Parallel imaging enhanced MR colonography using a phantom model

PURPOSE: To compare various Array Spatial and Sensitivity Encoding Technique (ASSET)-enhanced T2W SSFSE (single shot fast spin echo) and T1-weighted (T1W) 3D SPGR (spoiled gradient recalled echo) sequences for polyp detection and image quality at MR colonography (MRC) in a phantom model. Limitations of MRC using standard 3D SPGR T1W imaging include the long breath-hold required to cover the entire colon within one acquisition and the relatively low spatial resolution due to the long acquisition time. Parallel imaging using ASSET-enhanced T2W SSFSE and 3D T1W SPGR imaging results in much shorter imaging times, which allows for increased spatial resolution. MATERIALS AND METHODS: Using two porcine colon phantoms each with eight simulated 3-10-mm "polyps," baseline reference sequences acquired without ASSET (6-mm slices and readout bandwidth [BW] 62 kHz) were compared with 11 SSFSE and 8 SPGR sequences acquired with 2-fold ASSET acceleration. ASSET-enhanced SSFSE and SPGR sequences comprised BW/matrix combinations ranging from 20-62 kHz/256-352x256, respectively, with slice thicknesses adjusted from 3.0 to 4.5 mm to maintain a 23-26-second acquisition time and 30 cm slab thickness. Two experienced radiologists viewed the datasets in a randomized, blinded fashion. RESULTS: Compared to reference sequences, ASSET-enhanced SSFSE and SPGR sequences facilitated better polyp detection and had similar overall image quality and per-phantom specificity. The two best ASSET-enhanced SSFSE (3 and 4.5 mm slices, each with BW of 62.5 kHz and 352x256 matrices) and three best ASSET-enhanced SPGR BW/slice thickness/matrix combinations of 31 kHz/4.4 msec/192x256; 62/3.4/192x256; and 62/4.0/192x256, respectively, permitted detection of all polyps>or=5 mm. CONCLUSION: Parallel imaging using ASSET-enhanced T2W SSFSE and T1W 3D SPGR improves the ability to detect significant colon polyps in an MRC phantom model.     

Magnetic resonance imaging sialolithography: direct visualization of calculi in the submandibular gland using susceptibility-weighted imaging (SWI) at 3 Tesla

Magnetic resonance imaging (MRI) sialolithography is a useful technique for evaluating acute and chronic sialadenitis. However, its major weakness is that stones are not imaged directly. We have developed an MRI technique that allows specific identification and localization of calculi within the submandibular salivary gland or duct. This test is noninvasive and does not require ionizing radiation or a sialogogue. By using 3-dimensional susceptibility-weighted imaging, one can probe MRI signal phase changes. Corrected positive filtered phase and magnitude images, acquired using susceptibility-weighted imaging, allowed identification and anatomical localization of calcified calculi in the submandibular gland with efficacy comparable to computed tomography.     

Differentiation between Parkinson disease and other forms of Parkinsonism using support vector machine analysis of susceptibility-weighted imaging (SWI): initial results

Objectives

To diagnose Parkinson disease (PD) at the individual level using pattern recognition of brain susceptibility-weighted imaging (SWI).

Methods

We analysed brain SWI in 36 consecutive patients with Parkinsonism suggestive of PD who had (1) SWI at 3?T, (2) brain ¹²³I-ioflupane SPECT and (3) extensive neurological testing including follow-up (16 PD, 67.4?±?6.2?years, 11 female; 20 OTHER, a heterogeneous group of atypical Parkinsonism syndromes 65.2?±?12.5?years, 6 female). Analysis included group-level comparison of SWI values and individual-level support vector machine (SVM) analysis.

Results

At the group level, simple visual analysis yielded no differences between groups. However, the group-level analyses demonstrated increased SWI in the bilateral thalamus and left substantia nigra in PD patients versus other Parkinsonism. The inverse comparison yielded no supra-threshold clusters. At the individual level, SVM correctly classified PD patients with an accuracy above 86?%.

Conclusions

SVM pattern recognition of SWI data provides accurate discrimination of PD among patients with various forms of Parkinsonism at an individual level, despite the absence of visually detectable alterations. This pilot study warrants further confirmation in a larger cohort of PD patients and with different MR machines and MR parameters.

Key Points

? Magnetic resonance imaging data offers new insights into Parkinson??s disease ? Visual susceptibility-weighted imaging (SWI) analysis could not discriminate idiopathic from atypical PD ? However, support vector machine (SVM) analysis provided highly accurate detection of idiopathic PD ? SVM analysis may contribute to the clinical diagnosis of individual PD patients ? Such information can be readily obtained from routine MR data     

The prominent hypointense vessel sign on susceptibility-weighted imaging (SWI) as a potential imaging biomarker for poor clinical outcome in acute ischemic stroke (AIS)

Susceptibility-weighted imaging (SWI) is a relatively new magnetic resonance imaging (MRI) technique used in the workup and diagnosis of brain pathologies. In the context of acute ischemic stroke (AIS), it is increasingly becoming useful in the diagnosis, treatment, and further management of these patients. An elderly man with metabolic syndrome presented to us with an acute onset of right sided body weakness and aphasia. Urgent imaging via MRI noted a left middle cerebral artery (MCA) occlusion. Diffusion-weighted imaging (DWI)/fluid attenuated inversion recovery (FLAIR) mismatch was noted with an acute infarct involving the left MCA territory; hence, treatment with intravenous (IV) thrombolysis was administered. On SWI, the prominent hypointense vessel sign was noted. Recanalization of the occluded left MCA was seen on diagnostic cerebral angiography post IV thrombolysis, however, the patient was noted to have early neurological deterioration (END) and poor early stage clinical outcome, despite repeat MRI showing recanalization of the left MCA occlusion and reversal of the prominent hypointense vessel sign on SWI. Presence of the prominent hypointense vessel sign on SWI in AIS patients is associated with poor clinical outcome, unsuccessful recanalization rates, END, poor early stage clinical outcome, and infarct core progression. Some studies have shown an association between this imaging sign and poor collateral circulation status. Therefore, this imaging sign could potentially prove to be a useful imaging biomarker. However, more studies are needed to validate this theory.     

3.0T MRI相位对比法对流体模型的定量测量研究

目的：通过流体模型,检验3.0T磁共振相位对比法（3.0T PC-MRI）速度编码值（Venc）、编码方向的选择及其对流体动力学定量测量的准确性和稳定性。方法：固定流速和Venc,检验不同编码方向下的流体信息;固定Venc和编码方向,检验PC-MRI对不同注射流率（0-5 ml/s）所测流速值与标准流速的差别,采用配对样本t检验以检验二者差异性。结果：只有选择SLICE或SI编码方向才能正确反映质子流动的方向;在Venc适当大于实际流速时,不同注射流率PC-MRI流速测量值与流速真实值之间差异无统计学意义（t=-0.861,P〉0.05）,二者呈显著正相关关系（r=0.999,P〈0.001）。结论：选择正确的Venc和编码方向是准确测量流体动力学的前提,3.0T PC-MRI能客观、准确地测量流体模型的动力学信息,可用于临床血流定量测量研究。     

Magnetic susceptibility-weighted MR phase imaging of the human brain

BACKGROUND AND PURPOSE: MR gradient echo imaging is sensitive to the magnetic susceptibility of different tissue types. The purpose of this study was to investigate the diagnostic potential of MR phase imaging of the human brain. METHODS: High-spatial-resolution, T2*-weighted, single-echo images were acquired in five volunteers and one patient with a brain tumor on a 1.5T system by applying a 3D, first-order, velocity-compensated gradient echo sequence by using a quadrature transmit-receive head coil. Phase images were reconstructed from the raw data and unwrapped by using a region-growing phase-unwrapping algorithm. Low-spatial-frequency components originating from static background susceptibility effects were removed by high-pass filtering. RESULTS: Phase images showed excellent image contrast and revealed anatomic structures that were not visible on the corresponding magnitude images. CONCLUSION: Improved processing of susceptibility-weighted MR phase images offers a new means of contrast for neuroimaging applications.     

Predicting stroke evolution: comparison of susceptibility-weighted MR imaging with MR perfusion

Objectives

To investigate the ability of susceptibility-weighted imaging (SWI) to predict stroke evolution in comparison with perfusion-weighted imaging (PWI).

Methods

In a retrospective analysis of 15 patients with non-lacunar ischaemic stroke studied no later than 24?h after symptom onset, we used the Alberta Stroke Program Early CT Score (ASPECTS) to compare lesions on initial diffusion-weighted images (DWI), SWI, PWI and follow-up studies obtained at least 5?days after symptom onset. The National Institutes of Health Stroke Scale scores at entry and stroke risk factors were documented. The clinical–DWI, SWI–DWI and PWI–DWI mismatches were calculated.

Results

SWI–DWI and mean transit time (MTT)–DWI mismatches were significantly associated with higher incidence of infarct growth (P?=?0.007 and 0.028) and had similar ability to predict stroke evolution (P?=?1.0). ASPECTS values on initial DWI, SWI and PWI were significantly correlated with those on follow-up studies (P?≤?0.026) but not associated with infarct growth. The SWI ASPECTS values were best correlated with MTT ones (ρ?=?0.8, P?Conclusions SWI is an alternative to PWI to assess penumbra and predict stroke evolution. Further prospective studies are needed to evaluate the role of SWI in guiding thrombolytic therapy. Key Points ? SWI can provide perfusion information comparable to MTT ? SWI–DWI mismatch can indicate ischaemic penumbra ? SWI–DWI mismatch can be a predictor for stroke evolution     

预测脑卒中的进展:MR磁敏感加权成像与MR灌注成像的比较研究

摘要目的与MR灌注加权成像(PWI)比较,探讨磁敏感加权成像(SWI)在预测脑卒中进展方面的作用。方法对一组(15例)发病24h内的非腔隙性缺血性脑卒中病人的回顾性分析中,我们使用Alberta卒中项目早期CT评分标准(Alberta Stroke Program Early CT Score,ASPECTS)比较了发病初期和发病至少5d后扩散加权成像(DWI)、SWI、PWI上病变的变化。     

MR磁敏感加权成像检出肝硬化铁沉积结节的比较研究

目的:评价磁敏感加权成像(SWI)在肝硬化铁沉积结节(SN)检出中的价值。方法:40例病理证实的肝硬化患者及40例年龄、性别匹配的健康志愿者行MRI检查,扫描序列包括平扫T1WI、T2WI、T2*和SWI。2名腹部影像诊断医师共同评价,肝实质内局灶性低信号定义为SN,3mm以下SN定义为微小SN。计数SN总数目及微小SN数目,将其显示程度分为3个等级。结果:对照组均未见SN检出。40例肝硬化病例中T1WI上有10例无SN检出,T2WI、T2*和SWI均检出了SN。SWI检出SN的总数目、平均数目和微小SN明显高于T1WI、T2WI,SWI图像上表现为3级对比的SN(95%)明显高于T1WI(10%)、T2WI(10%)和T2*(33%)。结论:SWI能够较常规序列更为敏感的检出SN,且具有更好的图像对比,有望成为肝脏铁过载无创性评价的新方法。     

Posttraumatic syringomyelia: volumetric phantom and patient studies using MR imaging

The purpose of this study was to determine the volume of posttraumatic syringomyelia (PTS) based on standard MRI data acquisitions, and to compare the volumes with the neurological deficits of the patients. Firstly, phantom studies were performed using routine T1- (T1W; TR 500 ms, TE 13 ms) spin-echo (SE) images, 3D gradient-echo (GE) images and T2-weighted (T2W) turbo spin-echo (TSE) images (TR 3000 ms, TE 130 ms), in the sagittal plane. The slices were interleaved so that there was no gap. Twelve phantoms simulating a PTS were constructed and filled with fluid. Each volume was exactly measured immediately prior to filling (volumes: 3600–74,000 mm³, mean 27,500 mm³). In the clinical study 32 patients with PTS were examined using the same protocol. Patients were supine and a phased-array coil was used. The phantom studies revealed measurement errors of within 35%. There were problems defining the boundaries in the small and irregular phantoms as well as in small and irregular PTS, and due to the partial-volume averaging effect. The two small irregular phantoms could only be measured on the axial images. The T2W images in the axial plane showed the best results: measurement accuracy 92%. In the clinical study all examinations were technically successful. The volumes of the PTS ranged between 200 and 19,800 mm³; the mean volume was 4075 mm³. Our initial results show that the volume measurement of a PTS using standard MRI sequences can help generate more objective and accurate measures of spinal cord lesions, and this may enhance the sensitivity of MRI in detecting disease progression or regression after treatment. Electronic Publication     

Study of temperature rise in RF irradiation during MR imaging: measurement of local temperature using a loop phantom

Cases have been reported of a local rise in temperature at the patient's skin and of burns occurring during MR imaging. To verify this phenomenon, we created a phantom from agarose, saline, and preservative, and measured the increase in local temperature. In addition, phantoms of limbs of the human body shaped such that a closed loop was formed were also used. The temperature of the phantom was measured for 50 minutes in each state, i.e., where a closed loop was formed and where the loop was incomplete. Moreover, the radio frequency (RF) and gradient fields were set as respectively independent states, and the temperature of the phantom was measured. Results of the experiment showed that temperature changed from approximately 6 degrees to 11.5 degrees in the closed loop part of the phantom, whereas there was no significant change when the loop was incomplete. In addition, with exposure to RF, a significant rise in temperature occurred where the loop was closed, whereas there was no significant increase in temperature in gradient fields. This experiment demonstrated that the increase in temperature as a result of RF irradiation occurred in the closed part of the loop phantom. Consequently, a loop formed in the human body may be subject to burns in the area of contact.     

MR imaging using stimulated echoes (STEAM)

The introduction of STEAM (stimulated echo acquisition mode) magnetic resonance (MR) sequences provides access to a variety of MR parameters. T1-weighted and calculated T1 proton MR images of the head of healthy volunteers and a patient with an astrocytoma are presented. MR examinations were performed with a 2.0-T whole-body system. The STEAM T1 method can be used to characterize multiexponential relaxation behavior, to evaluate T1 relaxation times, and to improve the T1 contrast within MR images. Both the measuring time and the spatial resolution are the same as for a conventional image.     

Imaging putative foetal cerebral blood oxygenation using susceptibility weighted imaging (SWI)

Objective

To evaluate the magnetic susceptibility, ?χ _v , as a surrogate marker of venous blood oxygen saturation, S _v O ₂, in second- and third-trimester normal human foetuses.

Methods

Thirty-six pregnant women, having a mean gestational age (GA) of 31 2/7 weeks, underwent magnetic resonance imaging (MRI). Susceptibility-weighted imaging (SWI) data from the foetal brain were acquired. ?χ _v of the superior sagittal sinus (SSS) was quantified using MR susceptometry from the intra-vascular phase measurements. Assuming the magnetic property of foetal blood, ?χ _do , is the same as that of adult blood, S _v O ₂ was derived from the measured Δχ _v . The variation of ?χ _v and S _v O ₂, as a function of GA, was statistically evaluated.

Results

The mean ?χ _v in the SSS in the second-trimester (n = 8) and third-trimester foetuses (n = 28) was found to be 0.34± 0.06 ppm and 0.49 ±0.05 ppm, respectively. Correspondingly, the derived S _v O ₂ values were 69.4% ±3.27% and 62.6% ±3.25%. Although not statistically significant, an increasing trend (p = 0.08) in Δχ _v and a decreasing trend (p = 0.22) in S _v O ₂ with respect to advancing gestation was observed.

Conclusion

We report cerebral venous blood magnetic susceptibility and putative oxygen saturation in healthy human foetuses. Cerebral oxygen saturation in healthy human foetuses, despite a slight decreasing trend, does not change significantly with advancing gestation.

Key points

? Cerebral venous magnetic susceptibility and oxygenation in human foetuses can be quantified. ? Cerebral venous oxygenation was not different between second- and third-trimester foetuses. ? Foetal cerebral venous oxygenation does not change significantly with advancing gestation.

     

Ghost imaging for targeting breast masses with MR imaging: a phantom study

RATIONALE AND OBJECTIVES: The purpose of this study was to test the accuracy of ghost magnetic resonance (MR) imaging for guiding core biopsies of simulated breast masses in a tissue phantom. MATERIALS AND METHODS: A tissue MR phantom implanted with 20 grapes as targets was placed into an interventional breast MR coil. The locations of the centers of the targets were determined, recorded, and saved as ghost images. A nonmagnetic phantom needle was constructed to avoid imprecision secondary to magnetic field inhomogeneity and was used to determine the three-dimensional location of the needle tip in the center of each grape on the ghost image. Once the positions were determined, the true needle was placed and biopsy specimens were taken. The needle was inspected for the presence of pulp after each pass. Each grape was inspected to determine the location of the needle track in relation to the center of the grape. The duration of the procedure was recorded. RESULTS: All grapes were hit by the biopsy needle, as demonstrated either by pulp within the needle or by a needle track within the grape. Seventeen of the 20 grapes (85%) were hit centrally. Three were sampled eccentrically, up to 5-6 mm from the center. Each biopsy took approximately 1 hour. CONCLUSION: These results suggest that ghost imaging may be ideal for needle guidance in core biopsy or preoperative localization, as it extends the period of visibility after a bolus injection of contrast material. Additionally, using a phantom needle for localization appears to overcome imprecision due to magnetic field inhomogeneity of the needle.     

Intracranial mass lesions: dynamic contrast-enhanced susceptibility-weighted echo-planar perfusion MR imaging

Dynamic contrast agent-enhanced perfusion magnetic resonance (MR) imaging provides physiologic information that complements the anatomic information available with conventional MR imaging. Analysis of dynamic data from perfusion MR imaging, based on tracer kinetic theory, yields quantitative estimates of cerebral blood volume that reflect the underlying microvasculature and angiogenesis. Perfusion MR imaging is a fast and robust imaging technique that is increasingly used as a research tool to help evaluate and understand intracranial disease processes and as a clinical tool to help diagnose, manage, and understand intracranial mass lesions. With the increasing number of applications of perfusion MR imaging, it is important to understand the principles underlying the technique. In this review, the essential underlying physics and methods of dynamic contrast-enhanced susceptibility-weighted echo-planar perfusion MR imaging are described. The clinical applications of cerebral blood volume maps obtained with perfusion MR imaging in the differential diagnosis of intracranial mass lesions, as well as the pitfalls and limitations of the technique, are discussed. Emphasis is on the clinical role of perfusion MR imaging in providing insight into the underlying pathophysiology of cerebral microcirculation.