Real‐time flow with fast GPU reconstruction for continuous assessment of cardiac output

Purpose:

To demonstrate the feasibility of real‐time phase contrast magnetic resonance (PCMR) assessment of continuous cardiac output with a heterogeneous (CPU/GPU) system for online image reconstruction.

Materials and Methods:

Twenty healthy volunteers underwent aortic flow examination during exercise using a real‐time spiral PCMR sequence. Acquired data were reconstructed in online fashion using an iterative sensitivity encoding (SENSE) algorithm implemented on an external computer equipped with a GPU card. Importantly, data were sent back to the scanner console for viewing. A multithreaded CPU implementation of the real‐time PCMR reconstruction was used as a reference point for the online GPU reconstruction assessment and validation. A semiautomated segmentation and registration algorithm was applied for flow data analysis.

Results:

There was good agreement between the GPU and CPU reconstruction (?0.4 ± 0.8 mL). There was a significant speed‐up compared to the CPU reconstruction (15×). This translated into the flow data being available on the scanner console ≈9 seconds after acquisition finished. This compares to an estimated time using the CPU implementation of 83 minutes.

Conclusion:

Our heterogeneous image reconstruction system provides a base for translation of complex MRI algorithms into clinical workflow. We demonstrated its feasibility using real‐time PCMR assessment of continuous cardiac output as an example. J. Magn. Reson. Imaging 2012; 36:1477–1482. © 2012 Wiley Periodicals, Inc.

     

High spatial and temporal resolution cardiac cine MRI from retrospective reconstruction of data acquired in real time using motion correction and resorting

Cine MRI is used for assessing cardiac function and flow and is typically based on a breath‐held, segmented data acquisition. Breath holding is particularly difficult for patients with congestive heart failure or in pediatric cases. Real‐time imaging may be used without breath holding or ECG triggering. However, despite the use of rapid imaging sequences and accelerated parallel imaging, real‐time imaging typically has compromised spatial and temporal resolution compared with gated, segmented breath‐held studies. A new method is proposed that produces a cardiac cine across the full cycle, with both high spatial and temporal resolution from a retrospective reconstruction of data acquired over multiple heartbeats during free breathing. The proposed method was compared with conventional cine images in 10 subjects. The resultant image quality for the proposed method (4.2 ± 0.4) without breath holding or gating was comparable to the conventional cine (4.4 ± 0.5) on a five‐point scale (P = n.s.). Motion‐corrected averaging of real‐time acquired cardiac images provides a means of attaining high‐quality cine images with many of the benefits of real‐time imaging, such as free‐breathing acquisition and tolerance to arrhythmias. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

High efficiency multishot interleaved spiral‐in/out: Acquisition for high‐resolution BOLD fMRI

Growing demand for high spatial resolution blood oxygenation level dependent (BOLD) functional magnetic resonance imaging faces a challenge of the spatial resolution versus coverage or temporal resolution tradeoff, which can be addressed by methods that afford increased acquisition efficiency. Spiral acquisition trajectories have been shown to be superior to currently prevalent echo‐planar imaging in terms of acquisition efficiency, and high spatial resolution can be achieved by employing multiple‐shot spiral acquisition. The interleaved spiral in/out trajectory is preferred over spiral‐in due to increased BOLD signal contrast‐to‐noise ratio (CNR) and higher acquisition efficiency than that of spiral‐out or noninterleaved spiral in/out trajectories (Law & Glover. Magn Reson Med 2009; 62:829–834.), but to date applicability of the multishot interleaved spiral in/out for high spatial resolution imaging has not been studied. Herein we propose multishot interleaved spiral in/out acquisition and investigate its applicability for high spatial resolution BOLD functional magnetic resonance imaging. Images reconstructed from interleaved spiral‐in and ‐out trajectories possess artifacts caused by differences in T2* decay, off‐resonance, and k‐space errors associated with the two trajectories. We analyze the associated errors and demonstrate that application of conjugate phase reconstruction and spectral filtering can substantially mitigate these image artifacts. After applying these processing steps, the multishot interleaved spiral in/out pulse sequence yields high BOLD CNR images at in‐plane resolution below 1 × 1 mm while preserving acceptable temporal resolution (4 s) and brain coverage (15 slices of 2 mm thickness). Moreover, this method yields sufficient BOLD CNR at 1.5 mm isotropic resolution for detection of activation in hippocampus associated with cognitive tasks (Stern memory task). The multishot interleaved spiral in/out acquisition is a promising technique for high spatial resolution BOLD functional magnetic resonance imaging applications. Magn Reson Med 70:420–428, 2013. © 2012 Wiley Periodicals, Inc.     

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.

     

Highly accelerated real‐time cardiac cine MRI using k–t SPARSE‐SENSE

For patients with impaired breath‐hold capacity and/or arrhythmias, real‐time cine MRI may be more clinically useful than breath‐hold cine MRI. However, commercially available real‐time cine MRI methods using parallel imaging typically yield relatively poor spatio‐temporal resolution due to their low image acquisition speed. We sought to achieve relatively high spatial resolution (～2.5 × 2.5 mm²) and temporal resolution (～40 ms), to produce high‐quality real‐time cine MR images that could be applied clinically for wall motion assessment and measurement of left ventricular function. In this work, we present an eightfold accelerated real‐time cardiac cine MRI pulse sequence using a combination of compressed sensing and parallel imaging (k‐t SPARSE‐SENSE). Compared with reference, breath‐hold cine MRI, our eightfold accelerated real‐time cine MRI produced significantly worse qualitative grades (1–5 scale), but its image quality and temporal fidelity scores were above 3.0 (adequate) and artifacts and noise scores were below 3.0 (moderate), suggesting that acceptable diagnostic image quality can be achieved. Additionally, both eightfold accelerated real‐time cine and breath‐hold cine MRI yielded comparable left ventricular function measurements, with coefficient of variation <10% for left ventricular volumes. Our proposed eightfold accelerated real‐time cine MRI with k–t SPARSE‐SENSE is a promising modality for rapid imaging of myocardial function. J. Magn. Reson. Imaging 2013. © 2012 Wiley Periodicals, Inc.     

First‐pass contrast‐enhanced myocardial perfusion MRI in mice on a 3‐T clinical MR scanner

First‐pass contrast‐enhanced myocardial perfusion MRI in rodents has so far not been possible due to the temporal and spatial resolution requirements. We developed a new first‐pass perfusion MR method for rodent imaging on a clinical 3.0‐T scanner (Philips Healthcare, Best, The Netherlands) that employed 10‐fold k‐space and time domain undersampling with constrained image reconstruction, using temporal basis sets (k‐t principle component analysis) to achieve a spatial resolution of 0.2 × 0.2 × 1.5mm³ and an acquisition window of 43 msec. The method was successfully tested in five healthy and four infarcted mice (C57BL/6J) at heart rates of 495.1 ± 45.8 beats/min. Signal‐intensity‐time profiles showed a percentage myocardial signal increase of 141.3 ± 38.9% in normal mice, compared with 44.7 ± 32.4% in infarcted segments. Mean myocardial blood flow by Fermi function for constrained deconvolution in control mice was 7.3 ± 1.5 mL/g/min, comparable to published literature, with no significant differences between three myocardial segments. In infarcted segments, myocardial blood flow was significantly reduced to 1.2 ± 0.8 mL/g/min (P < 0.01). This is the first report of first‐pass myocardial perfusion MR in a mouse model on a clinical 3‐T MR scanner and using a k‐t undersampling method. Data were acquired on a 3‐T scanner, using an approach similar to clinical acquisition protocols, thus facilitating translation of imaging findings between rodent and human studies. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Myocardial perfusion MRI with sliding‐window conjugate‐gradient HYPR

First‐pass perfusion MRI is a promising technique for detecting ischemic heart disease. However, the diagnostic value of the method is limited by the low spatial coverage, resolution, signal‐to‐noise ratio (SNR), and cardiac motion‐related image artifacts. In this study we investigated the feasibility of using a method that combines sliding window and CG‐HYPR methods (SW‐CG‐HYPR) to reduce the acquisition window for each slice while maintaining the temporal resolution of one frame per heartbeat in myocardial perfusion MRI. This method allows an increased number of slices, reduced motion artifacts, and preserves the relatively high SNR and spatial resolution of the “composite images.” Results from eight volunteers demonstrate the feasibility of SW‐CG‐HYPR for accelerated myocardial perfusion imaging with accurate signal intensity changes of left ventricle blood pool and myocardium. Using this method the acquisition time per cardiac cycle was reduced by a factor of 4 and the number of slices was increased from 3 to 8 as compared to the conventional technique. The SNR of the myocardium at peak enhancement with SW‐CG‐HYPR (13.83 ± 2.60) was significantly higher (P < 0.05) than the conventional turbo‐FLASH protocol (8.40 ± 1.62). Also, the spatial resolution of the myocardial perfection images was significantly improved. SW‐CG‐HYPR is a promising technique for myocardial perfusion MRI. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Single‐shot magnetic resonance spectroscopic imaging with partial parallel imaging

A magnetic resonance spectroscopic imaging (MRSI) pulse sequence based on proton–echo‐planar‐spectroscopic‐imaging (PEPSI) is introduced that measures two‐dimensional metabolite maps in a single excitation. Echo‐planar spatial–spectral encoding was combined with interleaved phase encoding and parallel imaging using SENSE to reconstruct absorption mode spectra. The symmetrical k‐space trajectory compensates phase errors due to convolution of spatial and spectral encoding. Single‐shot MRSI at short TE was evaluated in phantoms and in vivo on a 3‐T whole‐body scanner equipped with a 12‐channel array coil. Four‐step interleaved phase encoding and fourfold SENSE acceleration were used to encode a 16 × 16 spatial matrix with a 390‐Hz spectral width. Comparison with conventional PEPSI and PEPSI with fourfold SENSE acceleration demonstrated comparable sensitivity per unit time when taking into account g‐factor–related noise increases and differences in sampling efficiency. LCModel fitting enabled quantification of inositol, choline, creatine, and N‐acetyl‐aspartate (NAA) in vivo with concentration values in the ranges measured with conventional PEPSI and SENSE‐accelerated PEPSI. Cramer–Rao lower bounds were comparable to those obtained with conventional SENSE‐accelerated PEPSI at the same voxel size and measurement time. This single‐shot MRSI method is therefore suitable for applications that require high temporal resolution to monitor temporal dynamics or to reduce sensitivity to tissue movement. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Time‐resolved angiography: Past,present, and future

The introduction of digital subtraction angiography (DSA) in 1980 provided a method for real time 2D subtraction imaging. Later, 4D magnetic resonance (MR) angiography emerged beginning with techniques like Keyhole and time‐resolved imaging of contrast kinetics (TRICKS) that provided frame rates of one every 5 seconds with limited spatial resolution. Undersampled radial acquisition was subsequently developed. The 3D vastly undersampled isotropic projection (VIPR) technique allowed undersampling factors of 30–40. Its combination with phase contrast displays time‐resolved flow dynamics within the cardiac cycle and has enabled the measurement of pressure gradients in small vessels. Meanwhile similar accelerations were achieved using Cartesian acquisition with projection reconstruction (CAPR), a Cartesian acquisition with 2D parallel imaging. Further acceleration is provided by constrained reconstruction techniques such as highly constrained back‐projection reconstruction (HYPR) and its derivatives, which permit acceleration factors approaching 1000. Hybrid MRA combines a separate phase contrast, time‐of flight, or contrast‐enhanced acquisition to constrain the reconstruction of contrast‐enhanced time frames providing exceptional spatial and temporal resolution and signal‐to‐noise ratio (SNR). This can be extended to x‐ray imaging where a 3D DSA examination can be used to constrain the reconstruction of time‐resolved 3D volumes. Each 4D DSA (time‐resolved 3D DSA) frame provides spatial resolution and SNR comparable to 3D DSA, thus removing a major limitation of intravenous DSA. Similar techniques have provided the ability to do 4D fluoroscopy. J. Magn. Reson. Imaging 2012; 36:1273–1286. © 2012 Wiley Periodicals, Inc.     

Time‐resolved absolute velocity quantification with projections

Quantitative information on time‐resolved blood velocity along the femoral/popliteal artery can provide clinical information on peripheral arterial disease and complement MR angiography as not all stenoses are hemodynamically significant. The key disadvantages of the most widely used approach to time‐resolve pulsatile blood flow by cardiac‐gated velocity‐encoded gradient‐echo imaging are gating errors and long acquisition time. Here, we demonstrate a rapid nontriggered method that quantifies absolute velocity on the basis of phase difference between successive velocity‐encoded projections after selectively removing the background static tissue signal via a reference image. The tissue signal from the reference image's center k‐space line is isolated by masking out the vessels in the image domain. The performance of the technique, in terms of reproducibility and agreement with results obtained with conventional phase contrast‐MRI was evaluated at 3 T field strength with a variable‐flow rate phantom and in vivo of the triphasic velocity waveforms at several segments along the femoral and popliteal arteries. Additionally, time‐resolved flow velocity was quantified in five healthy subjects and compared against gated phase contrast‐MRI results. To illustrate clinical feasibility, the proposed method was shown to be able to identify hemodynamic abnormalities and impaired reactivity in a diseased femoral artery. For both phantom and in vivo studies, velocity measurements were within 1.5 cm/s, and the coefficient of variation was less than 5% in an in vivo reproducibility study. In five healthy subjects, the average differences in mean peak velocities and their temporal locations were within 1 cm/s and 10 ms compared to gated phase contrast‐MRI. In conclusion, the proposed method provides temporally resolved arterial velocity with a temporal resolution of 20 ms with minimal post processing. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Fast gated EPR imaging of the beating heart: Spatiotemporally resolved 3D imaging of free‐radical distribution during the cardiac cycle

In vivo or ex vivo electron paramagnetic resonance imaging (EPRI) is a powerful technique for determining the spatial distribution of free radicals and other paramagnetic species in living organs and tissues. However, applications of EPRI have been limited by long projection acquisition times and the consequent fact that rapid gated EPRI was not possible. Hence in vivo EPRI typically provided only time‐averaged information. In order to achieve direct gated EPRI, a fast EPR acquisition scheme was developed to decrease EPR projection acquisition time down to 10–20 ms, along with corresponding software and instrumentation to achieve fast gated EPRI of the isolated beating heart with submillimeter spatial resolution in as little as 2–3 min. Reconstructed images display temporal and spatial variations of the free‐radical distribution, anatomical structure, and contractile function within the rat heart during the cardiac cycle. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Dynamic visualization of arachnoid adhesions in a patient with idiopathic syringomyelia using high‐resolution cine magnetic resonance imaging at 3T

A 39‐year‐old female patient with thoracic syringomyelia underwent routine magnetic resonance imaging (MRI) and 3 T MRI to investigate the value of retrospectively cardiac‐gated cine steady‐state free precession (SSFP) MRI in the preoperative and postoperative diagnosis of arachnoid membranes in the spinal subarachnoid space. Therefore, 3T MRI included sagittal and transverse retrospectively cardiac‐gated cine balanced fast‐field echo (balanced‐FFE) sequences both preoperatively and after microsurgical lysis of arachnoid adhesions and expansive duraplasty. Arachnoid membranes were detected and this result was correlated with intraoperative findings and the results of routine cardiac‐gated phase‐contrast cerebrospinal fluid (CSF) flow MRI. Retrospectively cardiac‐gated cine SSFP MRI enabled imaging of arachnoid membranes with high spatial resolution and sufficient contrast to delineate them from hyperintense CSF preoperatively and postoperatively. The images were largely unaffected by artifacts. Surgery confirmed the presence of arachnoid adhesions in the upper thoracic spine. Not all arachnoid membranes that were seen on cine balanced‐FFE sequences caused significant spinal CSF flow blockages in cardiac‐gated phase‐contrast CSF flow studies. In conclusion, retrospectively cardiac‐gated cine SSFP MRI may become a valuable tool for the preoperative detection of arachnoid adhesions and the postoperative evaluation of microsurgical adhesiolysis in patients with idiopathic syringomyelia. J. Magn. Reson. Imaging 2010. © 2010 Wiley‐Liss, Inc.     

Time‐resolved bolus‐chase MR angiography with real‐time triggering of table motion

Time‐resolved bolus‐chase contrast‐enhanced MR angiography with real‐time station switching is demonstrated. The Cartesian acquisition with projection reconstruction‐like sampling (CAPR) technique and high 2D sensitivity encoding (SENSE) (6× or 8×) and 2D homodyne (1.8×) accelerations were used to acquire 3D volumes with 1.0‐mm isotropic spatial resolution and frame times as low as 2.5 sec in two imaging stations covering the thighs and calves. A custom real‐time system was developed to reconstruct and display CAPR frames for visually guided triggering of table motion upon passage of contrast through the proximal station. The method was evaluated in seven volunteers. High‐spatial‐resolution arteriograms with minimal venous contamination were consistently acquired in both stations. Real‐time stepping table contrast‐enhanced MR angiography is a method for providing time‐resolved images with high spatial resolution over an extended field‐of‐view. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Increasing spatial coverage for high‐resolution functional MRI

For high‐resolution functional MRI (fMRI) studies, one of the major challenges is limited spatial coverage, which results because of the tradeoffs between temporal resolution and covering more k‐space. Given the same temporal resolution, fewer slices can be collected for high‐resolution fMRI. If the number of slices is not large enough to cover all regions of interest, additional scans may become necessary, which increases both the total scan time and the complexity of interpreting data. In this work, we propose a method that combines the undersampled 3D stack‐of‐spirals acquisition method with the UNFOLD technique to significantly increase the spatial coverage for high‐resolution fMRI. Undersampling allows more slices to be fit into a given temporal resolution. The signal‐to‐noise ratio (SNR) drop associated with undersampling can be compensated by the increase in the excited volume in 3D acquisitions. Theoretical analysis shows that although there is a negligible increase in temporal coherence due to spectral filtering in the UNFOLD technique, twice the spatial coverage can be achieved given the same total scan time and similar quality of activation maps, which was confirmed by experiments on normal subjects. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Single breath‐hold assessment of ventricular volumes using 32‐channel coil technology and an extracellular contrast agent

Purpose:

To evaluate the feasibility of a single breath‐hold 3D cine balanced steady‐state free precession (b‐SSFP) sequence after gadolinium diethylenetriamine penta‐acetic acid (Gd‐DTPA) injection for volumetric cardiac assessment.

Materials and Methods:

Fifteen adult patients routinely referred for cardiac magnetic resonance imaging (MRI) underwent quantitative ventricular volumetry on a clinical 1.5T MR‐scanner using a 32‐channel cardiac coil. A stack of 2D cine b‐SSFP slices covering the ventricles was used as reference, followed by a single breath‐hold 3D cine balanced SSFP protocol acquired before and after administration of Gd‐DTPA. The acquisition was accelerated using SENSE in both phase encoding directions. Volumetric and contrast‐to‐noise data for each technique were assessed and compared.

Results:

The 3D cine protocol was accomplished within one breath‐hold (mean acquisition time 20 sec; spatial resolution 2.1 × 2.1 × 10 mm; temporal resolution 51 msec). The contrast‐to‐noise ratio between blood and myocardium was 234 determined for the multiple 2D cine data, and could be increased for the 3D acquisition from 136 (3D precontrast) to 203 (3D postcontrast) after injecting Gd‐DTPA. In addition the endocardial definition was significantly improved in postcontrast 3D cine b‐SSFP. There was no significant difference for left and right ventricular volumes between standard 2D and 3D postcontrast cine b‐SSFP. However, Bland–Altman plots showed greater bias and scatter when comparing 2D with 3D cine b‐SSFP without contrast.

Conclusion:

3D cine b‐SSFP imaging of the heart using 32 channel coil technology and spatial undersampling allows reliable volumetric assessment within a single breath‐hold after application of Gd‐DTPA. J. Magn. Reson. Imaging 2010;31:838–844. ©2010 Wiley‐Liss, Inc.     

Highly efficient whole‐heart imaging using radial phase encoding‐phase ordering with automatic window selection

Three dimensional (3D) whole‐heart magnetic resonance imaging (MRI) has become an important imaging modality to assess cardiovascular diseases. The main challenges for 3D whole‐heart MRI are long acquisition times, required to achieve high spatial resolution, and image artefacts due to physiological motion. Here we propose to overcome these problems by the combination of an interleaved Radial Phase Encoding trajectory and the Phase Ordering with Automatic Window Selection method. This Radial Phase Encoding‐Phase Ordering with Automatic Window Selection approach yields fast 3D whole‐heart imaging with a high isotropic resolution and high navigator efficiency even for extremely irregular breathing. Numerical simulations were performed and Radial Phase Encoding‐Phase Ordering with Automatic Window Selection was implemented on a clinical scanner. A comparison between the proposed method and a respiratory gated 3D Cartesian approach was carried out. Radial Phase Encoding‐Phase Ordering with Automatic Window Selection leads to a better depiction of coronary arteries and an increase in navigator efficiency. In addition to a high resolution image, this method also provides dynamic respiratory information without an increase in scan time. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Whole‐heart contrast‐enhanced coronary magnetic resonance angiography using gradient echo interleaved EPI

Whole‐heart coronary MR angiography (MRA) is a promising method for detecting coronary artery disease. However, the imaging time is relatively long (on the order of 10–15 min). Such a long imaging time may result in patient discomfort and compromise the robustness of whole‐heart coronary MRA due to increased respiratory and cardiac motion artifacts. The goal of this study was to optimize a gradient echo interleaved echo planar imaging (GRE‐EPI) acquisition scheme for reducing the imaging time of contrast‐enhanced whole‐heart coronary MRA. Numerical simulations and phantom studies were used to optimize the GRE‐EPI sequence parameters. Healthy volunteers were scanned with both the proposed GRE‐EPI sequence and a 3D TrueFISP sequence for comparison purposes. Slow infusion (0.5 cc/sec) of Gd‐DTPA was used to enhance the signal‐to‐noise ratio (SNR) of the GRE‐EPI acquisition. Whole‐heart images with the GRE‐EPI technique were acquired with a true resolution of 1.0 × 1.1 × 2.0 mm³ in an average scan time of 4.7 ± 0.7 min with an average navigator efficiency of 44 ± 6%. The GRE‐EPI acquisition showed excellent delineation of all the major coronary arteries with scan time reduced by a factor of 2 compared with the TrueFISP acquisition. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

An improved coverage and spatial resolution—using dual injection dynamic contrast‐enhanced (ICE‐DICE) MRI: A novel dynamic contrast‐enhanced technique for cerebral tumors

A new dual temporal resolution‐based, high spatial resolution, pharmacokinetic parametric mapping method is described ‐ improved coverage and spatial resolution using dual injection dynamic contrast‐enhanced (ICE‐DICE) MRI. In a dual‐bolus dynamic contrast‐enhanced‐MRI acquisition protocol, a high temporal resolution prebolus is followed by a high spatial resolution main bolus to allow high spatial resolution parametric mapping for cerebral tumors. The measured plasma concentration curves from the dual‐bolus data were used to reconstruct a high temporal resolution arterial input function. The new method reduces errors resulting from uncertainty in the temporal alignment of the arterial input function, tissue response function, and sampling grid. The technique provides high spatial resolution 3D pharmacokinetic maps (voxel size 1.0 × 1.0 × 2.0 mm³) with whole brain coverage and greater parameter accuracy than that was possible with the conventional single temporal resolution methods. High spatial resolution imaging of brain lesions is highly desirable for small lesions and to support investigation of heterogeneity within pathological tissue and peripheral invasion at the interface between diseased and normal brain. The new method has the potential to be used to improve dynamic contrast‐enhanced‐MRI techniques in general. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Nonrigid retrospective respiratory motion correction in whole‐heart coronary MRA

A nonrigid retrospective respiratory motion correction scheme is presented for whole‐heart coronary imaging with interleaved acquisition of motion information. The quasi‐periodic nature of breathing is exploited to populate a 3D nonrigid motion model from low‐resolution 2D imaging slices acquired interleaved with a segmented 3D whole‐heart coronary scan without imposing scan time penalty. Reconstruction and motion correction are based on inversion of a generalized encoding equation. Therein, a forward model describes the transformation from the motion free image to the motion distorted k‐space data, which includes nonrigid spatial transformations. The effectiveness of the approach is demonstrated on 10 healthy volunteers using free‐breathing coronary whole‐heart scans. Although conventional respiratory‐gated acquisitions with 5‐mm gating window resulted in an average gating efficiency of 51% ± 11%, nonrigid motion correction allowed for gate‐free acquisitions, and hence scan time reduction by a factor of two without significant penalty in image quality. Image scores and quantitative image quality measures for the left coronary arteries showed no significant differences between 5‐mm gated and gate‐free acquisitions with motion correction. For the right coronary artery, slightly reduced image quality in the motion corrected gate‐free scan was observed as a result of the close vicinity of anatomical structures with different motion characteristics. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Volume‐targeted and whole‐heart coronary magnetic resonance angiography using an intravascular contrast agent

Purpose

To compare volume‐targeted and whole‐heart coronary magnetic resonance angiography (MRA) after the administration of an intravascular contrast agent.

Materials and Methods

Six healthy adult subjects underwent a navigator‐gated and ‐corrected (NAV) free breathing volume‐targeted cardiac‐triggered inversion recovery (IR) 3D steady‐state free precession (SSFP) coronary MRA sequence (t‐CMRA) (spatial resolution = 1 × 1 × 3 mm³) and high spatial resolution IR 3D SSFP whole‐heart coronary MRA (WH‐CMRA) (spatial resolution = 1 × 1 × 2 mm³) after the administration of an intravascular contrast agent B‐22956. Subjective and objective image quality parameters including maximal visible vessel length, vessel sharpness, and visibility of coronary side branches were evaluated for both t‐CMRA and WH‐CMRA.

Results

No significant differences (P = NS) in image quality were observed between contrast‐enhanced t‐CMRA and WH‐CMRA. However, using an intravascular contrast agent, significantly longer vessel segments were measured on WH‐CMRA vs. t‐CMRA (right coronary artery [RCA] 13.5 ± 0.7 cm vs. 12.5 ± 0.2 cm; P < 0.05; and left circumflex coronary artery [LCX] 11.9 ± 2.2 cm vs. 6.9 ± 2.4 cm; P < 0.05). Significantly more side branches (13.3 ± 1.2 vs. 8.7 ± 1.2; P < 0.05) were visible for the left anterior descending coronary artery (LAD) on WH‐CMRA vs. t‐CMRA. Scanning time and navigator efficiency were similar for both techniques (t‐CMRA: 6.05 min; 49% vs. WH‐CMRA: 5.51 min; 54%, both P = NS).

Conclusion

Both WH‐CMRA and t‐CMRA using SSFP are useful techniques for coronary MRA after the injection of an intravascular blood‐pool agent. However, the vessel conspicuity for high spatial resolution WH‐CMRA is not inferior to t‐CMRA, while visible vessel length and the number of visible smaller‐diameter vessels and side‐branches are improved. J. Magn. Reson. Imaging 2009;30:1191–1196. © 2009 Wiley‐Liss, Inc.