Parallel Acquisition Technique Research Articles

Preoperative Evaluation of Potential Living Related Kidney Donors with High-Spatial-Resolution Magnetic Resonance (MR) Angiography at 3 Tesla

The purpose of this prospective study was to determine the feasibility and accuracy of high-spatial-resolution MR imaging at 3 Tesla (T) in the preoperative evaluation of potential living related kidney donors. Eighteen potential donors (8 men, 10 women; mean age, 50.1 +/- 14.2 years) for renal transplantation were evaluated with 3 T MR imaging. A high-spatial-resolution 3-dimensional (3D) gradient-echo MR angiography (repetition time/echo time, 3.0/1.14 ms; flip, 19-23 degrees; matrix, 512; slice thickness, 1.0 mm) using parallel acquisition technique (GRAPPA) with an acceleration factor of 3 was performed on a whole body scanner. Images were evaluated in a prospective and blinded fashion by 2 MR radiologists. The number of renal arteries, presence of early branches (defined as a branch arising within 2 cm of the main renal ostium), and renal artery stenosis were analyzed. The renal parenchyma, collecting system and ureters, were evaluated on the MR urograms. Interpretation of MR images were compared with surgical findings. Based on MR angiography data sets, a total of 36 main and 9 accessory renal arteries was found. There were 5 renal arteries presenting an early branching (<or=2 cm). The correct venous anatomy was identified in 13 of 14 patients (93%), including a single left renal vein anterior to the aorta (n = 3), retroaortic left renal vein (n = 2), and single right renal vein (n = 9). A single collecting system in all harvested kidneys was identified correctly with MR imaging. Overall, the sensitivity and positive predictive value of MRI in correctly determining the vascular and parenchymal anatomy in the harvested kidney was 85% and 93%, respectively. High-spatial-resolution contrast-enhanced MR angiography at 3 T can predict successful donor nephrectomy in potential living related kidney donors.

Single‐ vs. Double‐dose in contrast‐enhanced magnetic resonance angiography of the carotid arteries

WE READ WITH great interest the article by Jourdan et al (1), published in the March 2007 issue of JMRI, in which the authors reported their results of dose comparison of single- vs. double-dose in contrast-enhanced magnetic resonance angiography (CE-MRA) of the carotid arteries. Their intraindividual crossover trial performed with 11 healthy volunteers confirms to a large extent our own conclusions after prospectively including 119 patients with suspected vascular occlusive disease in a similar study (2), stating that a single-dose of 0.1 mmol/kg body weight (BW) of an extracellular gadolinium agent may be the preferable dosage for the carotid arteries. Initially, we were surprised when reading that a double dose resulted in significantly (P < 0.05) higher signal intensity (SI), signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) compared to a single dose, while our patient study could not reveal such a difference. To the contrary, dosing did not influence our contrast enhancement when comparing double-dose (mean CNR = 70.01 ± 20.7) and single-dose (CNR = 69.12 ± 19.8). The most probable explanation for this difference seems to be the different contrast administration scheme—in our case both dosages were administered at 3.0 mL/second followed by a saline flush of 30 mL at the same speed. In the study of Jourdan et al (1), the infusion time was generally fixed for the gadolinium bolus to 10 seconds—followed by a 20-mL saline flush at the same rate. Overall, one can calculate a mean injection speed of 1.5 mL/second for single-dose and 3.0 mL/second for double-dose considering the mean weight of 71 ± 16 kg. Apparently, a higher injection speed results in higher CNR values, even though these have little impact on the diagnostic quality, as stated in both studies. It would have been interesting to retrieve some more information about the total acquisition time and the k-space filling techniques used by the authors, as these are of growing importance when evaluating and improving MRA techniques (3). In fact, centric-elliptic reordering or radial k-space techniques using segmented k-space filling should allow further dose decrease while improving spatial resolution and suppressing venous overlay. Using initial central k-space encoding and fixed acquisition times of 19 seconds, we could not observe a decreased coverage in case of single-dose or higher frequencies of venous overlay within the double-dose group. This confirms the high robustness of the MR angiographic technique we had implemented with little differences in our qualitative results despite the presence of all kinds of vascular pathologies in both dose groups. Mean CNR levels were considerably lower in the study of Jourdan et al (1), but this is probably attributed to the higher spatial resolution they report, and in part to the unequal formulas they used to calculate SNR and CNR. Nevertheless, the SNR penalty due to higher spatial resolution must perhaps be corrected by higher dosing? Furthermore, our study results could show that CNR is inversely related to BW both for single- and double-dose groups. Apparently, patients with higher BW present lower CNR values. This did not seem to lead to a decay of diagnostic quality. It remains to be clarified if patients with higher BW are rather underdosed, or the opposite, if patients with lower BW could benefit from a dose reduction even beyond single-dose. Influence of dosing on the degree of overstaging of stenosis in CE-MRA compared to digital subtraction angiography. Optimal dosing when segmented k-space techniques are implemented. (4) Dose influence on resolution or in other words on outer k-space trajectories. Adaptation of dosing to BW or better to body-surface of the patients. Dosing at high field. (3) Dosing when using parallel acquisition techniques with high acceleration factors. To conclude, data of both studies together are suggestive that single-dose, when ideally timed for central k-space would likely be optimal, both from the diagnostic and economic point of view for CE-MRA of the carotids. Johannes M. Froehlich PhD*, Martin Unterweger MD*, Rahel A. Kubik-Huch MD*, * Institute of Radiology, Cantonal Hospital Baden, Baden, Switzerland.

Comparison of three accelerated pulse sequences for semiquantitative myocardial perfusion imaging using sensitivity encoding incorporating temporal filtering (TSENSE)

To investigate the parallel acquisition technique sensitivity encoding incorporating temporal filtering (TSENSE) with three saturation-recovery (SR) prepared pulse sequences (SR turbo fast low-angle shot [SR-TurboFLASH], SR true fast imaging with steady precession [SR-TrueFISP], and SR-prepared segmented echo-planar-imaging [SR-segEPI]) for semiquantitative first-pass myocardial perfusion imaging. In blood- and tissue-equivalent phantoms the relationship between signal intensity (SI) and contrast-medium concentration was evaluated for the three pulse sequences. In volunteers, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and normalized upslopes (NUS) were calculated from signal-time curves (STC). Moreover, artifacts, image noise, and overall image quality were qualitatively evaluated. Phantom data showed a 40% increased linear range of the relation between SI and contrast-medium concentration with TSENSE. In volunteers, TSENSE introduced significantly residual artifacts and loss in SNR and CNR. No differences were found for NUS values with TSENSE. SR-TrueFISP yielded highest SNR, CNR, and quality scores. However, in SR-True-FISP images, dark-banding artifacts were most pronounced. NUS values obtained with SR-TrueFISP were significantly higher and with SR-segEPI significantly lower than with SR-TurboFLASH. Semiquantitative myocardial perfusion imaging can significantly benefit from TSENSE due to shorter acquisition times and increased linearity of the pulse sequences. Among the three pulse sequences tested, SR-TrueFISP yielded best image quality. SR-segEPI proved to be an interesting alternative due to shorter acquisition times, higher linearity and fewer dark-banding artifacts.

Fast High-Spatial-Resolution MRI of the Ankle with Parallel Imaging Using GRAPPA at 3 T

The purpose of our study was to compare an autocalibrating parallel imaging technique at 3 T with standard acquisitions at 3 and 1.5 T for small-field-of-view imaging of the ankle. MRI of the ankle was performed in three fresh human cadaver specimens and three healthy volunteers. Axial and sagittal T1-weighted, axial fat-saturated T2-weighted, and coronal intermediate-weighted fast spin-echo sequences, as well as a fat-saturated spoiled gradient-echo sequence, were acquired at 1.5 and 3 T. At 3 T, reduced data sets were reconstructed using a generalized autocalibrating partially parallel acquisition (GRAPPA) technique, with a scan time reduction of approximately 44%. All images were assessed by two radiologists independently concerning image quality. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were measured in every data set. In the cadaver specimens, macroscopic findings after dissection served as a reference for the pathologic evaluation. SNR and CNR in the GRAPPA images were comparable to the standard acquisition at 3 T. The image quality was rated significantly higher at 3 T with both normal and parallel acquisition compared with 1.5 T. There was no significant difference in ligament and cartilage visualization or in image quality between standard and GRAPPA reconstruction at 3 T. Ankle abnormalities were better seen at 3 T than at 1.5 T for both normal and parallel acquisitions. Using higher field strength combined with parallel technique, MR images of the ankle were obtained with excellent diagnostic quality and a scan time reduction of about 44%. In addition, parallel imaging can provide more flexibility in protocol design.

High-Resolution Magnetic Resonance Angiography of the Lower Extremities With a Dedicated 36-Element Matrix Coil at 3 Tesla

Recent developments in hard- and software help to significantly increase image quality of magnetic resonance angiography (MRA). Parallel acquisition techniques (PAT) help to increase spatial resolution and to decrease acquisition time but also suffer from a decrease in signal-to-noise ratio (SNR). The movement to higher field strength and the use of dedicated angiography coils can further increase spatial resolution while decreasing acquisition times at the same SNR as it is known from contemporary exams. The goal of our study was to compare the image quality of MRA datasets acquired with a standard matrix coil in comparison to MRA datasets acquired with a dedicated peripheral angio matrix coil and higher factors of parallel imaging. Before the first volunteer examination, unaccelerated phantom measurements were performed with the different coils. After institutional review board approval, 15 healthy volunteers underwent MRA of the lower extremity on a 32 channel 3.0 Tesla MR System. In 5 of them MRA of the calves was performed with a PAT acceleration factor of 2 and a standard body-matrix surface coil placed at the legs. Ten volunteers underwent MRA of the calves with a dedicated 36-element angiography matrix coil: 5 with a PAT acceleration of 3 and 5 with a PAT acceleration factor of 4, respectively. The acquired volume and acquisition time was approximately the same in all examinations, only the spatial resolution was increased with the acceleration factor. The acquisition time per voxel was calculated. Image quality was rated independently by 2 readers in terms of vessel conspicuity, venous overlay, and occurrence of artifacts. The inter-reader agreement was calculated by the kappa-statistics. SNR and contrast-to-noise ratios from the different examinations were evaluated. All 15 volunteers completed the examination, no adverse events occurred. None of the examinations showed venous overlay; 70% of the examinations showed an excellent vessel conspicuity, whereas in 50% of the examinations artifacts occurred. All of these artifacts were judged as none disturbing. Inter-reader agreement was good with kappa values ranging between 0.65 and 0.74. SNR and contrast-to-noise ratios did not show significant differences. Implementation of a dedicated coil for peripheral MRA at 3.0 Tesla helps to increase spatial resolution and to decrease acquisition time while the image quality could be kept equal. Venous overlay can be effectively avoided despite the use of high-resolution scans.

High-Resolution Whole-Body Magnetic Resonance Imaging Applications at 1.5 and 3 Tesla

To analyze the impact of altered magnetic field properties on image quality and on potential artifacts when an established whole-body magnetic resonance imaging (WB-MRI) protocol at 1.5 Tesla (T) is migrated to 3 T. Fifteen volunteers underwent noncontrast magnetic resonance imaging (MRI) on 32-channel whole body-scanners at 1.5 and 3 T with the use of parallel acquisition techniques (PAT). Coronal T1-weighted TSE- and short tau inversion recovery (STIR)-sequences at 4 body levels including sagittal imaging of the whole spine were performed. Additional axial HASTE-imaging of lung and abdomen, T1-/T2-weighted-TSE- and EPI-sequences of the brain and T2-weighted respiratory-triggered imaging of the liver was acquired. Both data sets were compared by 2 independent readers in respect to artifacts and image quality using a 5-point scale. Regions of pronounced artifacts were defined. Overall image impression was both qualitatively rated as "good" at 1.5 and 3 T for T1-w-TSE- and STIR-imaging of the whole body and spine. At 1.5 T, significantly better quantitative values for overall image quality were found for WB-STIR, T2-w-TSE imaging of the liver and brain (Wilcoxon Mann-Whitney U Test; P < 0.05), overall rated as good at 3 T. Significantly higher dielectric effects at 3 T were affecting T1-w- and STIR-WB-MRI, and HASTE of the abdomen and better image homogeneity at 1.5 T was observed for T1-weighted-/STIR-WB-MRI and T1-w-TSE-imaging of the spine. Pulsation artifacts were significantly increased at 3 T for T1-w WB-MRI. Significantly higher susceptibility artifacts were found for GRE-sequences of the brain at 3 T. Motion artifacts, Gibbs-Ringing, and image distortion was not significantly different and showed slightly higher quantitative values at 3 T (except for HASTE imaging of the abdomen). Overall scan time was 45 minutes and 44 seconds at 1.5 T and 40 minutes and 28 seconds at 3 T at identical image resolution. Three Tesla WB-MRI is feasible with good image quality comparable to 1.5 T. 3.0 T WB-MRI shows significantly more artifacts with a mild to moderate impact on image assessment. Therefore 1.5 T WB-MRI is the preferred image modality. Overall scan time at 3 T is reduced with the use of parallel imaging at a constant image resolution.

White Matter Damage in Alzheimer Disease and Mild Cognitive Impairment: Assessment with Diffusion-Tensor MR Imaging and Parallel Imaging Techniques

To prospectively determine regional differences in fiber tract integrity between elderly patients with Alzheimer disease (AD), those with mild cognitive impairment (MCI), and healthy elderly subjects by using diffusion-tensor imaging with parallel imaging techniques and a new eight-element receiving coil. Institutional review board approval and informed consent were obtained. Fifteen patients with AD (seven men, eight women; mean age; 68.8 years), 16 patients with MCI (nine men, seven women; mean age, 68.9 years) and 19 healthy control subjects (eight men, 11 women; mean age, 63.9 years) underwent diffusion-tensor imaging performed with a 1.5-T magnetic resonance system. An echo-planar imaging diffusion sequence was used with an integrated parallel acquisition technique (PAT) and an eight-element head coil. The mean apparent diffusion coefficient (ADC), fractional anisotropy (FA), and relative anisotropy (RA) values of several white matter (WM) regions were determined. The Kruskal-Wallis test was used initially to test for overall equality of median values in each data group. Single posttest comparisons were performed with the Mann-Whitney U test, with an overall statistical significance level of .05. FA and RA values were significantly (P < .05) decreased, whereas ADC values in the splenium of the corpus callosum were higher in patients with AD than in patients with MCI. Evidence of higher ADC values in the WM of the temporal lobe was observed in patients with AD compared with the ADC values in patients with MCI and in control subjects. ADC values in the parietal WM were significantly (P < .05) elevated in patients with MCI compared with those in control subjects. The images obtained with integrated PAT showed fewer susceptibility artifacts and were less distorted than images acquired without parallel imaging techniques. Reduced FA and RA values in patients with AD suggest that diffusion-tensor imaging of the brain can be used to confirm clinical manifestation of AD but is less applicable in the detection of MCI.

Intraarterial Contrast-Enhanced MR Aortography With and Without Parallel Acquisition Technique in Patients with Peripheral Arterial Occlusive Disease

Repeated intraarterial gadolinium injections are necessary in endovascular MRI-guided interventions; therefore a low-dose protocol with a short acquisition time is preferable. The purpose of this study was to conduct a quantitative comparison of intraarterial MR aortograms obtained with and without high-speed parallel acquisition technique. Intraarterial MR aortography was performed at 1.5 T on nine patients with peripheral arterial occlusive disease and in an aortic phantom with pulsatile flow. A 3D fast low-angle shot MRI sequence was used for standard technique (acquisition time, 20 seconds) and for parallel acquisition technique (acquisition time, 14 seconds). In all patients, a pigtail catheter was left in the suprarenal position after digital subtraction angiography. Contrast-enhanced intraarterial MR aortography was performed after automated injection of 50 mmol/L gadoterate dimeglumine at an injection rate of 4 mL/s. Contrast-to-noise ratio (CNR) and image quality were evaluated in both imaging series at different locations. In an aortic phantom with pulsatile flow, CNR was determined 1, 30, and 60 cm distal to the catheter tip with standard and parallel acquisition techniques. In all patients, intraarterial MR aortography was feasible with both acquisition techniques. No significant difference in CNR or image quality was observed in the patient study. Similar results were calculated for the pulsatile aortic flow phantom at all locations. Intraarterial MR aortography is feasible with parallel acquisition technique without a significant loss of CNR. This technique reduces contrast agent consumption approximately 30% owing to an approximately 30% reduction in acquisition time.

Cardiac functional analysis with time-adaptive sensitivity encoding technique: comparison with generalized autocalibrating partially parallel acquisitions technique

目的 对比分析单次屏气时间调整敏感度编码(TSENSE)和多次屏气全局自动校准部分并行采集(GRAPPA)技术结合真实稳态进动(true FISP)电影,在评价左心室功能中的价值.方法 对健康组30例健康成年人和疾病组32例心脏器质性病变患者用数字表法随机行单次屏气TSENSE和同层数多次屏气GRAPPA左心室短轴电影进行心功能检查.所得图像经心功能分析软件分析后分别获得个体两种方法的左心室射血分数(EF)值、左心室舒张末容积(EDV)、左心室收缩末容积(ESV)、心搏量(SV)、心输出量(CO)、心肌平均重量(MM)和心率(HR),采用配对t检验法对两组两种方法所得各心功能指标进行比较.结果 健康组TSENSE CO较GRAPPA组低[分别为(3.8±0.8)、(4.0±0.9)L/min],差异有统计学意义(t=2.262,P＜0.05);其余心功能参数两种方法差异均无统计学意义(P＞0.05).疾病组左心室功能各指标,包括EF值、EDV、ESV、SV、CO和MM两种方法差异均无统计学意义(P＞0.05).结论 单次屏气TSENSE true FISP电影分析左心室功能简便、快速、准确,基本可以替代多次屏气GRAPPA电影,具有重要的临床实用价值。

MR-Sialographie: Prospektive Evaluation ultraschneller Sequenzen mit paralleler Bildgebung und oraler Stimulation bei Patienten

To evaluate an ultra-fast sequence for MR sialography requiring no post-processing and to compare the acquisition technique regarding the effect of oral stimulation with a parallel acquisition technique in patients with salivary gland diseases. 128 patients with salivary gland disease were prospectively examined using a 1.5-T superconducting system with a 30 mT/m maximum gradient capability and a maximum slew rate of 125 mT/m/sec. A single-shot turbo-spin-echo sequence (ss-TSE) with an acquisition time of 2.8 sec was used in transverse and oblique sagittal orientation. All images were obtained with and without a parallel imaging technique. The evaluation of the ductal system of the parotid and submandibular gland was performed using a visual scale of 1-5 for each side. The images were assessed by two independent experienced radiologists. An ANOVA with post-hoc comparisons and an overall two tailed significance level of p = 0.05 was used for the statistical evaluation. An intraclass correlation was computed to evaluate interobserver variability and a correlation of > 0.8 was determined, thereby indicating a high correlation. Depending on the diagnosed diseases and the absence of abruption of the ducts, all parts of excretory ducts were able to be visualized in all patients using the developed technique with an overall rating for all ducts of 2.70 (SD +/- 0.89). A high correlation was achieved between the two observers with an intraclass correlation of 0.73. Oral application of a sialogogum improved the visibility of excretory ducts significantly (p < 0.001). In contrast, the use of a parallel imaging technique led to a significant decrease in image quality (p = 0,011). The applied ss-TSE for MR sialography allows fast and sufficient visualization of the excretory ducts of the main salivary glands in patients, and no elaborate post-processing is required. Use of an oral sialogogum is suggested to improve the results of MR sialography.

Image Quality Using Dynamic MR Imaging of the Temporomandibular Joint with True-FISP Sequence

We evaluated the quality of dynamic magnetic resonance (MR) imaging of the temporomandibular joint (TMJ) in 20 normal volunteers with 40 TMJs. To confirm TMJ, we obtained static proton density weighted images (PDWI) before performing dynamic MR imaging with true-fast imaging in a steady-state precession (true-FISP) sequence. Four sequences of the first 10 volunteers were examined to determine the optimal sequence. The 4 sequences included the integrated parallel acquisition technique (iPAT) and/or fat saturation technique. The optimal sequence was then determined and performed in all 20 volunteers. The quality of imaging was evaluated, especially with respect to the conspicuity of the articular disk, mandibular condyle, articular eminence and lateral pterygoid muscle. One of 3 confidence levels was assigned for this evaluation. Neither iPAT nor fat saturation obtained the best quality imaging. Detection rates in the 20 volunteers were 83% for the articular disk, 95% for the mandibular condyle, 96% for the articular eminence and 7.5% for the lateral pterygoid muscle. We recommend dynamic MR imaging of the TMJ with the true-FISP sequence using neither iPAT nor fat saturation. Nevertheless, dynamic MR imaging was inferior to static imaging in detecting the articular disk and still requires improvement.

Isotropic 3D T2-Weighted MR Cholangiopancreatography with Parallel Imaging: Feasibility Study

The aim of this study was to compare the quality of images obtained with fast 3D T2-weighted turbo spin-echo (TSE) MR cholangiopancreatography (MRCP) sequences and 1-mm isotropic voxels with the quality of conventional 2D MRCP images. Thirty consecutively registered patients (14 women, 16 men; average age, 60.2 years; age range, 32-87 years) underwent imaging at 1.5 T with a 6-element body array coil. All imaging was performed with three MRCP techniques: free-breathing 3D T2-weighted TSE (TR/TE, 1,300/680; flip angle, 180 degrees; field of view, 250-300 mm; matrix size, 256 x 256; slice thickness, 1 mm; parallel acquisition technique factor, 2); breath-hold 3D T2-weighted TSE (same parameters as the free-breathing 3D technique); breath-hold coronal and oblique coronal thick-slab 2D TSE without parallel acquisition technique (2,800/1,100; flip angle, 150-180 degrees). Quantitative measures of image signal and contrast were evaluated by analysis of variance and paired Student's t tests. A 5-point scale (1, nondiagnostic, to 5, high diagnostic confidence) was used to compare the 3D and 2D data sets for image quality and definition of biliary and pancreatic ductal anatomic features. Friedman's nonparametric and Wilcoxon's rank sum tests were performed for statistical analysis of the qualitative assessments. Quantitative results showed free-breathing and breath-hold 3D TSE images had significantly higher relative signal intensity and contrast than 2D TSE images (p < 0.0001). The qualitative findings showed that both free-breathing and breath-hold 3D TSE techniques gave better delineation of biliary anatomy (p < 0.0001) than the 2D technique. The overall quality of 3D images was better than that of 2D images, and 3D imaging was better at depicting pancreatic ducts, although the difference did not reach statistical significance. Three-dimensional volumetric MRCP images are of superior quality and give better delineation of pancreaticobiliary anatomy than conventional 2D images and have the added advantage of multiplanar and postprocessing capabilities.

Whole-body three-dimensional contrast-enhanced magnetic resonance (MR) angiography with parallel imaging techniques on a multichannel MR system for the detection of various systemic arterial diseases

Using a 1.5-T magnetic resonance (MR) imager equipped with 32 receiving channels and integrated parallel acquisition techniques, 37 patients underwent whole-body three-dimensional (3D) contrast-enhanced MR angiography (WB 3D CE MRA). The patients included had clinically documented or suspected peripheral arterial occlusive disease (PAOD, n = 19), Takayasu arteritis (n = 8), polyarteritis nodosa (n = 1), type-B dissection (n = 4), thoracic and/or abdominal aneurysm (n = 5). Sixty-eight surface coils were employed to encompass the whole body. Four 3D CE MRA stations were acquired successively through automatic table moving. The spatial resolution was 1.6 x 1.0 mm and slice thickness was 1.5 mm for all stations. A total scan range of 188 cm was acquired. Overall image quality of each arterial segment and venous overlay were assessed. The depiction of various systemic arterial diseases was evaluated and compared, in 20 patients, with other imaging modalities. This WB 3D CE MRA yielded a detailed display of the arterial system with an average MR room time of 17.4 min. The image quality was considered diagnostic in 99.3% of the arterial segments. In 7 of 19 patients with PAOD, WB MRA showed additional vascular narrowing apart from peripheral arterial disease. In nine patients with vasculitis, WB MRA depicted luminal irregularity, narrowing or occlusion, aneurysm, and collateral circulation involving multiple vascular segments. WB MRA also clearly revealed the severity and extent of dissection and aortic aneurysm. In 20 cases where additional imaging investigations have been carried out, the vascular pathologies demonstrated by WB MRA agree with these additional imaging investigations.

2D‐GRAPPA‐operator for faster 3D parallel MRI

When using parallel MRI (pMRI) methods in combination with three-dimensional (3D) imaging, it is beneficial to subsample the k-space along both phase-encoding directions because one can then take advantage of coil sensitivity variations along two spatial dimensions. This results in an improved reconstruction quality and therefore allows greater scan time reductions as compared to subsampling along one dimension. In this work we present a new approach based on the generalized autocalibrating partially parallel acquisitions (GRAPPA) technique that allows Fourier-domain reconstructions of data sets that are subsampled along two dimensions. The method works by splitting the 2D reconstruction process into two separate 1D reconstructions. This approach is compared with an extension of the conventional GRAPPA method that directly regenerates missing data points of a 2D subsampled k-space by performing a linear combination of acquired data points. In this paper we describe the theoretical background and present computer simulations and in vivo experiments.

Autocalibrating parallel imaging of in vivo trabecular bone microarchitecture at 3 Tesla

In this work the generalized autocalibrating partially parallel acquisition (GRAPPA) technique was implemented with modified reconstruction and applied to in vivo high-resolution (HR) magnetic resonance imaging (MRI) of the trabecular bone microarchitecture at 3 Tesla (T) with a multiple-acquisition balanced steady-state free precession (b-SSFP) sequence. Trabecular bone is made up of a network of microstructures (80-140 microm), and its structural deterioration is associated with the skeletal metabolic disorder osteoporosis. HR-MRI is a promising noninvasive tool for assessing the trabecular microarchitecture in vivo, but it involves long acquisition times. Using partially parallel imaging (PPI) to accelerate the acquisition may help mitigate this shortcoming and allow more flexibility in protocol design. In this study the effects of GRAPPA-based reconstruction on image characteristics and the measurement of trabecular bone structural parameters were evaluated. Initial studies showed that image quality and depiction of microstructure were preserved in the GRAPPA-based reconstruction, indicating the feasibility of PPI in HR-MRI of trabecular bone. The results also demonstrated the potential of PPI for increasing the signal-to-noise ratio (SNR) efficiency of multiple-acquisition b-SSFP imaging protocols.

Respiratory‐triggered MRCP applying parallel acquisition techniques

To evaluate the influence of parallel imaging on the image quality of respiratory triggered magnetic resonance cholangiopancreatography (MRCP). A total of 30 consecutive patients underwent MRCP applying a respiratory triggered T2-weighted (T2w) turbo spin-echo (TSE) sequence without and with parallel imaging (acceleration factor of 2). Acquisition times of both sequences were recorded. Quantitative evaluation included measurement of a contour sharpness index of two segments of the pancreaticobiliary tree as well as calculation of the relative contrast between ductal structures and organ parenchyma at four different segments. The qualitative evaluation was performed by two independent radiologists who graded overall image quality, depiction of eight segments of the pancreaticobiliary tree, and the frequency of artifacts. The application of parallel imaging significantly (P<0.05) reduced the acquisition time of the respiratory triggered MRCP sequence by 37.7% (six minutes and two seconds+/-one minute and 26 seconds vs. three minutes and 46 seconds+/-58 seconds). The quantitative and qualitative evaluation revealed no statistically significant differences between the two sequences (P>0.05). The frequency of artifacts was at the same level for both sequences as well. The application of parallel imaging for respiratory triggered MRCP significantly reduces the acquisition time without relevant influence on image quality.

Assessment of the abdominal aorta and its visceral branches by contrast-enhanced dynamic volumetric hepatic parallel magnetic resonance imaging: feasibility, reliability and accuracy

The purpose of this study was to evaluate a new three-dimensional gradient-echo (GRE) MR sequence performed with a parallel acquisition technique to shorten breath-hold times (parallel GRE MRI) in the detection of arterial variants and stenosis of the abdominal aorta and its visceral branches. A total of 102 patients underwent dynamic parallel GRE MRI, timed to the arterial phase by a test bolus (mean breath-hold time, 17 s). For both quantitative and qualitative analysis, the abdominal aorta and its visceral branches were divided into 13 arterial segments. In a subanalysis of 55/102 patients, the accuracy of parallel GRE MRI compared to MDCT in the detection arterial variants and stenosis was calculated for two independent readers. Mean SNRs and CNRs were 47.2 and 35.6, respectively. Image quality was rated good or excellent in 1,234/1,326 segments (93%). Hepatic and renal arterial variants were identified with an accuracy of 93 and 95%, respectively (reader 1) and 98 and 100%, respectively (reader 2). Both readers detected arterial stenosis with an accuracy of 98%. Interobserver agreement was good to excellent for the detection of hepatic (kappa=0.69) and renal (kappa=0.92) variants and for the diagnosis of stenosis (kappa=0.96). Dynamic three-dimensional parallel GRE MRI is feasible and allows a reliable and accurate diagnosis of arterial variants and stenosis of the abdominal aorta and its visceral branches in a short breath-hold-time.

High-grade stenoses of the internal carotid artery: Comparison of high-resolution contrast enhanced 3D MRA, duplex sonography and power Doppler imaging

Purpose The objective of this study was to determine the agreement and diagnostic accuracy of high-resolution contrast enhanced magnetic resonance angiography (MRA) with integrated parallel acquisition techniques (iPAT), color coded duplex ultrasound (CCDS) and power Doppler ultrasound (PD) in the assessment of high-grade stenoses of the internal carotid artery (ICA). Methods Forty-four patients with 52 known or suspected stenoses of the internal carotid artery (ICA) were included in this prospective study. High-resolution MRA scans with a spatial resolution of 0.9 mm × 0.7 mm × 0.9 mm were acquired with an iPAT acceleration factor of 2 on a 1.5 T MR system (Sonata Maestro Class, Siemens Medical Solutions, Erlangen, Germany) with a head, neck and body coil. For the 3D-CE MRA a fast spoiled gradient echo sequence (FLASH) was used. To compensate for the inherent signal loss with parallel imaging, a 1 M contrast agent (gadobutrol, Gadovist ®, Schering, Berlin, Germany) was used. Stenoses were quantified by two readers in consensus in cross-sectional area measurements and graded according to the NASCET criteria. Using color coded duplex ultrasound (CCDS) and power Doppler (PD; Logiq 9, GE), the stenoses were also graded by two readers in consensus according to the NASCET criteria from intra- and post-stenotic diameter measurements. The results of MRA, CCDS and PD were compared to intraoperative findings or to follow-up examinations. Results High-resolution MRA allowed an excellent grading of vascular stenoses. In 70–90% degrees of stenosis there was an underestimation of the degree of stenosis in MRA as well as in CCDS. However, there was an overestimation of 90% stenoses in both MRA and CCDS. Pseudoocclusions with a lumen of less than one millimeter were occasionally rated as a complete occlusion in MRA. Conclusion A combination of MRA and duplex sonography seems reasonable for the accurate grading of stenoses and determination of distal stenoses downstream. However, the accuracy of duplex ultrasound depends on the examiner's experience.

High-Spatial-Resolution Contrast-Enhanced MR Angiography of Abdominal Arteries with Parallel Acquisition at 3.0 T: Initial Experience in 32 Patients

The objective of our study was to evaluate an isotropic high-spatial-resolution 3D contrast-enhanced MR angiography (CE-MRA) protocol with high acceleration parallel acquisition at 3.0 T for the display of the abdominal vasculature. Thirty-two consecutive patients (13 men, 19 women; age range, 28-88 years) with suspected abdominal arterial disease underwent abdominal 3D CE-MRA on a 3.0-T MR system, using a high-spatial-resolution (0.7 x 0.82 x 0.8 mm3) 3D gradient-refocused echo (GRE) sequence, integrated with a generalized autocalibrating partially parallel acquisitions (GRAPPA) technique with an acceleration factor of 3. Two vascular radiologists evaluated image quality and the presence and degree of arterial stenoses. Interobserver variability was calculated, using the kappa coefficient. The sensitivity and specificity of the technique were calculated and comparative analysis was performed with those of conventional catheter angiography (in eight patients) as the standard of reference. The abdominal arterial vasculature was visualized with diagnostic image quality in all subjects. Arterial stenoses were detected in 148 and 142 arterial segments by observer 1 and observer 2, respectively, with good interobserver agreement (kappa = 0.75; 95% confidence interval [CI]: 0.69-0.81). The sensitivity and specificity values for CE-MRA for the detection of significant (> 50%) arterial stenoses were 100% and 96% for observer 1 and 100% and 92% for observer 2, respectively. There was a significant correlation between CE-MRA and conventional angiography (R = 0.96 and 0.93 for observers 1 and 2, respectively) for the assessment of the degree of stenosis. The outlined MR angiography protocol at 3.0 T combined with parallel acquisition technique renders highly reliable and isotropic high-spatial-resolution imaging of the abdominal vasculature.

Accelerated volumetric MRI with a SENSE/GRAPPA combination

To combine the specific advantages of the generalized autocalibrating partially parallel acquisitions (GRAPPA) technique and sensitivity encoding (SENSE) with two-dimensional (2D) undersampling. By splitting the 2D reconstruction process into multiple one-dimensional (1D) reconstructions, the normal 1D GRAPPA method can be used for image reconstruction. Due to this data-handling process, a GRAPPA reconstruction is performed along the phase-encoding (PE) direction and effectively a SENSE reconstruction is performed along the partition-encoding (PAE) direction. In vivo experiments demonstrate the successful implementation of the SENSE/GRAPPA combination. Experimental results with up to 9.6-fold acceleration using a prototype 32-channel receiver head coil array are presented. The proposed SENSE/GRAPPA combination for 3D imaging allows the GRAPPA method to be applied in combination with 2D undersampling. Because the SENSE/GRAPPA combination is not based on knowledge of spatial coil sensitivities, it should be the method of choice whenever it is difficult to extract the sensitivity information.