Reduction of magnetic field inhomogeneity artifacts in echo planar imaging with SENSE and GESEPI at high field.

Geometric distortion, signal-loss, and image-blurring artifacts in echo planar imaging (EPI) are caused by frequency shifts and T(2)(*) relaxation distortion of the MR signal along the k-space trajectory due to magnetic field inhomogeneities. The EPI geometric-distortion artifact associated with frequency shift can be reduced with parallel imaging techniques such as SENSE, while the signal-loss and blurring artifacts remain. The gradient-echo slice excitation profile imaging (GESEPI) method has been shown to be successful in restoring tissue T(2)(*) relaxation characteristics and is therefore effective in reducing signal-loss and image-blurring artifacts at a cost of increased acquisition time. The SENSE and GESEPI methods are complementary in artifact reduction. Combining these two techniques produces a method capable of reducing all three types of EPI artifacts while maintaining rapid acquisition time.     

The effect of injection rate on time-resolved contrast-enhanced peripheral MRA

In contrast-enhanced (CE) magnetic resonance (MR) angiography (MRA), lower injection rates of a fixed contrast agent dose provide longer contrast agent bolus at the expense of lower intravascular signal. This study evaluated the effect of different injection rates in imaging of the vasculature of the lower extremities with time-resolved, CE MRA. In three volunteers, injection rates of 0.5, 1.5 and 3.0 mL/second were administered in a randomized order and imaged in two separate sessions. Contrast agent bolus dynamics measured in volunteers were used in computer simulations to confirm variations in contrast agent concentration as a source of vessel ringing and blurring artifacts. To validate the effect of injection rate in pathologic vessels, 37 patients with peripheral vascular disease were imaged with a time-resolved technique using an injection rate of 0.5 mL/second or 1.5 mL/second and retrospectively divided into two groups. In volunteers, higher injection rates caused a stronger modulation of k-space and resulted in increased ringing artifacts in time-resolved CE MRA. These results were reproduced with computer simulations. In the qualitative patient study, significantly less vessel blurring was observed using a lower injection-rate, without a significant loss of vessel contrast.     

Comparison of spin-echo MR pulse sequences for imaging of the brain.

PURPOSETo determine the value of the gradient- and spin-echo (GRASE) technique as compared with the fast spin-echo and conventional spin-echo techniques in MR imaging of the brain.METHODSSixty-six patients with ischemic and neoplastic brain lesions were examined with T2-weighted spin-echo, fast spin-echo, and GRASE sequences. Three independent observers evaluated the contrast characteristics of anatomic and pathologic structures and of artifacts. Quantitative image analysis included region-of-interest measurements of anatomic structures and lesions.RESULTSThe contrast of anatomic structures was superior in images obtained with conventional and fast spin-echo techniques as compared with those obtained with the GRASE technique. Extended lesions, such as tumors and territorial infarcts, were identified equally with all techniques. For delineation of small ischemic lesions, GRASE was slightly inferior to fast and conventional spin-echo sequences. Flow artifacts were considerably reduced with fast spin-echo and GRASE sequences. Chemical-shift artifacts were significantly reduced, but ringing artifacts were more pronounced with GRASE.CONCLUSIONFast spin-echo remains the standard technique in MR imaging of the brain. However, GRASE might be useful in special cases, such as with uncooperative patients whose conventional or fast spin-echo images show severe motion artifacts.     

Gibbs artifact reduction by nonnegativity constraint

This paper proposes a 2-step image reconstruction method in which the nonnegativity constraint in the iterative maximum-likelihood expectation maximization (MLEM) algorithm is used to effectively reduce Gibbs ringing artifacts. Methods: Gibbs artifacts are difficult to control during imaging reconstruction. The proposed method uses the postprocessing strategy to suppress Gibbs artifacts. In the first step, a raw image is reconstructed from projections without correction for point spread function (PSF). The attenuation correction can be performed in the first step by using, for example, the iterative MLEM or ordered-subsets expectation maximization (OS-EM) algorithm. The second step is a postprocessing procedure that corrects for the PSF blurring effect. If the target features (e.g., hot lesions) have a positive background, removing the background before application of the postprocessing filter significantly helps with target deblurring and Gibbs artifact suppression. This postprocessing filter is the image-domain MLEM algorithm. The background activity is attached back to the foreground after lesion sharpening. Results: Computer simulations and PET phantom studies were performed using the proposed 2-step method. The background removal strategy significantly reduced Gibbs artifacts. Conclusion: Gibbs ringing artifacts generated during image reconstruction are difficult to avoid if compensation for the PSF of the system is needed. The strategy of separating image reconstruction from PSF compensation has been shown effective in removal of Gibbs ringing artifacts.     

High-resolution T2-weighted abdominal magnetic resonance imaging using respiratory triggering: impact of butylscopolamine on image quality

Background: Respiratory triggering allows the acquisition of high-resolution magnetic resonance (MR) images of the upper abdomen. However, the depiction of organs close to the gastrointestinal tract can be considerably impaired by ghosting artifacts and blurring caused by bowel peristalsis.

Purpose: To evaluate the effect of gastrointestinal motion suppression by intramuscular butylscopolamine administration on the image quality of a respiratory-triggered T2-weighted turbo spin-echo (T2w TSE) sequence of the upper abdomen.

Material and Methods: Images of 46 patients were retrospectively analyzed. Twenty-four patients had received intramuscular injection of 40 mg butylscopolamine immediately before MR imaging. Fourteen of the 24 patients in the butylscopolamine group underwent repeat imaging after a mean of 29 min. Quantitative analysis of the ghosting artifacts was done by measuring signal intensities in regions of interest placed in air anterior to the patient. In addition, image quality was assessed qualitatively by two radiologists by consensus.

Results: Spasmolytic medication with butylscopolamine reduced ghosting artifacts and significantly improved image quality of the respiratory-triggered T2w TSE sequence. The most pronounced effect of butylscopolamine administration on image quality was found for the pancreas and the left hepatic lobe. The rate of examinations with excellent or good depiction of the pancreas and the left hepatic lobe in the group without premedication and in the butylscopolamine group was 55% vs. 96% (pancreatic head), 35% vs. 88% (pancreatic body), 43% vs. 96% (pancreatic tail), and 45% vs. 83% (left hepatic lobe), respectively. Regarding the duration of the effect of intramuscular butylscopolamine, repeat imaging after a mean of 29 min did not result in a significant deterioration of image quality.

Conclusion: Intramuscular butylscopolamine administration significantly improves image quality of respiratory-triggered T2-weighted abdominal MR imaging by persistent reduction of peristaltic artifacts. MR imaging of the liver and pancreas in particular benefits from the suppression of gastrointestinal peristalsis by butylscopolamine.     

Partial Fourier imaging in multi-dimensions: a means to save a full factor of two in time.

We present an improvement to the traditional one-dimensional partial Fourier method by extending the method to multi-dimensions. The modified method allowed a full factor of two savings in time with much better coverage of the central k-space information and, because of this, smaller reconstruction artifacts. The residual magnitude error was found to correlate strongly with the residual phase error. Numerical simulation also indicated that with a priori perfect phase information, the original magnitude image could be perfectly reconstructed with half of the k-space data points in the multi-dimensional case. Simulated, phantom, and human data sets were tested with edge differences ranging from 10% (consistent with variable Gibbs ringing) to 25% (consistent with a blurred version of the object). The method was found to be a valuable adjunct to human imaging for short TR, T1-weighted three-dimensional gradient-echo imaging and magnetic resonance (MR) angiographic methods, especially when short echo times were used.     

Enhancement efficacy of magnetic starch microspheres (MSM) in conventional spin-echo and turbo spin-echo sequences at 0.5 T and 1.5 T

The efficacy of the superparamagnetic contrast agent magnetic starch microspheres (MSM) was evaluated in vitro by NMR relaxometry and in vivo by MR imaging using T2-weighted spin-echo (SE) and turbo spin-echo (TSE) sequences at 0.5 T and 1.5 T in 60 normal rats who received MSM in doses of 10–50 μmol/kg. MR imaging was performed using T2-weighted SE and TSE sequences. The relaxation rates 1/T1 and 1/T2 for liver and spleen increased linearly with MSM concentrations up to 30 μmol/kg body weight, and approached almost constant levels for higher doses. The slopes in the linear part of the 1/T2 diagram were 0.62 Hz ± 0.03 for the liver and 0.51 Hz ± 0.06 × kg/μmol for the spleen. On all T2-weighted sequences at 0.5 T and 1.5 T, liver signal-to-noise ratio (SNR) decreased by a factor of 2-3 already at the lowest dose of 10 μmol/kg. SNR values of TSE sequences exceeded values for SE sequences by 50–80%. The SNR decrease was not significantly different between SE and TSE sequences. Our results show that MSM is well suited as a T2 contrast agent at both magnetic field strengths when using conventional SE and fast TSE sequences.     

八次激发SE-EPI在肝脏MRI的应用

目的；比较八次激发SE－EPI与呼吸门控FSE及SSFSE T2WI在肝脏的应用。方法：对14例志愿者及21例肝病患者行上腹部呼吸门控FSE及SSFSE和屏气八次激发SE－EPI扫描。所有T2WI序列均运用脂肪抑制技术。定量分析肝脏、病灶的信噪比及肝脏－病灶的对比噪声比，评价各序列的图像质量及伪影。结果：八次激发SE－EPI与SSFSE及FSE在肝脏及病灶信噪比，肝脏－病灶对比度噪声比和图像质量方面无明显差异（P＞0．05）。其磁敏感伪影较FSE及SSFSE重（P＜0．01），SE－EPI化学位移伪影与SSFSE及FSE相比无明显差别（P＞0．05）。SE－EPI及FSE运动伪影明显比SSFSE重（P＜0．01），但SE－EPI运动伪影与FSE相比无明显差别（P＞0．05）。SE－EPI与FSE及SSFSE的图像质量无明显差别（P＞0．05）。结论：八次激发SE－EPI能够在较短时间里提供较高质量的上腹部T2WI。被检查者在扫描时可自由平静呼吸或屏气，可作为肝脏T2WI的补充序列。     

Water and lipid MRI of the Xenopus oocyte.

Oocytes of Xenopus laevis are large, single cells that provide a promising model system for the exploration of the MR biophysics fundamental to more complex living systems. Previous studies have generally employed 2D spin-echo sequences with an image slice thickness greater than the thickness of the cellular volumes of interest. Also, the large cytoplasmic lipid signal has typically been ignored. This study describes separate, high-resolution 3D measurements of the water and lipid spin densities, T(1) and T(2) relaxation time constants, and the water apparent diffusion rate constant (ADC) in the Xenopus oocyte without significant partial volume artifacts. The lipid spin-density and values for water MR properties varied monotonically from the vegetal to animal poles, indicating that the border between the poles is not sharply demarcated. Regional water MR property values correlated with lipid signal intensity. Lipid-specific imaging is shown for which water suppression is achieved via high diffusion weighting in the imaging sequence.     

Fast magnetic resonance imaging of liver.

Recent magnetic resonance (MR) units with a stronger gradient system have allowed various fast MR imaging techniques to develop. These fast scan techniques have easily realized breath-holding acquisition in the liver and the image quality has been greatly improved without sacrificing spatial resolution. The majority of the fast imaging techniques have been devoted to T2-weighted imaging to obtain useful T2-weighted images in the shortest possible time. Among the fast sequences, fast spin-echo (FSE) sequence is the most promising technique and allows high-quality T2-weighted images with reduced motion artifacts. However, FSE sequences using multiple refocused pulses may essentially realize only poor soft-tissue contrast due to magnetization transfer and T2-filtering effects, and therefore, echo-planar (EP) imaging is expected to provide high image contrast. In addition, single-shot EP imaging allows even diffusion-weighted (DW) and perfusion-weighted (PW) imaging in the liver due to its short scanning time. Recent development of fast gadolinium-enhanced 3D MR angiography has also impacted liver imaging. Combined with such gadolinium-enhanced 3D-MRA sequences and zerofilling image interpolation technique, biphasic gadolinium-enhanced 3D-MRA (whole-liver dynamic MR imaging in the arterial phase and MR portography in the portal phase) can be obtained.     

High‐resolution spiral imaging on a whole‐body 7T scanner with minimized image blurring

High‐resolution (～0.22 mm) images are preferably acquired on whole‐body 7T scanners to visualize minianatomic structures in human brain. They usually need long acquisition time (～12 min) in three‐dimensional scans, even with both parallel imaging and partial Fourier samplings. The combined use of both fast imaging techniques, however, leads to occasionally visible undersampling artifacts. Spiral imaging has an advantage in acquisition efficiency over rectangular sampling, but its implementations are limited due to image blurring caused by a strong off‐resonance effect at 7T. This study proposes a solution for minimizing image blurring while keeping spiral efficient. Image blurring at 7T was, first, quantitatively investigated using computer simulations and point‐spread functions. A combined use of multishot spirals and ultrashort echo time acquisitions was then employed to minimize off‐resonance‐induced image blurring. Experiments on phantoms and healthy subjects were performed on a whole‐body 7T scanner to show the performance of the proposed method. The three‐dimensional brain images of human subjects were obtained at echo time = 1.18 ms, resolution = 0.22mm (field of view = 220mm, matrix size = 1024), and in‐plane spiral shots = 128, using a home‐developed ultrashort echo time sequence (acquisition‐weighted stack of spirals). The total acquisition time for 60 partitions at pulse repetition time = 100 ms was 12.8 min without use of parallel imaging and partial Fourier sampling. The blurring in these spiral images was minimized to a level comparable to that in gradient‐echo images with rectangular acquisitions, while the spiral acquisition efficiency was maintained at eight. These images showed that spiral imaging at 7T was feasible. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Tissue temperature monitoring for thermal interventional therapy: Comparison of T1-weighted MR sequences

For thermal interventional therapy, near real-time monitoring of temperature changes in the treated area is desirable. In this study, various fast T1-weighted magnetic resonance (MR) imaging protocols were compared to determine the sensitivity and resolution of signal intensity for temperatures within the range of 36°C–66°C in gel phantoms and in vitro porcine liver specimens. The results showed that a T1-weighted fast spin-echo sequence with a TR of 100 msec had better temperature sensitivity and resolution than other sequences with comparable temporal resolutions. The longer imaging times required for fast spin-echo sequences with a TR of 300 msec did not improve temperature sensitivity. The methods introduced to evaluate temperature sensitivity and resolution should prove useful in selecting appropriate MR protocols for monitoring thermal treatment modalities such as interstitial laser therapy, focused ultrasound therapy, or radio-frequency heating.     

Effect of gadolinium concentration on renal signal intensity: An in vitro study with a saline bag model

PURPOSE: To compare the effects of gadolinium concentration on intermediate-weighted fast inversion-recovery and T2-weighted fast spin-echo magnetic resonance (MR) images by using a saline bag model of the kidney. MATERIALS AND METHODS: Normal-saline bags containing gadopentetate dimeglumine in concentrations of 0-20 mmol/L were imaged by using a variety of pulse sequences. Signal intensity was measured. RESULTS: Signal intensity loss at high gadolinium concentrations (negative enhancement) was demonstrated with all MR sequences. Increasing T2 weighting increased the negative enhancement effect and reduced the minimum gadolinium concentration at which negative enhancement was seen. The results of this study match theoretic predictions. CONCLUSION: The negative enhancement of normal renal tissue seen in intermediate-weighted fast inversion-recovery MR imaging is caused by T2 shortening at high gadolinium concentrations. Thus, gadolinium-enhanced T2-weighted fast spin-echo imaging also is expected to show negative enhancement and may prove to be a superior sequence for MR imaging of pyelonephritis. Further clinical investigation is warranted.     

Dynamic gadolinium-enhanced rapid acquisition spin-echo MR imaging of the liver

Rapid acquisition spin-echo (RASE) magnetic resonance (MR) imaging allows for coverage of the entire liver with highly T1-weighted SE images during a single 23-second breath-holding period. The RASE sequence was implemented in conjunction with rapid intravenous injection of gadopentetate dimeglumine to enable performance of dynamic contrast material-enhanced MR imaging of the liver. Prospective evaluation of 24 patients with 62 liver lesions 1 cm or greater in diameter was performed. Images obtained with RASE were devoid of respiratory-related ghost artifacts or edge blurring. The dynamic contrast-enhanced RASE technique resulted in contrast-to-noise and contrast-to-artifact values and time efficiency measures significantly greater (P less than .05) than those obtained with use of conventional T1- and T2-weighted pulse sequences, indicating a higher likelihood for lesion detectability. Lesion conspicuity was maximal during or immediately following bolus administration of gadopentetate dimeglumine, with lesions often becoming obscured at delayed postcontrast imaging.     

Utilizing SENSE to reduce scan duration in high-resolution contrast-enhanced renal MR angiography

PURPOSE: To evaluate the use of sensitivity encoding (SENSE) to reduce scan time and decrease detrimental artifacts arising from motion and bolus profile effects during contrast-enhanced MR angiography (CE-MRA) of the renal arteries (RAs). MATERIALS AND METHODS: A direct comparison of conventional and SENSE (acceleration factor 2) CE-MRA protocols was performed on 20 patients. Each patient underwent both scans. Both protocols achieved the same resolution, but the SENSE protocol was 50% faster and utilized a faster injection than the conventional scan. Three radiologists graded the images for image quality, artifact levels, and reader confidence. RESULTS: While the signal-to-noise ratio (SNR) decreased (26+/-5 vs. 30+/-10; P=0.04) with the SENSE protocol, the image-quality scores for four identified segments of the RAs increased or were unchanged. The largest improvements in image quality occurred in the more distal segments of the RAs. Parenchymal ringing (P=0.005) and RA blurring (P=0.006) were significantly reduced, and there was a trend toward improvement of RA ringing despite the increased injection rate. CONCLUSION: The faster SENSE scan maintained nearly the same SNR (due to faster injection of Gd-chelate), reduced artifact levels, and improved image quality ratings for the distal renal vessels.     

Artifacts and pitfalls in MR imaging of the pelvis

Artifacts are intimately intertwined with MRI. For the practicing radiologist, effective supervision, troubleshooting, and interpretation of diagnostic MR studies require a solid knowledge of the pertinent artifacts. This article seeks to familiarize the reader with commonly encountered artifacts and pitfalls in pelvic imaging, the mechanism behind their generation, and methods of minimizing their negative impact or maximizing their diagnostic yield. It also serves as an exciting tool to learn many aspects of basic and advanced MR physics. Artifacts are categorized into patient- and sequence-related artifacts. Various manifestations of motion and vascular artifacts, susceptibility, altered tissue contrast, blurring, chemical shift artifact, volume averaging, and gadolinium (Gd) pseudolayering are explained, along with their proposed remedies.     

Liver: T2-weighted MR imaging with breath-hold fast-recovery optimized fast spin-echo compared with breath-hold half-Fourier and non-breath-hold respiratory-triggered fast spin-echo pulse sequences

At liver magnetic resonance (MR) imaging in 38 patients, a breath-hold T2-weighted fast spin-echo (SE) pulse sequence optimized with fast recovery was compared with a conventional respiratory-triggered fast SE sequence and a breath-hold single-shot fast SE sequence. Mean signal-to-noise ratios for liver and contrast-to-noise ratios for hepatic lesions were higher with the breath-hold fast-recovery fast SE sequence than with the respiratory-triggered fast SE sequence (P <.05). Breath-hold fast-recovery images displayed better lesion clarity than did single-shot fast SE images (P <.05) and fewer image artifacts than did respiratory-triggered fast SE images (P <.05). The ability to determine lesion size and the overall image quality was best with the breath-hold fast-recovery sequence (P <.05). These results may justify use of the breath-hold fast-recovery fast SE pulse sequence for first-line T2-weighted MR imaging of the liver.     

半FOV匀场技术纠正FIESTA序列腹部磁敏感伪影的初步研究

目的:探讨快速稳态进动采集(fast imaging employing steady-state acquisition,FIESTA)序列腹部冠状位MRI成像时磁敏感伪影的形成机制,以及匀场技术对减少磁敏感伪影的应用价值。方法:在1.5T MRI,采用不同FOV匀场策略的FIESTA序列对患者行腹部冠状位扫描。以膈肌以下肝脏为ROI,对匀场后ROI部位伪影的改善效果进行评分,分析和比较不同匀场策略对于改善肝脏部位磁敏感伪影的差异。结果:1FIESTA序列腹部冠状位扫描时在膈肌附近易产生磁敏感伪影。2施加匀场之后可以改善伪影,半FOV匀场效果优于全FOV匀场效果(Z=-6.211,P0.05)。结论:FIESTA序列腹部扫描时正确施加适当范围的局部匀场可以减少磁敏感伪影,提高图像质量。     

Three-dimensional time-of-flight MR angiography using selective inversion recovery RAGE with fat saturation and ECG-triggering: Application to renal arteries

A three-dimensional (3D), ECG-triggered, selective inversion recovery (SIR) rapid gradient-echo (RAGE) technique is proposed to obtain MR angiograms of the main renal arteries. By using the selective inversion recovery and fat saturation, the background is significantly suppressed while blood maintains a high signal intensity as compared with conventional 3D time-of-flight (TOF) MR angiography. The sequence is ECG-triggered so that blood in-flow is maximized during systole, and intravoxel dephasing and pulsatile flow artifacts are minimized by collecting data during diastole. As a result, vessel boundary blurring and ghosting artifacts due to background motion are dramatically reduced, and the conspicuity and lumen definition of the arteries are significantly improved. High-quality MR angiograms of the main renal arteries with excellent blood/tissue contrast and suppression of motion artifacts have been consistently obtained for normal volunteers, with the length of visualization being 51 ± 07 mm for the left, and 57 ± 06 mm for the right renal arteries, significantly greater than using conventional 3D TOF pulse sequences. Statistical analysis was performed by using a one-sided Student's t test. Key words: renal artery; MR angiography; three-dimensional MR imaging.     

PROPELLER FSE磁共振扩散加权技术在诊断超急性和急性脑梗死中的价值

目的比较螺旋桨扫描技术(PROPELLER FSE)和平面回波成像(EPI)两种扩散加权成像(DWI)序列,探讨PROPELLER技术在DWI的应用价值。资料与方法选取67例临床疑诊急性脑梗死的患者分别行PROPEL-LER FSE DWI、EPI DWI和常规T1WI、T2WI、T2WI液体衰减反转恢复序列(FLAIR)检查,比较两种不同的DWI序列的图像质量,病灶检出率和对病变的显示程度。结果67例EPI DWI图像均不同程度地存在磁化率伪影,67例PROPELLER FSE DWI图像未见明显伪影及变形。67例中共发现并确诊46个急性脑梗死灶(4例为颅内多发病灶),其中病灶位于颅底区域12个,非颅底区域34个。PROPELLER FSE DWI和EPI DWI均可分辩出所有非颅底区域梗死灶。对12个颅底病灶PROPELLER FSE DWI检出率达100%,而EPI DWI可以明确诊断的7个,检出率达58.3%,病灶形态显示不充分或变形者3个,占25%,漏诊2个,漏诊率为16.7%。所有颅底区域病灶EPI DWI上信噪比(SNR)低于相应PROPELLER FSE DWI的SNR(t=-2.874,P<0.05)。结论PROPELLER FSE DWI能提高图像SNR,有效消除颅底磁化率伪影,改善图像质量,提高颅底病变的检出率。