Single-breathhold 3D-trueFISP cine cardiac imaging.

Cardiac MRI function measurements are typically based on multiple breathhold 2D sequences to acquire images of the entire heart. In the present study, the feasibility of a cine 3D TrueFISP technique in which several complete volumetric measurements may be obtained during a single breathhold is demonstrated. In contrast to 3D FLASH, the TrueFISP sequence offers an excellent contrast between the myocardium and the intraventricular cavity without the use of contrast agent. An ECG-gated 3D cine TrueFISP sequence was implemented with a repetition time of 2.4-2.8 ms, which allows imaging of the complete heart within a single breathhold throughout 20-46 heartbeats with a 3D frame rate of 8-13 volumes per cardiac cycle and a spatial resolution of about 1.5 x 3.5 x 3.5 mm(3). Breathhold volumetric cine imaging with the 3D TrueFISP technique holds promise for rapid and accurate evaluation of the cardiac regional wall motion and the calculation of cardiac volume and ejection fraction.     

心脏MRI检查中自由呼吸单次激发FIESTA序列的应用

目的:研究SS-SSFP序列在心脏MRI检查中从图像质量和扫描时间方面与IR-FGRE、3D IR-FGRE序列的优势.方法:本文搜集疑似或确诊心脏病患者52例进行心脏MRI检查,延迟期扫描采用SS-SSFP、IR-FGRE、3D IR-FGRE序列.通过测量扫描时间和图像质量相关参数评价新序列与传统序列的差别.结果:SS-SSFP序列(24～36s)和3D IR-FGRE序列(27～35s)扫描时间明显低于IR FGRE序列(116～154s),两者有统计学差异(P＜0.001).统计学结果显示SS-SSFP序列图像质量、异常强化体积及百分比与IR-FGRE序列相比较无差异,而心肌信噪比、强化心肌与心肌对比信噪比有差异,SS-SSFP序列高于IR-FGRE序列(P＜0.001).3D IR-FGRE序列的所有测量值均高于IR-FGRE序列,由于有更多的呼吸运动伪影,二者在图像质量方面有统计学差异(P＜0.001).结论:SS-SSFP序列加快扫描时,由于其较高的信噪比、对比信噪比,且图像质量与IR-FGRE序列无统计学差异,SS-SSFP序列更适合患有严重心脏病和不能屏气的患者.     

Segmented k-space fast cardiac imaging using an echo-train readout.

A segmented k-space fast gradient-echo pulse sequence with an echo-train readout (FGRE-ET) was developed for high-quality cine imaging of the heart in reduced scan times. Using segmented FGRE-ET, cine images of the heart can be acquired in as few as 1-5 heart beats and do not display the geometric distortion and flow-related artifacts typically associated with cardiac echoplanar imaging (EPI). Segmented FGRE-ET was compared with conventional segmented FGRE and with conventional multi-phase EPI in normal volunteers. Segmented FGRE-ET was found to have reduced temporal blurring compared with segmented FGRE for cine imaging in 4 heart beats (P<0.05). Also, segmented FGRE-ET did not display geometric distortion characteristic of conventional EPI (P<0.05). Segmented FGRE-ET may be particularly applicable to functional cardiac stress testing because it allows versatile cine imaging in very short breath-holds.     

Single breathhold cardiac CINE imaging with multi‐echo three‐dimensional hybrid radial SSFP acquisition

Purpose:

To achieve single breathhold whole heart cardiac CINE imaging with improved spatial resolution and temporal resolution by using a multi‐echo three‐dimensional (3D) hybrid radial SSFP acquisition.

Materials and Methods:

Multi‐echo 3D hybrid radial SSFP acquisitions were used to acquire cardiac CINE imaging within a single breathhold. An optimized interleaving scheme was developed for view ordering throughout the cardiac cycle.

Results:

Whole heart short axis views were acquired with a spatial resolution of 1.3 × 1.3 × 8.0 mm³ and temporal resolution of 45 ms, within a single 17 s breathhold. The technique was validated on eight healthy volunteers by measuring the left ventricular volume throughout the cardiac cycle and comparing with the conventional 2D multiple breathhold technique. The left ventricle functional measurement bias of our proposed 3D technique from the conventional 2D technique: end diastolic volume ?3.3 mL ± 13.7 mL, end systolic volume 1.4 mL ± 6.1 mL, and ejection fraction ?1.7% ± 4.3%, with high correlations 0.94, 0.97, and 0.91, accordingly.

Conclusion:

A multi‐echo 3D hybrid radial SSFP acquisition was developed to allow for a whole heart cardiac CINE exam in a single breathhold. Cardiac function measurements in volunteers compared favorably with the standard multiple breathhold exams. J. Magn. Reson. Imaging 2010;32:434–440. © 2010 Wiley‐Liss, Inc.

     

High temporal resolution breathheld 3D FIESTA CINE imaging: validation of ventricular function in patients with chronic myocardial infarction

PURPOSE: To develop a gated single-breathhold, high temporal resolution three-dimensional (3D) CINE imaging technique and to evaluate its accuracy in volumetric and functional quantification in patients with chronic myocardial infarction. MATERIALS AND METHODS: A 3D CINE steady-state free precession (SSFP) pulse sequence was developed incorporating variable temporal sampling of the low and high spatial frequency k-space data to reduce breathhold time and parallel imaging to increase temporal resolution. Reconstruction with retrospective interpolation enabled complete R-R interval coverage. Feasibility was assessed in eight patients with chronic myocardial infarction and ventricular functional values were compared to those of a 2D CINE acquisition. RESULTS: There was no significant difference between the 3D CINE and 2D CINE for end-diastolic volume (168 +/- 73 vs. 177 +/- 59 mL, respectively; P < 0.27), end-systolic volume (81 +/- 62 vs. 79 +/- 53 mL; P < 0.81), and ejection fraction (EF) measurements (55 +/- 14% vs. 58 +/- 14%; P < 0.14). The mean difference in EF was less than 2.5%. A wall motion assessment indicated a good agreement, with a weighted kappa value of 0.62. CONCLUSION: High temporal resolution 3D CINE SSFP imaging of the whole heart can be obtained in a single breathhold and yield ventricular function measurements similar to 2D CINE methods.     

Accelerating cardiac cine 3D imaging using k-t BLAST.

By exploiting spatiotemporal correlations in cardiac acquisitions using k-t BLAST, gated cine 3D acquisitions of the heart were accelerated by a net factor of 4.3, making single breathhold acquisitions possible. Sparse sampling of k-t space along a sheared grid pattern was implemented into a cine 3D SSFP sequence. The acquisition of low-resolution training data, which was required to resolve aliasing in the k-t BLAST method, was either interleaved into the sampling process or obtained in a separate prescan to allow for shorter breathhold durations in patients with heart disease. Volumetric datasets covering the heart with 20 slices at a spatial resolution of 2 x 2 x 5 mm3 were recorded with 20 cardiac phases in a total breathhold duration of 25-27 sec, or 18 sec if partial Fourier sampling was additionally employed. The feasibility of the method was demonstrated on healthy volunteers and on patients. The comparison of endocardial area derived from single slices of the 3D dataset with values extracted from separate single-slice acquisitions showed no significant differences. By shortening the acquisition substantially, k-t BLAST may greatly facilitate volumetric imaging of the heart for evaluation of regional wall motion and the assessment of ventricular volume and ejection fraction.     

Head-to-head comparison of eight late gadolinium-enhanced cardiac MR (LGE CMR) sequences at 1.5 tesla: from bench to bedside

Purpose:

To compare—theoretically and experimentally—clinically available two‐dimensional/three‐dimensional (2D/3D), breathhold and non‐breathhold, inversion‐recovery (IR) gradient‐echo (GRE) sequences used to differentiate between nonviable injured and normal myocardium with late gadolinium‐enhanced techniques (IR‐GRE2D sequence is used as a reference), and to evaluate their respective clinical benefit.

Materials and Methods:

Six breathhold (2D‐IR‐GRE, 3D‐IR‐GRE, balanced steady‐state free precession 2D‐IR‐bSSFP and 3D‐IR‐bSSFP, phase‐sensitive 2D‐PSIR‐GRE, and 2D‐PSIR‐bSSFP) and two non‐breathhold late gadolinium‐enhanced techniques (single‐shot 2D‐ssbSSFP and 2D‐PSIR‐ssbSSFP) were consecutively performed in 32 coronary artery disease patients with chronic myocardial infarction. Qualitative assessment and manual planimetry were performed by two independent observers. Quantitative assessment was based on percentage signal intensity elevation between injured and normal myocardium and contrast‐to‐noise ratio. Theoretical simulations were compared with experimental measurements performed on phantoms with various concentrations of gadolinium.

Results:

The 3D‐IR‐GRE image quality appeared better than the other 2D and 3D sequences, showing better delineation of complex nontransmural lesions, with significantly higher percentage signal intensity and contrast‐to‐noise ratio. PSIR techniques appeared more limited in differentiating sub‐endocardial lesions and intracavity blood pool, but in all other cases were comparable to the other techniques. Single‐shot PSIR‐ssbSSFP appeared to be a valuable alternative technique when breathhold cannot be achieved.

Conclusion:

We recommend 3D‐IR‐GRE as the method of choice for late gadolinium‐enhanced cardiac magnetic resonance imaging in clinical practice. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.     

Four-dimensional single breathhold magnetic resonance imaging using kt-BLAST enables reliable assessment of left- and right-ventricular volumes and mass

PURPOSE: To prospectively determine the accuracy of four-dimensional (4D) kt-broad-use linear acquisition speed-up technique (BLAST) accelerated MRI (kt-BLAST) for the assessment of left-ventricular (LV) volumes and mass as well as right-ventricular (RV) volumes in comparison to standard multiple breathhold cine imaging. MATERIALS AND METHODS: A total of 40 patients with suspected or known coronary artery disease (CAD) underwent cardiac MRI. In each patient a standard multislice cine steady-state free precession (SSFP) sequence was performed with complete ventricular coverage during multiple breathholds. Additionally, a kt-BLAST-accelerated 4D sequence with complete ventricular coverage was acquired during one single breathhold. For comparison of SSFP and kt-BLAST, the following LV parameters were determined: end-diastolic and end-systolic volumes, ejection fraction, end-diastolic diameter and mass. For comparison of RV dimensions, end-diastolic and end-systolic volumes and ejection fraction were assessed. RESULTS: LV volumes, ejection fraction, diameter, and mass showed a strong correlation between SSFP and kt-BLAST (r=0.98-0.99; P<0.01). In addition, RV parameters demonstrated a high correlation (r=0.97-0.98; P<0.01). For all parameters, the calculated bias between both methods was found to be minimal (0.4-4%). CONCLUSION: 4D kt-BLAST-accelerated MRI enabled the accurate assessment of LV and RV quantitative parameters during one single breathhold when compared to standard multislice, multiple breathhold SSFP imaging.     

Single breath-hold 3D contrast-enhanced method for assessment of cardiac function.

Cardiac MRI function measurements are typically performed using 2D sequences and require multiple breath-holds to image the entire heart. A single 3D acquisition using a T(1)-shortening agent has many potential advantages over techniques that acquire multiple 2D images, including more consistent contrast and precise slice coverage. However, 3D techniques currently require much longer than a single breath-hold to complete. It has been shown that for MR angiography undersampled projection reconstruction can acquire much higher resolution per unit time than Fourier imaging with acceptable artifacts. By employing a gated, undersampled projection technique, high-resolution 3D multiphase volumes of the heart can be acquired in a single breath-hold. Short repetition times result in good myocardial suppression and a temporal aperture of 60 ms.     

Coronary MRA with 3D undersampled projection reconstruction TrueFISP.

Undersampled projection reconstruction (PR) offers improved imaging efficiency allowing a relative tradeoff between signal-to-noise ratio (SNR) or streak artifact and the number of acquired k-space views rather than the tradeoff between resolution or aliasing artifact and the number of acquired k-space lines inherent to Fourier imaging techniques. TrueFISP (true fast imaging with steady state precession) is ideally suited for undersampled PR imaging because of its inherently high SNR. The purpose of this work was to investigate the feasibility of using undersampled three-dimensional (3D) PR TrueFISP for breathhold coronary artery imaging. Phantom studies and a preliminary in vivo comparison demonstrated the improved spatial resolution of PR over Fourier TrueFISP with the same imaging time. In a 24-heartbeat coronary imaging scheme, segmented 3D PR TrueFISP provided a 1.0 x 1.0 mm(2) isotropic in-plane voxel size while acquiring between 93 and 153 views per partition. Streak artifacts were present in some images but were not found to impede coronary artery delineation. In conclusion, 3D undersampled PR TrueFISP provides isotropic in-plane voxel size within a single breathhold and is a promising technique for coronary artery imaging.     

Evaluating contrast kinetics by acquiring 2D images during 3D contrast-enhanced MR angiography

PURPOSE: To monitor contrast kinetics by acquiring multiple 2D images during 3D contrast-enhanced magnetic resonance angiography (CE MRA). MATERIALS AND METHODS: A 2D real-time autotriggering tool was integrated into a 3D sequence, enabling it to run multiple times during 3D acquisition. Several dummy scans were applied after each transition to maintain the steady state condition of both sequences. The number of the acquired 2D images and their distribution can be adjusted. Each 2D image was saved along with its associated timing. Contrast signal variations over time were plotted, reflecting selective signal measurement over an artery and vein from the saved 2D images. RESULTS: Different contrast kinetics timings were calculated from the resulting plot. Contrast arrival time to the internal cerebral artery was 13.2 +/- 1.2 seconds and the peak arterial to peak venous (at the confluence of sinuses) enhancement was 6.7 +/- 0.6 seconds. The observed timing could be used for 3D sequence optimization; the saved 2D images are useful in detecting and characterizing vascular abnormalities. CONCLUSION: Integrating 2D and 3D sequences into one sequence to monitor contrast kinetics through the neurovasculature is feasible without the need for extra injections or reduced spatial resolution. The technique can also be used in different parts of the body to extract useful clinical information.     

Cardiac cine MRI: Quantification of the relationship between fast gradient echo and steady-state free precession for determination of myocardial mass and volumes

PURPOSE: To determine the correlation function between the steady-state free precession (SSFP) and fast gradient echo (FGRE) cine MRI pulse sequences for measuring the myocardial mass and volumes. MATERIALS AND METHODS: Cardiac cine MRI examinations were acquired in 50 individuals (female: 35, male: 15, mean age 64.1 +/- 9.1 years, range 48-83) using SSFP and FGRE cardiac pulse sequences. RESULTS: The mean (standard deviation [SD]) left ventricular end diastolic volume measured by SSFP was significantly larger (4.5%) than by FGRE (P < 0.001); this was also the case for end systolic volume (15.0%, P < 0.001). The relationship between SSFP and FGRE measures were linear and highly correlated (P < 0.001) for both left ventricular end diastolic and end systolic volumes (r(2) = 0.90 vs. 0.91, respectively). We determined linear regression models to estimate the SSFP values based on the FGRE measures. Slope (intercept) for ejection fraction, stroke volume, and cardiac output were 0.99 (-2.79), 0.77 (17.5), and 0.76 (1.29), respectively. CONCLUSION: Linear relationships exist for key LV function parameters when comparing SSFP and FGRE cine MRI. These results indicate that existing databases and normal values for FGRE LV function may be converted to corresponding LV function values for SSFP MRI.     

Time-resolved 3D MR angiography of the abdomen with a real-time system.

This study reports on the implementation of a real-time MR acquisition, reconstruction, and display system on standard hardware for 3D contrast-enhanced MR abdominal angiography. The system allows for dynamic imaging of the contrast passage with good spatial resolution in a single breathhold. It is capable of synchronously acquiring and processing multiple coil data. Bolus arrival can be detected confidently from the display of MIP images from sagittal 3D slabs with moderate spatial resolution. Upon detection of the contrast agent, the patient is asked to hold their breath and a novel 3D acquisition is started for a coronal volume. Temporal imaging within the breathhold is achieved through applying a modified time-resolved imaging of contrast kinetics (TRICKS) technique with elliptical centric view ordering. The technique displays contrast passage from the arterial phase through enhancement of the hepatic venous system. It also provides the ability to quantify motion of the diaphragm over a long breathhold.     

Application of refocused steady-state free-precession methods at 1.5 and 3 T to in vivo high-resolution MRI of trabecular bone: simulations and experiments

PURPOSE: To evaluate the potential of fully-balanced steady-state free-precession (SSFP) sequences in in vivo high-resolution (HR) MRI of trabecular bone at field strengths of 1.5 and 3 T by simulation and experimental methods. MATERIALS AND METHODS: Using simulation studies, refocused SSFP acquisition was optimized for our imaging purposes with a focus on signal-to-noise ratio (SNR) and SNR efficiency. The signal behavior in trabecular bone was estimated using a magnetostatic model of the trabecular bone and marrow. Eight normal volunteers were imaged at the proximal femur, calcaneus, and the distal tibia on a GE Signa scanner at 1.5 and at 3 T with an optimized single-acquisition SSFP sequence (three-dimensional FIESTA) and an optimized multiple-acquisition SSFP sequence (three-dimensional FIESTA-c). Images were also acquired with a fast gradient echo (FGRE) sequence for evaluation of the SNR performance of SSFP methods. RESULTS: Refocused SSFP images outperformed FGRE acquisitions in both SNR and SNR efficiency at both field strengths. At 3 T, susceptibility effects were visible in FIESTA and FGRE images and much reduced in FIESTA-c images. The magnitude of SNR boost at 3 T was closely predicted by simulations. CONCLUSION: Single-acquisition SSFP (at 1.5 T) and multiple-acquisition SSFP (at 3 T) hold great potential for HR-MRI of trabecular bone.     

Multiple breath-hold averaging (mba) method for increased snr in abdominal mri

Breath-holding during MR imaging eliminates respiratory motion artifacts but places a major time constraint on data acquisition. This constraint limits image signal-to-noise ratio and hence spatial resolution. A new method, multiple breathhold averaging, is presented that overcomes these time limitations. Several images are acquired in sequential breath-hold periods, separated by periods of normal breathing, and averaged. This averaged image shows the expected increase in SNR with surprisingly little blurring due to misregistration. SNR improvements can be traded for increased spatial resolution. The MBA methodology can also be applied to 3D data acquisitions, dynamic contrast acquisitions, and image subtractions.     

Phase-sensitive inversion recovery single-shot balanced steady-state free precession for detection of myocardial infarction during a single breathhold

RATIONALE AND OBJECTIVES: We sought to show that phase-sensitive detection and a single-shot technique allow imaging of the heart for detection of myocardial infarction during a single breathhold without adaptation of the inversion time. MATERIALS AND METHODS: Thirty-five patients at 2 weeks to 3 months after Q-wave myocardial infarction were examined on a 1.5-T MR system 10 minutes after the administration of a double-dose extravascular contrast agent. In order to determine the optimal inversion recovery time (TI), a TI scout sequence was performed. An IR-turboFlash sequence with optimized TI was used as standard of reference. A phase-sensitive inversion recovery (PSIR) single-shot TrueFISP sequence, which allows imaging of nine slices during one breathhold (TR/TE/FA/BW: 2.2 ms/1.1 ms/60 degrees , 8 degrees /1220 Hz/Px) was used with a nominal TI of 200 ms. Spatial resolution was identical for both techniques: 1.3 mm x 1.8 mm x 8 mm. Infarct volumes, area of infarction on a selected slice, and scan time for imaging delayed contrast enhancement (DCE) were compared. RESULTS: The mean values for the time of imaging DCE were 10 minutes 43 seconds for the IR turboFLASH and 17 seconds (P<.001) for the PSIR single-shot TrueFISP sequence. No significant difference was found for the mean values of the infarct volumes with 18.7 ml (IR turboFLASH) and 17.3 ml (PSIR single-shot TrueFISP). The values for the correlation coefficients of the infarct volumes and infarct areas of the two different techniques were r=0.95 (P<.004) and r=0.97 (P<.002). The regression equations were y=0.76+0.92*x and y=0.07+0.93*x, respectively. CONCLUSIONS: PSIR single-shot TrueFISP allows for accurate identification of myocardial infarction during a single breathhold with reduction of scan time by a factor of 38.     

Myocardial tagging with SSFP.

This work presents the first implementation of myocardial tagging with refocused steady-state free precession (SSFP) and magnetization preparation. The combination of myocardial tagging (a noninvasive method for quantitative measurement of regional and global cardiac function) with the high tissue signal-to-noise ratio (SNR) obtained with SSFP is shown to yield improvements in terms of the myocardium-tag contrast-to-noise ratio (CNR) and tag persistence when compared to the current standard fast gradient-echo (FGRE) tagging protocol. Myocardium-tag CNR and tag persistence were studied using numerical simulations as well as phantom and human experiments. Both quantities were found to decrease with increasing imaging flip angle (alpha) due to an increased tag decay rate and a decrease in myocardial steady-state signal. However, higher alpha yielded better blood-myocardium contrast, indicating that optimal alpha is dependent on the application: higher alpha for better blood-myocardium boundary visualization, and lower alpha for better tag persistence. SSFP tagging provided the same myocardium-tag CNR as FGRE tagging when acquired at four times the bandwidth and better tag- and blood-myocardium CNRs than FGRE tagging when acquired at equal or twice the receiver bandwidth (RBW). The increased acquisition efficiency of SSFP allowed decreases in breath-hold duration, or increases in temporal resolution, as compared to FGRE.     

Navigator-gated three-dimensional MR angiography of the pulmonary arteries using steady-state free precession

PURPOSE: To assess the quality of a navigator-gated, free breathing, steady-state free precession (SSFP) technique in comparison to a single breathhold for pulmonary artery imaging in normal volunteers. MATERIALS AND METHODS: Sagittal sections of the left pulmonary arteries of 10 volunteers were obtained with a three-dimensional SSFP sequence using both a single breathhold of 30 seconds and a navigator-gated version of the same sequence. The images were compared and rated by a blinded cardiovascular radiologist for image quality, sharpness, and artifact. RESULTS: On a scale ranging from -2 to 2, in which positive numbers denote that the navigator method was favorable compared to the single breathhold method, image quality was rated 0.7+/-1.4, sharpness 0.6+/-1.5, and artifact 0.1+/-1.4. Thus, there was no statistical difference between the two methods. CONCLUSION: The navigator-gated SSFP sequence is able to acquire images equal in quality to the breathhold sequence. This may be of clinical importance for pulmonary imaging in patients who are unable to sustain a long breathhold.     

动态增强容积内插序列在肝脏占位性病变的临床应用价值

目的:评价MRI三维动态增强容积内插序列在肝脏局灶性病变的临床应用价值.方法:91例肝脏占位性病变患者进行常规MR T1WI和T2WI扫描后,采用三维扰相梯度序列行屏气全肝3期动态增强扫描并进行图像重组,观察病灶的增强特点,并对肝动脉的显示程度进行分级.结果:91例中原发性肝癌17例,肝血管瘤24例,肝转移性肿瘤16例,局灶性结节增生2例,肝脓肿11例,肝囊肿21例.肝动脉显示为2级86例94.5%,1级3例3.3%,0级2例2.2%.结论:MR 动态增强容积内插技术可以获得高质量的图像(尤其是动脉期),有利于肝脏局灶性病变的定性、定位诊断和指导临床治疗.     

Numerical simulation of in vitro pulsatile flow and its study using FRISK, a rapid phase contrast technique

PURPOSE: To test the potential of a phase contrast magnetic resonance (PC-MR) sparse sampling technique, fragmented regional interpolation segmentation for k-space (FRISK), to capture complex flow features within a breathhold duration by using numerical simulations and experimental approaches. MATERIALS AND METHODS: Computational fluid dynamics (CFD) data of three models were generated: a two-chamber orifice flow model simulating valvular regurgitation, a femoral artery model, and a U-shaped model simulating the aortic arch. These data were used to simulate conventional and FRISK PC-MR data acquisitions. FRISK parameters can be adapted for different flow fields to capture either high temporal information or complexly varying spatial information with a temporal component or a mixture of both. In vivo PC-MR images on a healthy volunteer were sampled to compare conventional PC-MR with novel FRISK imaging. RESULTS: In our simulations of three representative models, when only the errors from different sampling sequences were considered, FRISK was shown to maintain or even improve data accuracy while cutting the scan time by at least 50% compared to corresponding conventional PC-MR. By adapting the FRISK parameters for flowfields with different features, FRISK was capable of capturing in-plane and through-plane velocity information with excellent detail in approximately 20 heartbeats breathhold duration. The results of the in vivo MR experiment were consistent with the simulation results, showing that breathhold FRISK imaging improved spatial resolution of the data and maintained adequate temporal resolution compared with breathhold conventional imaging. CONCLUSION: FRISK, a new MRI sampling sequence that sparsely samples data and aligns acquired data during postprocessing, provides a scan time advantage of approximately a factor of 2 compared to conventional scans, and allowed rapid or breathhold scanning while obtaining acceptable accuracy.