISP）对瓣膜病进行定性评价，15例患者还进行小角度快速激发（FLASH）电影序列和流速编码电影（VEC）定量分析，并将MR结果与UCG进行对比。结果 UCG和MRI诊断二尖瓣狭窄（MS）2例，二尖瓣关闭不全（MI）23例；主动脉瓣狭窄（AS）7例，其中二瓣畸形5例；主动脉瓣关闭不全（AI）13例；三尖瓣关闭不全（TI）2例；复合或联合瓣膜病9例。受累心腔的增大和升主动脉的扩张是其主要形态学改变，异常的血液湍流信号是受累瓣膜的直接征象。VEC-MR定量分析与多普勒超声一致性好：6例AS，MR和UCG检查结果相关性分析，相关系数和校正相关系数分别为：R=0，975、Rsq=0．951（P〈0．01），AI9例，分别为R=0．965、Rsq=0．932（P〈0．01）。结论 MR多序列综合扫描可对心脏瓣膜病特别是主动脉瓣疾病进行准确的定性及定量评价。     

MR flow mapping in coronary artery bypass grafts: a validation study with Doppler flow measurements.

PURPOSE: To validate fast magnetic resonance (MR) flow mapping with intravascular Doppler flow measurements in vitro and in patients with nonstenotic and stenotic coronary artery bypass grafts. MATERIALS AND METHODS: MR and Doppler flow measurements were performed in a small-diameter flow phantom with physiologic flow conditions and at baseline and during adenosine stress in 27 grafts in 23 patients, who were scheduled for cardiac catheterization. At invasive analysis, the grafts were divided into those with stenosis of less than 50% (nonstenotic) and those with stenosis greater than or equal to 50% (stenotic). In vitro velocity values and velocity values in nonstenotic and stenotic grafts were compared with linear regression analysis, and the in vitro interstudy variability was determined. RESULTS: Excellent correlations in average peak velocity (r = 0.99, P <.001) and diastolic peak velocity (r = 0.99, P <.001) were demonstrated in vitro between MR and Doppler flow measurements, with less than 5% interstudy variability. MR and Doppler flow measurements revealed good correlations in peak velocity and velocity reserve both in nonstenotic (n = 20) (average peak velocity: r = 0.81, P <.001; diastolic peak velocity: r = 0.83, P <.001; velocity reserve: r = 0.56, P =.010) and stenotic (n = 7) (average peak velocity: r = 0.83, P <.001; diastolic peak velocity: r = 0.78, P =.001; velocity reserve: r = 0.70, P =.078) grafts. CONCLUSION: Fast MR flow mapping provides noninvasive measures of peak velocity and velocity reserve, which closely correlate with Doppler values both in vitro and in nonstenotic and stenotic grafts.     

Analysis of complex cardiovascular flow with three‐component acceleration‐encoded MRI

Functional information regarding cardiac performance, pressure gradients, and local flow derangement are available from blood acceleration fields. Thus, this study examines a 2D and 3D phase contrast sequence optimized to efficiently encode three‐directional, time‐resolved acceleration in vitro and in vivo. Stenosis phantom acceleration measurements were compared to acceleration derived from standard velocity encoded phase contrast‐magnetic resonance imaging (i.e., “velocity‐derived acceleration”). For in vivo analysis, three‐directional 2D acceleration maps were compared to velocity‐derived acceleration using regions proximal and distal to the aortic valve in six healthy volunteers at 1.5 and 3.0 T (voxel size = 1.4 × 2.1 × 8 mm, temporal resolution = 16–20 ms). In addition, a 4D acceleration sequence was evaluated for feasibility in a healthy volunteer and postrepair biscuspid aortic valve patient with an ascending aortic aneurysm. The phantom magnetic resonance acceleration measurements were more accurate (nonturbulent root mean square error = 2.2 vs. 5.1 m/s² for phase contrast‐magnetic resonance imaging) and 10 times less noisy (nonturbulent σ = 0.9 vs. 13.6 m/s² for phase contrast‐magnetic resonance imaging) than velocity‐derived acceleration. Acceleration mapping of the left ventricular outflow tract and aortic arch exhibited signal voids colocated with complex flow events such as vortex formation and high order motion. 4D acceleration data, visualized in combination with the velocity data, may provide new insight into complex flow phenomena. Magn Reson Med 67:50–61, 2012. © 2011 Wiley Periodicals, Inc.     

Accurate measurement of pulsatile flow velocity in a small tube phantom: comparison of phase-contrast cine magnetic resonance imaging and intraluminal Doppler guidewire

Purpose

We compared the accuracy of magnetic resonance imaging (MRI) measurements of pulsatile flow velocity in a small tube phantom using different spatial factors versus those obtained by intraluminal Doppler guidewire examination (as reference).

Materials and methods

We generated pulsatile flow velocities averaging about 20–290 cm/sec in a tube of 4 mm diameter; we performed phase-contrast cine MRI on pixels measuring 1.00²–2.50² mm². We quantified spatial peak flow velocities of a single pixel and a cluster of five pixels and spatial mean velocities within regions of interest enclosing the entire lumen in the phantom’s cross-section. Finally, we compared the measurements of temporally mean and maximum flow velocity with the Doppler measurements.

Results

Linear correlation was excellent between both measurements of spatial peak flow velocities in one pixel. The highest spatial resolution using spatial peak flow velocities of a single pixel allowed the most accurate MRI measurements of both temporally mean and maximum pulsatile flow velocity (r = 0.97 and 0.99, respectively: MRI measurement = 0.95x + 8.9 and 0.88x + 24.0 cm/s, respectively). Otherwise, MRI measurements were significantly underestimated at lower spatial resolutions.

Conclusion

High spatial resolution allowed accurate MRI measurement of temporally mean and maximum pulsatile flow velocity at spatial peak velocities of one pixel.     

Relationship among RR interval, optimal reconstruction phase, temporal resolution, and image quality of end-systolic reconstruction of coronary CT angiography in patients with high heart rates: in search of the optimal acquisition protocol

The purpose of this study is to elucidate the relationship among RR interval (RR), the optimal reconstruction phase, and adequate temporal resolution (TR) to obtain coronary CT angiography images of acceptable quality using 64-MDCT (Aquilion 64) of end-systolic reconstruction in 407 patients with high heart rates. Image quality was classified into 3 groups [rank A (excellent): 161, rank B (acceptable): 207, and rank C (unacceptable): 39 patients]. The optimal absolute phase (OAP) significantly correlated with RR [OAP (ms)=119-0.286RR (ms), r=0.832, p<0.0001], and the optimal relative phase (ORP) also significantly correlated with RR [ORP (%)=62-0.023RR (ms), r=0.656, p<0.0001], and the correlation coefficient of OAP was significantly (p<0.0001) higher than that of ORP. The OAP range (±2SD) in which it is highly possible to get a static image was from [119-0.286RR (ms)-46] to [119-0.286RR (ms)+46]. The TR was significantly different among ranks A (97 ± 22 ms), B (111 ± 31 ms) and C (135 ± 34 ms). The TR significantly correlated with RR in ranks A (TR=-16+0.149RR, r=0.767, p<0.0001), B (TR=-15+0.166RR, r=0.646, p<0.0001), and C (TR=52+0.117RR, r=0.425, p=0.0069). Rank C was distinguished from ranks A or B by linear discriminate analysis (TR=-46+0.21RR), and the discriminate rate was 82.6%. In conclusion, both the OAP and adequate TR depend on RR, and the OAP range (±2SD) can be calculated using the formula [119-0.286RR (ms)-46] to [119-0.286RR (ms)+46], and an adequate TR value would be less than (-46+0.21RR).     

3-dimensional magnetic resonance angiography in apnea with the rapid infusion of a paramagnetic contrast medium in studying the thoracic aorta

INTRODUCTION: We investigated the diagnostic accuracy of gadolinium-enhanced 3D MRA in the assessment of thoracic aortic diseases. MATERIAL AND METHODS: Thirty-eight patients with diagnosed or suspected conditions of thoracic aorta were examined with contrast-enhanced MRA. All the examinations were performed with a 1.5 T superconductive magnet acquiring breath-hold 3D fast Gradient-Echo (GE) sequences (TR = 5.9 ms; TE = 1.2 ms; FA = 45 degrees; FOV = 48 cm; thickness = 2-2.5 mm; locs = 30-32; TA = 22-24 s; MA = 512) on the coronal plane. The contrast agent was injected bolus after a bolus-test to evaluate circulation time. RESULTS: Three-dimensional gadolinium-enhanced MRA permitted to correctly diagnose aneurysm in 18 patients, dissection in 13 patients and coarctation in 3 patients. In the former the size and extent of the aneurysmal lumen and its relationship to aortic side branches was demonstrated. As for dissections we evaluated the following parameters: 1) type; 2) presence of intimal flap; 3) thrombosis of the false lumen; 4) dilatation of the aorta; 5) assessment of great vessel origins. MRA data were correlated with those of biplane transesophageal esophageal echocardiography, conventional MRI and spiral CT. In the three patients with aortic coarctation the site of coarctation was correctly identified, the degree of aortic narrowing evaluated and the collateral vessels demonstrated. CONCLUSIONS: In our opinion contrast-enhanced three-dimensional MR angiography should be the screening technique of choice in the evaluation of thoracic aorta thanks to its low invasiveness, short acquisition time, large field of view and morphologic resolution. ECG gating is not needed. Limitations are found in the study of wall and periaortic region which are better evaluated with conventional MR imaging.