Susceptibility-weighted imaging: technical aspects and clinical applications, part 1

White matter lesions defined by diffusion tensor imaging in older adults

OBJECTIVE

The cellular and molecular mechanisms underlying magnetic resonance imaging-defined white matter (WM) changes associated with age-related cognitive decline remain poorly defined. We tested the hypothesis that WM lesions in older adults, defined by diffusion tensor imaging (DTI), arise in the setting of vascular brain injury (VBI) and are characterized by increased free radical injury and aberrant oligodendrocyte lineage (OL) cell response to injury.

METHODS

We undertook a multimodal analysis of prefrontal cortex (PFC) WM from 25 autopsies derived from a population-based cohort where VBI and Alzheimer disease (AD) frequently coincide. Ex vivo high field strength DTI measurements of fractional anisotropy (FA), apparent diffusion coefficient, and axial and radial (D(⊥) ) diffusivity were measured at high magnetic field strength (11.7T) and analyzed relative to quantitative in vivo biomarkers of free radical injury, an OL-specific marker Olig2, and histologic evaluation of hyaluronan (HA), an inhibitor of OL maturation.

RESULTS

Coincident AD and VBI showed significant association with lower FA and a robust relationship between decreasing FA and increasing D(⊥) . Free radical injury to docosahexaenoate and adrenate in PFC WM was significantly elevated in cases with VBI independent of AD, and was inversely correlated with FA. Similarly, increased density of Olig2-immunoreactive cells in PFC WM was significantly associated with VBI independent of AD and colocalized with regions enriched in HA.

INTERPRETATION

DTI-defined PFC WM lesions in older individuals are characterized by free radical injury to myelin and neuroaxonal elements that coincides with pronounced expansion of the pool of OL cells in HA-rich regions.

Putaminal hypointensity in the parkinsonian variant of multiple system atrophy: simple visual assessment using susceptibility-weighted imaging

Background and Purpose

Susceptibility-weighted imaging (SWI) has been shown to be superior in its ability to demonstrate brain mineralization than other conventional MR imaging. The goal of our study was therefore to assess the frequency and extent of putaminal hypointensity in parkinsonian variant MSA using SWI.

Methods

11 patients with multiple system atrophy-parkinsonian type (MSA-p), 30 patients with Parkinson’s disease (PD), and age matched 30 controls were investigated using 3 Tesla MRI. The pattern of putaminal hypointensity was measured using a visual grading scale and scored from 0 to 3.

Results

Hemi- or bilateral putaminal hypointensity (a score of ≥ 2) and hyperintense rim were recognized in 81.8% and 54.5% of 11 MSA-p, respectively. The scores of putaminal hypointensity of MSA-p were significantly higher than other groups (p < 0.001), a score of ≥ 2 differentiated MSA-p from other groups. And all five patients with early disease stage also showed these characteristic findings.

Conclusions

SWI appears to be useful for depicting putaminal hypointensity even in early stage of MSA-p. This finding suggests that iron deposition associated putaminal degeneration can occur early in the disease process.

Numerical Analysis of Human Sample Effect on RF Penetration and Liver MR Imaging at Ultrahigh Field

Magnetic resonance imaging (MRI) can provide clinically-valuable images for hepatic diseases and has become one of the most promising noninvasive methods in evaluating liver lesions. To facilitate the ultrahigh field human liver MRI, in this work, the RF penetration behavior in the conductive and high dielectric human body at the ultrahigh field of 7 Tesla (7T) is investigated and evaluated using the finite-difference time-domain numerical analysis. The study shows that in brain imaging at the ultrahigh field of 7T, the "dielectric resonance" effect dominates among other factors, resulting in improved B(1) penetration; while in liver imaging, due to its irregular geometry of the liver, the "dielectric resonance" effect is not readily to be established, leading to a reduced B(1) penetration or limited image coverage comparing to that in the brain. Therefore, it is necessary to build a large size coil to have deeper penetration to image human liver although the coil design may become more challenging due to the required high frequency. Based on this study, a bisected microstrip coil operating at 300 MHz range is designed and constructed. Three-dimensional in vivo liver images in axial, sagittal and coronal orientations are then acquired from healthy volunteers using this dedicated RF coil on a 7T whole body MR scanner.

Brain iron deposition and sequence characteristics in Parkinsonism: comparison of SWI, T₂* maps, T₂-weighted-, and FLAIR-SPACE

OBJECTIVES

To compare quantitatively T2- and T2*-based magnetic resonance imaging sequences in patients with symptoms of Parkinson disease and to evaluate the information content of those sequences regarding brain iron concentration.

MATERIALS AND METHODS

We imaged 51 patients with symptoms of Parkinson disease on 3-T magnetic resonance imaging with T2-weighted sampling perfection with application optimized contrasts using different flip-angle evolution (SPACE), fluid attenuation inversion recovery (FLAIR)-SPACE, susceptibility-weighted imaging (SWI), and parametric T2* sequence (MapIt). Signal analysis was performed in 22 regions of interest in the brain.

RESULTS

Correlations (r2 = 0.82...0.96) with brain iron concentration were excellent. Contrast and tissue separability ratios were best in the T2* maps and FLAIR-SPACE, respectively. Good correlations of contrast were reached between SWI and both T2-weighted SPACE and FLAIR-SPACE. Their relation to quantitative T2* values was reminiscent of a quadratic curve shape. However, separation into gray and white matter revealed a linear positive and negative correlation, respectively.

CONCLUSIONS

SWI showed potential in differentiating illnesses characterized by brain iron deposition. Closely similar information was given by T2-weighted SPACE and FLAIR-SPACE, whereas other sequence comparisons revealed dispersion from intersequence agreement.

Postcontrast susceptibility-weighted imaging: a novel technique for the detection of arteriovenous shunting in vascular malformations of the brain

BACKGROUND AND PURPOSE

The purpose of this study was to determine the utility of postcontrast susceptibility-weighted MRI (PCSWI) in the evaluation of vascular malformations of the brain (BVM).

METHODS

We retrospectively evaluated PCSWI and digital subtraction angiography data from 16 consecutive patients with known or suspected BVM, which had been entered into a prospectively maintained database during a 1-year period. There had been no intervening treatment or change in patients' symptoms between the PCSWI and digital subtraction angiography studies. The use of PCSWI in the detection of arteriovenous shunting was compared with that of routine noncontrast susceptibility-weighted imaging, time-of-flight MR angiography, and contrast-enhanced MR angiography using digital subtraction angiography results as the reference standard. The presence of arteriovenous shunting in PCSWI or susceptibility-weighted imaging sequences was defined by the presence of abnormal signal hyperintensity in the venous structures adjacent to the BVM.

RESULTS

A total of 17 BVMs were identified by digital subtraction angiography (9 newly diagnosed arteriovenous malformations, 3 dural arteriovenous fistulas, 4 treated arteriovenous malformations with residual arteriovenous shunting, and 1 complex developmental venous anomaly). PCSWI was 100% sensitive and 100% specific with 100% positive predictive value and 100% negative predictive value for the detection of arteriovenous shunting in these BVMs. The PCSWI/susceptibility-weighted imaging signal intensity ratio in the most prominent early draining venous structure was 1.2±0.32.

CONCLUSIONS

PCSWI appears to be superior to susceptibility-weighted imaging, time-of-flight MR angiography, and contrast-enhanced MR angiography in detecting arteriovenous shunting in BVMs and may be useful in the initial diagnosis and follow-up of patients with BVMs.

Relaxation time mapping in multiple sclerosis

Several relaxation mapping techniques have been proposed to quantitatively assess disease-related brain tissue changes in multiple sclerosis. Newer developments also account for the distribution of hydrogen protons in different tissue compartments, and therefore provide markers for myelin and macromolecular content. This article will cover the broad spectrum of the pulse sequences and analysis techniques related to this topic that are currently available. Various technical and practical limitations linked with specific approaches will be discussed. These include acquisition time, accuracy and precision, radiofrequency absorption and limited coverage of the brain. Finally, the application of these techniques in the context of multiple sclerosis will be reviewed.

Simultaneous visualization of nerves and vessels of the lower extremities using magnetization-prepared susceptibility weighted magnetic resonance imaging at 3.0 T

BACKGROUND

Identifying the extent of involvement of the vessel and nerve, particularly in regard to preoperative evaluation and precise localization of the tumor and its relation to the structures of the extremities, has important applications for advancing the treatment of lower extremity diseases.

OBJECTIVE

To review the technical feasibility of simultaneous visualization of nerves and vessels of the lower extremities by using magnetization-prepared susceptibility-weighted magnetic resonance (MR) imaging (MP-SWI) at 3.0T.

METHODS

Ten healthy volunteers and 10 patients were studied. Optimized MP-SWI, MR neurography (MRN) based on 3D diffusion-weighted steady-state free precession imaging and contrast-enhanced MR angiography (CE-MRA) sequences were performed for each subject. The means of signal-to-noise ratio (SNR)n, SNRv, SNRm, contrast-to-noise ratio (CNR)n,m and CNRv,m were calculated and the certainty of identifying nerves and vessels was determined. CNRn,m between MP-SWI and MRN, and CNRv,m between MP-SWI and CE-MRA were compared.

RESULTS

MP-SWI provides slightly poorer CNRv,m than CE-MRA, whereas MP-SWI provides a better CNRn,m than MRN. In thin-slice-thickness maximum-intensity projection arbitrary planes, the sciatic nerve and its branches were clearly identified (score 1 or 2 of 2) in 17 subjects (85%); the femoral artery and the main branches were identified (score 1 or 2 of 2) in all 20 subjects (100%). The nerves are isointense to slightly hypointense to muscle, and the vessels show a more obvious hyperintense signal than muscle in MP-SWI.

CONCLUSION

The proposed MP-SWI demonstrates the feasibility of simultaneously visualizing nerves and vessels of the lower extremities without using an exogenous contrast agent. It may enable straightforward localization of a disease process to a specific nerve and vessel.

Spin dephasing under nonlinear gradients: implications for imaging and field mapping

This work examines the prototypical MR echo that would be expected for a voxel of spins evolving in a strong nonlinear field, specifically focusing on the quadratic z(2) - ½(x(2) + y(2) ) field. Dephasing under nonlinear gradients is increasingly relevant given the growing interest in nonlinear imaging, and here, we report several notable differences from the linear case. Most notably, in addition to signal loss, intravoxel dephasing under gradients creating a wide and asymmetric frequency distribution across the voxel can cause skewed and nonlinear phase evolution. After presenting the qualitative and analytical origins of this difference, we experimentally demonstrate that neglecting these dynamics can lead to significant errors in sequences that assume phase evolution is proportional to voxel frequency, such as those used for field mapping. Finally, simplifying approximations to the signal equations are presented, which not only provide more intuitive forms of the exact expression but also result in simple rules to predict key features of the nonlinear evolution.

Pictorial essay: Susceptibility-weighted imaging in cerebral ischemia

The susceptiblity effect in magnetic resonance imaging (MRI) has been recognized for long and often has been considered undesirable, producing unnecessary noise. Susceptibility-weighted imaging (SWI) aims at exploiting this effect to provide a different type of contrast that is suited for vascular imaging. We describe five different cases in which SWI was found useful to delineate the underlying ischemia or to arrive at the corect diagnosis.

Clinical correlates of white matter blood flow perfusion changes in Sturge-Weber syndrome: a dynamic MR perfusion-weighted imaging study

BACKGROUND AND PURPOSE

Low brain tissue perfusion due to abnormal venous drainage is thought to be a central mechanism of brain damage in SWS. Here, HR-PWI was used to quantify WM perfusion abnormalities and to correlate these with brain atrophy and clinical variables.

MATERIALS AND METHODS

Fourteen children (age range, 0.8-10.0 years) with unilateral SWS underwent MR imaging examinations, including HR-PWI. rCBV, rCBF, and MTT in the affected WM and in contralateral homotopic WM were measured. AI for each perfusion parameter was correlated with age, brain atrophy, and motor and seizure variables as well as IQ.

RESULTS

Increased perfusion was seen in the affected hemisphere in 5 children and decreased perfusion in 9 children. Brain atrophy was more severe in the low-perfusion group (P = .01) and was related to both CBF-AI and CBV-AI (r = -0.69, P = .007; r = -0.64, P = .014, respectively). Older children had lower CBV values on the affected side (r = -0.62, P = .02). Longer duration of epilepsy was related to lower CBF (more negative CBF-AI, r = -0.58, P = .03) and low CBV (r = -0.55, P = .04) on the affected side. Lower perfusion was associated with more frequent seizures (rCBF-AI: r = -0.56, P = .04; rCBV-AI: r = -0.63, P = .02).

CONCLUSIONS

Increased perfusion in the affected cerebral WM may indicate an early stage of SWS without severe brain atrophy. Decreased perfusion is associated with frequent seizures, long duration of epilepsy, and brain atrophy.

Dissected sentinel lymph nodes of breast cancer patients: characterization with high-spatial-resolution 7-T MR imaging

PURPOSE

To investigate the association of 7-T magnetic resonance (MR) imaging characteristics with metastatic nodal invasion, determined with histopathologic assessment in dissected sentinel lymph nodes of breast cancer patients.

MATERIALS AND METHODS

Institutional review board approval and informed consent were obtained. From November 2008 to July 2010, 114 dissected lymph nodes from 33 women (mean age, 57 years; range, 31-80 years) with breast cancer were included. For morphological analysis, three-dimensional (3D) T1-weighted fat-suppressed fast field- (gradient-) echo (isotropic resolution, 180 μm) MR was performed; 3D nodal dimensions, maximum cortical thickness, and presence of fatty hilum were noted. For quantitative parametric analysis, two-dimensional T1-weighted and 3D T2-, T2*-, and diffusion-weighted images were acquired. Statistical analysis included generalized estimating equations (GEEs), forward and backward stepwise regression analyses, and calculation of positive predictive value (PPV) and negative predictive value (NPV).

RESULTS

Of 114 nodes, 26 (23%) were malignant. Morphological criteria showed weak discriminatory power: A fatty center was absent in 35% of malignant nodes and 30% of benign nodes (P = .9). Nodal volume and length-width ratio were not significantly different (P = .11 and .75, respectively). Cortical thickness (threshold level, 3 mm; P = .02) showed 91% NPV for malignancy and 95% NPV for presence of macrometastases. Quantitative parametric analyses showed comparable mean T1, T2, and T2* relaxation time constants and apparent diffusion coefficient for metastatic and benign nodes: 991 msec, 30 msec, and 18 msec and 0.17 mm²/sec versus 1035 msec (P = .14), 31 msec (P = .001; not significant after GEE), and 15 msec (P = .002) and 0.20 mm²/sec (P = .38), respectively. Mean T2* alone offered an additive discriminatory effect for identification of metastatic nodes. Consistent with the notion of pannodal changes accompanying tumor infiltration, mean T2* differed significantly even if only micrometastases were present. The interindividual differences were small, precluding easy clinical implementation.

CONCLUSION

Morphological criteria showed poor discriminatory power, even with very-high-spatial-resolution imaging. T2* quantification allowed identification of metastatic nodal invasion.

Whole brain susceptibility mapping using compressed sensing

The derivation of susceptibility from image phase is hampered by the ill-conditioned filter inversion in certain k-space regions. In this article, compressed sensing is used to compensate for the k-space regions where direct filter inversion is unstable. A significantly lower level of streaking artifacts is produced in the resulting susceptibility maps for both simulated and in vivo data sets compared to outcomes obtained using the direct threshold method. It is also demonstrated that the compressed sensing based method outperforms regularization based methods. The key difference between the regularized inversions and compressed sensing compensated inversions is that, in the former case, the entire k-space spectrum estimation is affected by the ill-conditioned filter inversion in certain k-space regions, whereas in the compressed sensing based method only the ill-conditioned k-space regions are estimated. In the susceptibility map calculated from the phase measurement obtained using a 3T scanner, not only are the iron-rich regions well depicted, but good contrast between white and gray matter interfaces that feature a low level of susceptibility variations are also obtained. The correlation between the iron content and the susceptibility levels in iron-rich deep nucleus regions is studied, and strong linear relationships are observed which agree with previous findings.

Clinical relevance of improved microbleed detection by susceptibility-weighted magnetic resonance imaging

BACKGROUND AND PURPOSE

Susceptibility-weighted imaging (SWI) has been shown to be more sensitive in detecting cerebral microbleeds (MBs) than is conventional T2*-weighted gradient-recalled echo imaging (GRE). However, the clinical relevance of this improved detection in terms of associations with clinical measures and risk factors is unclear. We sought to determine whether associations of MBs with clinical characteristics, risk factors, white-matter hyperintensities, and lacunes were different on GRE versus SWI in memory clinic patients.

METHODS

One hundred forty-one patients presenting at our memory clinic were included and underwent clinical evaluation and a magnetic resonance imaging protocol including both GRE and SWI. Images were analyzed for numbers and locations of MBs and white-matter hyperintensities. In a subset of patients, apolipoprotein E status was determined. Negative binomial regression was used to assess clinical and radiologic associations with MB number.

RESULTS

MB prevalence was 23% on GRE and 40% on SWI. A total of 219 and 284 MBs were detected on GRE and SWI, respectively. Within groups with MBs, the median MB count was 1 (range, 1 to 144) on GRE and 2 (range, 1 to 129) on SWI (P<0.001). The increase in the number of MBs on SWI was equally distributed among brain regions. Strengths of the associations with age, sex, white-matter hyperintensities, and presence of lacunes with higher MB numbers were comparable for GRE and SWI (all P<0.05); no differential independent associations were detected.

CONCLUSIONS

SWI detected more MBs in more patients, irrespective of MB location. However, this enhanced detection had no improved clinical relevance in terms of associations with vascular risk factors or radiologic markers of small-vessel disease.

Punctate white matter lesions in infants: new insights using susceptibility-weighted imaging

INTRODUCTION

Punctate white matter lesions (PWML) are recognized with magnetic resonance imaging (MRI) as hypersignal on T1-weighted imaging and hyposignal on T2-weighted imaging. Our aim was to assess how often a hemorrhagic component was present in PWML using susceptibility-weighted imaging (SWI).

METHODS

Seventeen preterm (gestational age, 25-34 weeks) and seven full-term infants (age at MRI, 37-42 weeks) with PWML were included. Seven preterm infants had sequential MRIs. PWML were diagnosed with conventional MRI and compared with SWI, where signal loss is suggestive of hemorrhage. The pattern of associated brain lesions was taken into account, and the percentage of lesions with signal loss on SWI was calculated for each infant.

RESULTS

A significantly higher percentage of signal loss on SWI (median, 93.9%) was found among infants with germinal matrix and intraventricular hemorrhage as the primary diagnosis (n = 8) compared to those with a primary diagnosis of white matter injury (n = 14; median, 14.2%; p < 0.01). In the infants with serial MRIs, a reduction in the number of PWML and/or signal loss on SWI was noted at term equivalent age. In the patient who died, cystic lesions, associated with hemorrhage and gliosis, were demonstrated on histology.

CONCLUSIONS

SWI can distinguish hemorrhagic and non-hemorrhagic PWML. Signal loss on SWI was more common when PWML were associated with an intraventricular hemorrhage. Longitudinal imaging showed a decrease in the number of PWML over time, with some PWML no longer showing signal loss on SWI, suggesting early gliosis.

Cerebral microbleeds on MR imaging: comparison between 1.5 and 7T

BACKGROUND AND PURPOSE

The detection of microbleeds differs strongly between studies, due to differences in scan protocol. This study aims to compare the visualization of microbleeds with 3D T2*-weighted imaging at 1.5T with 3D dual-echo T2*-weighted imaging at 7T.

MATERIALS AND METHODS

Thirty-four patients (29 male; mean age, 58 ± 12 years) with atherosclerotic disease from the Second Manifestations of ARTerial Disease study were included. 3D T2*-weighted imaging at 1.5T and dual-echo T2*-weighted imaging at 7T were done in all patients. The presence and number of definite microbleeds were recorded on minimal intensity projections. Inter- and intraobserver reliability was assessed with Cohen κ test and the ICC. The difference in presence and number of microbleeds was tested with the McNemar test and Wilcoxon signed rank test.

RESULTS

The interobserver ICC at 7T was 0.61 and the intraobserver ICC was 0.94, whereas at 1.5T the interobserver ICC was 0.50 and the intraobserver ICC was 0.59. Microbleeds were detected in significantly more patients on 7T (50%) than on 1.5T scans (21%) (P = .001). The number of microbleeds was also higher at 7T (median, 0.5; range, 0-5) than on 1.5T (median, 0.0; range, 0-6) (P = .002).

CONCLUSIONS

3D dual-echo T2*-weighted imaging at 7T results in better and more reliable detection of microbleeds compared with 3D T2*-weighted imaging at 1.5T.

Evaluation of parenchymal neuro-behçet disease by using susceptibility-weighted imaging

BACKGROUND AND PURPOSE

Neurologic involvement in Behçet disease, also known as NBD, is one of the most devastating manifestations of the disease. The precise pathologic mechanism of parenchymal NBD lesions has not been established. We evaluated lesion characteristics and probable venous hemorrhage in parenchymal NBD by using SWI, and we compared the imaging results with conventional MR imaging sequences.

MATERIALS AND METHODS

We performed cranial MR imaging by using a 1.5T scanner in 23 patients with a definitive diagnosis of parenchymal NBD. We compared the proportion of lesion detection and the performance of hemorrhagic detection with the T2 FSE, T2*GE, and SWI magnitude, and SWI mIP by using the χ(2) test.

RESULTS

The proportion of lesion detection with both SWI magnitude and SWI MinMIP was significantly larger than that with T2*GE. The proportions of lesion detection among all other pairs of methods were not significantly different according to the corresponding P value (χ(2) = 17.4929, df = 3, P = .0006). Proportions of hypointense hemorrhagic lesions with T2 FSE and T2*GE were not significantly different, and likewise for the proportions of hypointense hemorrhagic lesions with SWI magnitude and SWI mIP. In contrast, the proportions of hypointense hemorrhagic lesions with SWI magnitude and SWI mIP were significantly larger than that with T2 FSE and T2*GE (χ(2) = 108.5396, df = 3, P < .0001).

CONCLUSIONS

Most of the lesions in parenchymal NBD were found to be hemorrhagic with SWI, supporting the proposed venous theory in pathology. In addition, compared with T2 FSE and T2*GE sequences, SWI was more successful in the determination of widespread involvement of the disease, particularly in nonchronic cases.

Susceptibility phase imaging with comparison to R2 mapping of iron-rich deep grey matter

Magnetic resonance imaging with susceptibility phase is seeing increasing use, especially at high magnetic fields. Tissue susceptibility can produce unique phase contrast for qualitative or quantitative imaging of iron-rich deep grey matter. However, phase imaging has several established sources of error including inherent susceptibility field effects and artifacts from background phase removal. These artifacts have led to inconsistent findings in past works relating iron to phase in healthy deep grey matter. This study seeks to determine the relative artifactual contributions from inherent susceptibility fields and from high pass phase filtering, currently the most common and accessible background phase removal method. In simulation, phase is compared to a known susceptibility distribution, while R2 maps are used as the in vivo gold standard surrogate for iron in healthy volunteers. The results indicate phase imaging depends highly on filtering, structure size, shape and local environment. Using in vivo phase and R2 profiles, it is shown that different filtering values, commonly seen in the literature, can lead to substantially different phase measures. Correlations between phase and R2 mapping are shown to be highly variable between structures. For example, using a standard filter of 0.125 the slopes and correlation coefficients were 4.28×10(-4) ppm s and R=0.88 for the putamen, 0.81×10(-4) ppm s and R=0.08 for the globus pallidus, 5.48×10(-4) ppm s and R=0.72 for the red nucleus, and -14.64×10(-4) ppm s and R=0.54 for the substantia nigra. To achieve the most effective correlation to R2 we recommend using a filter width of 0.094 for the globus pallidus and putamen and 0.125 for the substantia nigra and red nucleus. The baseline phase measure should be obtained directly adjacent to the substantia nigra, and red nucleus to yield the most accurate phase values as demonstrated in simulation and in vivo. Different regression slopes are seen between subROIs within structures suggesting that regional iron accumulation within a structure is best studied with subROIs between different subject groups, not differences in phase values relative to the overall phase in one structure. Phase imaging with the standard high pass filter method has the potential to differentiate subtle iron changes in pathological processes compared to normal tissues with more reliability if specific filter strengths and measurement areas are appropriately applied on a structure dependent basis.

Susceptibility-weighted angiography (SWAN) of cerebral veins and arteries compared to TOF-MRA

PROBLEM

High resolution, non-contrast imaging of both cerebral veins and arteries by use of gradient echo T2 star weighted angiography (SWAN) is a new method for susceptibility-weighted imaging with short acquisition times. We assessed the potential of this sequence for the depiction of both cerebral veins and arteries.

METHODS

15 healthy volunteers were included in the study. MRI was performed on a 3T MR scanner using the following sequences: (1) a 3D multi-echo gradient echo T2 star weighted angiography (SWAN), (2) an arterial 3D TOF MR angiography and (3) a venous 2D TOF. With regard to the SWAN sequence, both MinIP and MIP images were reconstructed and systematically compared to MIP reconstructions of the artTOF and the venTOF. To suggest possible clinical implications of our findings, we additionally included two illustrative cases.

RESULTS

With regard to the visualization of the cerebral veins, the MinIP reconstructions of the SWAN sequence were considerably superior compared to the venTOF. Concerning the depiction of the main segments of the big cerebral arteries the value of the MIP reconstructions of the SWAN was comparable to that of the artTOF with limitations in the homogenity and in the depiction of smaller arteries.

CONCLUSIONS

SWAN allows for high-resolution visualization of both cerebral veins and arteries in one sequence without application of contrast agent and with significantly shortened scan time compared to the combined scan time of TOF-MRA and TOF-MRV. By use of either MinIP or MIP reconstructions, the arteries can be distinguished from the veins.

Multimodality intraoperative MRI for brain tumor surgery

Intraoperative MRI has already fundamentally changed the way current brain tumor surgery is performed. The ability to integrate high-field MRI into the operating room has allowed intraoperative MRI to emerge as an important adjunct to CNS tumor treatment. Furthermore, the ability of MRI to successfully couple with molecular imaging (PET and/or optical imaging), neuroendoscopy and therapeutic devices, such as focused ultrasound, will allow it to emerge as an important image-guidance modality for improving brain tumor therapy and outcomes.

New MR sequences in daily practice: susceptibility weighted imaging. A pictorial essay

Background

Susceptibility-weighted imaging (SWI) is a relatively new magnetic resonance (MR) technique that exploits the magnetic susceptibility differences of various tissues, such as blood, iron and calcification, as a new source of contrast enhancement. This pictorial review is aimed at illustrating and discussing its main clinical applications.

Methods

SWI is based on high-resolution, three-dimensional (3D), fully velocity-compensated gradient-echo sequences using both magnitude and phase images. A phase mask obtained from the MR phase images is multiplied with magnitude images in order to increase the visualisation of the smaller veins and other sources of susceptibility effects, which are displayed at best after post-processing of the 3D dataset with the minimal intensity projection (minIP) algorithm.

Results

SWI is very useful in detecting cerebral microbleeds in ageing and occult low-flow vascular malformations, in characterising brain tumours and degenerative diseases of the brain, and in recognizing calcifications in various pathological conditions. The phase images are especially useful in differentiating between paramagnetic susceptibility effects of blood and diamagnetic effects of calcium. SWI can also be used to evaluate changes in iron content in different neurodegenerative disorders.

Conclusion

SWI is useful in differentiating and characterising diverse brain disorders.

Suspicious neuroimaging pattern of thrombotic microangiopathy

The TMAs are a group of microvascular occlusive disorders characterized by thrombocytopenia and intravascular hemolysis. Literature review reveals a spectrum of neuroimaging findings, including a single case report of multifocal hemorrhagic infarctions. We present a series of 12 patients with TMA demonstrating a similar pattern of multifocal cortical and subcortical hemorrhagic infarctions.

Clinical Application of Susceptibility Weighted Imaging: Exploring from Routine Service

To evaluate the usefulness of susceptibility weighted imaging (SWI) in clinical brain MRI. Retrospective study was performed after approval from institution Ethical Committee. The brain MRIs with SWI were selected from data base of the radiology department. Only cases with no abnormality in extra-axial location were included into the study. Two neuroradiologists revealed the images without knowledge of patients' history and diagnosis. The SWI was first interpreted. Then conventional MRI (cMRI) was interpreted after finishing data collection from SWI. Clinical data and final diagnosis were collected from information given on requested forms and followed up imaging studies. Descriptive analysis was performed. From January 2007 to December 2009, 82 cases were satisfied the inclusion criteria. There were 40 males and 42 females with age 7–79 years old (means = 47.45). The final diagnosis were normal brain imaging 4 cases (4.9%), dementia/atypical Parkinson disease 2 cases (2.4%), cerebrovascular disease 24 cases (29.3%), parenchymal brain tumors 35 cases (42.7%), infection 4 cases (4.9%), multiple sclerosis (MS) 6 cases (7.3%) and inconclusive diagnosis 7 cases (8.5%). The abnormalities found on SWI were related to cMRI in 67 cases (81.7%). Three cases (3.7%) had lesions on SWI not demonstrated on cMRI The information got from SWI added on cMRI for interpretation and diagnosis in 43 cases (52.4%). SWI were shown more detectable microbleed and changing visualization of cortical and transmedullary veins. Microvascular structure inside the mass was demonstrated on SWI in brain tumor group. The increased transmedullary veins on SWI helped to confirm the non-neoplastic lesions. In selected cases with suspected or diagnosis of vascular disease and tumor, SWI added more information on cMRI especially microbleed and microvascular structure.

Susceptibility Weighted Imaging and Cerebrovascular Disorders

To investigate the correlation between the SWI findings and prognosis of the cerebrovascular disorders. From July 2008 to July 2010, 299 ischemic stroke patients were found in our hospital. The gender ratio is as male and female being 157 to 142. The mean age of all patients is 65.4, mean female age is 69.1, and mean male age is 62.6. There were 86 patients who had satisfactory pre-and post-treatment of CT, MRI with SWI. 23 of these 86 patients had catheter cerebral angiography. 50 of these 86 patients had MR angiogram or CT angiogram. 13 of these 86 patients did not have angiogram. We have also collected 7 severe cardiac arrested and cessation of cerebral circulation and 2 patients with chronic venous hypertension. Among the 86 patients, 23 patients who had negative with deoxygenated vessel on SWI were with small infarction on DWI Thirty-one patients had negative on initial CT head scan. CT finding did not accord with presence of hypointense vessel on SWI. Sixty-three patients had varied degree of abnormal hypointense vessels on SWI as deoxygenated vessels. The initial small foci on DWI may result with a larger infarction if there were with prominent hypointense vessels.

Susceptibility-weighted imaging of the brain: current utility and potential applications

BACKGROUND AND PURPOSE

Susceptibility-Weighted Imaging (SWI) is a relatively new magnetic resonance imaging (MRI) sequence relying on susceptibility differences between adjacent tissues to produce an image. It is extremely sensitive for detection of blood products (hemosiderin, ferritin), deoxygenated blood, calcium, iron, and small vein depiction. Little information is available in the literature to describe common findings seen using this imaging sequence. This article is an comprehensive review of appearances across the spectrum of brain pathology encountered in routine clinical practice.

MATERIALS AND METHODS

A retrospective review of 400 MRI Brain examinations was performed by one fellowship trained Neuroradiologist with 5 years experience (SB) and one general radiologist (RR). Agreement was by consensus observing normal and pathological imaging features seen in SWI sequences.

RESULTS

Results are presented as a comprehensive pictorial review highlighting the key imaging findings observed and new directions using SWI.

CONCLUSION

SWI is an extremely useful adjunct to current MRI sequences of the brain and is advocated for inclusion into routine Neuroimaging protocols.

Susceptibility weighted imaging in detecting hemorrhage in acute cervical spinal cord injury

BACKGROUND AND PURPOSE

Susceptibility weighted imaging (SWI) is sensitive to deoxyhemoglobin and blood products such as hemosiderin in detecting microbleeds in the brain. However, there are no studies on SWI in the spine cord injury so far. The purpose of this study was to evaluate the role of SWI in detecting hemorrhage in acute cervical spinal cord injury (SCI).

MATERIALS AND METHODS

Twenty-three patients with a history of acute cervical spine trauma were studied. High-resolution SWI, gradient-echo (GRE) T2* weighted-image (T2*WI) and conventional magnetic resonance imaging (MRI) were performed on all patients within 15 days of the onset of injury. On the basis of the MRI findings, the patients were classified into four patterns: normal cord, spinal cord edema, spinal cord contusion and spinal cord hemorrhage. Quantitative analysis was performed by calculating and comparing the signal ratio of the hemorrhage to normal spinal cord on the same slice of T2*WI and SWI. All patients were clinically evaluated in follow-up. Twenty volunteers were also scanned as a control group.

RESULTS

Out of 23 patients with a history of acute cervical spine trauma, 4 patients showed normal spinal cord on both conventional MRI and SWI, 8 had only spinal cord edema and 5 had contusion on conventional MRI, but SWI showed hemorrhage in 2 of the 5 patients with spinal contusion on conventional MRI; the other 6 patients had intraspinal hemorrhage on conventional MRI, and SWI proved hemorrhage in all these 6 patients. There was a significant difference between the signal ratios of hemorrhage to normal tissue on T2*WI and SWI (Z=2.34, P=.02).

CONCLUSION

Susceptibility weighted imaging is more sensitive than conventional MRI in detecting hemorrhage in acute cervical SCI. This technique could prove to be a useful tool in the routine evaluation of cervical SCI patients.

Correlation of putative iron content as represented by changes in R2* and phase with age in deep gray matter of healthy adults

PURPOSE

To establish a correlation between putative iron content using susceptibility weighted imaging (SWI) phase and T2* weighted magnitude values in the basal ganglia and the thalamus as a function of age in healthy human brains.

MATERIALS AND METHODS

One hundred healthy adults (range, 20-69 years; mean, 43 years) were evaluated for this study using a gradient echo sequence. The original magnitude and high pass filtered phase data were analyzed as proxy variables for iron content in the substantia nigra, red nucleus, globus pallidus, putamen, caudate nucleus, thalamus, and pulvinar thalamus. Each structure was broken into two parts, a high iron content region and a low iron content region.

RESULTS

Both magnitude and phase data showed an increase in putative iron content with age. However, the high iron content region revealed two new pieces of information: both the average iron content per pixel and the area of high iron increased with age. Furthermore, significant increase in iron uptake as a function of age was found past the age of 40.

CONCLUSION

A two region of interest analysis of iron is a much more sensitive means to evaluate iron content change over time. Contrary to the current belief that iron content increases level off with age, the putative iron deposition in the high iron content region is seen to increase with age.

Quantitative susceptibility mapping of human brain reflects spatial variation in tissue composition

Image phase from gradient echo MRI provides a unique contrast that reflects brain tissue composition variations, such as iron and myelin distribution. Phase imaging is emerging as a powerful tool for the investigation of functional brain anatomy and disease diagnosis. However, the quantitative value of phase is compromised by its nonlocal and orientation dependent properties. There is an increasing need for reliable quantification of magnetic susceptibility, the intrinsic property of tissue. In this study, we developed a novel and accurate susceptibility mapping method that is also phase-wrap insensitive. The proposed susceptibility mapping method utilized two complementary equations: (1) the Fourier relationship of phase and magnetic susceptibility; and (2) the first-order partial derivative of the first equation in the spatial frequency domain. In numerical simulation, this method reconstructed the susceptibility map almost free of streaking artifact. Further, the iterative implementation of this method allowed for high quality reconstruction of susceptibility maps of human brain in vivo. The reconstructed susceptibility map provided excellent contrast of iron-rich deep nuclei and white matter bundles from surrounding tissues. Further, it also revealed anisotropic magnetic susceptibility in brain white matter. Hence, the proposed susceptibility mapping method may provide a powerful tool for the study of brain physiology and pathophysiology. Further elucidation of anisotropic magnetic susceptibility in vivo may allow us to gain more insight into the white matter micro-architectures.

Susceptibility-weighted imaging of the brain: does gadolinium administration matter?

OBJECTIVE

Susceptibility-weighted MR imaging (SWI) is usually obtained without administration of intravenous gadolinium (Gd). However, it is occasionally necessary to perform SWI after Gd is injected. The effects of Gd on SWI have not been systematically examined. The aim of this prospective study was to investigate whether performing SWI after Gd would influence the diagnostic image quality, parenchymal signal and vascular enhancement. An additional aim is to suggest potential future applications for Gd-enhanced SWI.

METHODS

SWI was performed in 31 subjects before and after Gd administration. 17 cases were examined in a 1.5T scanner and the remaining 14 were scanned at 3T. The pre- and post-Gd images were analysed for signal changes in the cerebral grey matter (GM), white matter (WM) as well as for enhancement in the superficial and deep venous system. The visibility of the veins was graded on subtraction maps.

RESULTS

The Gd-enhanced images showed no image quality degradation and no significant signal intensity change in the GM and WM as compared to the pre-Gd images (p>0.05). After Gd-administration significant enhancement of the venous sinuses was noticed (p<0.005), while the deep and cortical veins were poorly enhanced as confirmed by the calculated subtraction maps. The results showed no significant difference at variable MRI field strengths.

CONCLUSION

It is possible to perform SWI after Gd injection without information loss or signal change in the parenchyma. The most significant difference is the enhancement of the cerebral venous sinuses. Potential future applications are discussed.

Neuroimaging in rabies

Rabies remains a virtually incurable disease once symptoms develop. Neuroimaging studies demonstrate lesions in the different parts of the neuroaxis, even before brain symptoms are evident. These abnormalities have been detailed in both rabies virus-infected humans and dogs with magnetic resonance imaging (MRI). MRI disturbances were similar in both forms (furious or paralytic) in human rabies; however, they were more pronounced in paralytic than in furious rabies virus-infected dogs in which examination was done early in the disease course. Abnormalities were not confined only to neuronal structures of hippocampus, hypothalamus, basal ganglia, and brain stem but also extended to white matter. The blood-brain barrier (BBB) has been clearly shown to be intact during the time rabies virus-infected patients and dogs remained conscious, whereas leakage was demonstrated as soon as they became comatose. Although the location of MRI abnormalities can help diagnosing rabies, the intensities of signals are usually not very distinct and sometimes not recognizable. Newer techniques and protocols have been developed and utilized, such as diffusion-weighted imaging and diffusion tensor imaging, and the latter provides both qualitative and quantitative data. These techniques have been applied to normal and rabies virus-infected dogs to construct fractional anisotropy and mean diffusivity maps. Results showed clear-cut evidence of BBB intactness with absence of vasogenic brain edema and preservation of most neuronal structures and tracts except at the level of brainstem in paralytic rabies-infected dogs. Neuroimaging is one of the most useful tools for the in vivo study of central nervous system infections.

MRI in rodent models of brain disorders

Magnetic resonance imaging (MRI) is a well-established tool in clinical practice and research on human neurological disorders. Translational MRI research utilizing rodent models of central nervous system (CNS) diseases is becoming popular with the increased availability of dedicated small animal MRI systems. Projects utilizing this technology typically fall into one of two categories: 1) true "pre-clinical" studies involving the use of MRI as a noninvasive disease monitoring tool which serves as a biomarker for selected aspects of the disease and 2) studies investigating the pathomechanism of known human MRI findings in CNS disease models. Most small animal MRI systems operate at 4.7-11.7 Tesla field strengths. Although the higher field strength clearly results in a higher signal-to-noise ratio, which enables higher resolution acquisition, a variety of artifacts and limitations related to the specific absorption rate represent significant challenges in these experiments. In addition to standard T1-, T2-, and T2*-weighted MRI methods, all of the currently available advanced MRI techniques have been utilized in experimental animals, including diffusion, perfusion, and susceptibility weighted imaging, functional magnetic resonance imaging, chemical shift imaging, heteronuclear imaging, and (1)H or (31)P MR spectroscopy. Selected MRI techniques are also exclusively utilized in experimental research, including manganese-enhanced MRI, and cell-specific/molecular imaging techniques utilizing negative contrast materials. In this review, we describe technical and practical aspects of small animal MRI and provide examples of different MRI techniques in anatomical imaging and tract tracing as well as several models of neurological disorders, including inflammatory, neurodegenerative, vascular, and traumatic brain and spinal cord injury models, and neoplastic diseases.

Pediatric hemiplegic migraine: susceptibility weighted and MR perfusion imaging abnormality

We report on an 11-year-old girl suffering from a typical attack of hemiplegic migraine with characteristic abnormalities in perfusion MR and susceptibility-weighted MR imaging findings. The imaging abnormalities were resolved 48 h after the attack. Susceptibility-weighted MR imaging findings correlated well with the MR perfusion, thus it can be used along with conventional MRI for evaluation of children with complex migraine attacks. Susceptibility-weighted MR imaging might have a diagnostic role in assessing the vascular events in hemiplegic migraine.

Accuracy of susceptibility-weighted imaging for the detection of arteriovenous shunting in vascular malformations of the brain

BACKGROUND AND PURPOSE

to determine the accuracy of susceptibility-weighted MRI (SWI) for the detection of arteriovenous shunting (AVS) in vascular malformations of the brain (BVM).

METHODS

we retrospectively identified 60 patients who had been evaluated for known or suspected BVM by both SWI and digital subtraction angiography, without intervening treatment, during a 3-year period. SWI images were retrospectively assessed by 2 independent reviewers for the presence of AVS as determined by the presence of signal hyperintensity within a venous structure in the vicinity of the BVM. Discrepancies were resolved by consensus among a panel of 3 neuroradiologists. Accuracy parameters of SWI for the detection of AVS were calculated using digital subtraction angiography as the reference standard.

RESULTS

a total of 80 BVM were identified in the 60 patients included in our study. Of the 29 BVM with AVS on digital subtraction angiography, 14 were untreated arteriovenous malformations, 10 were previously treated arteriovenous malformations, and 5 were untreated dural arteriovenous fistulas. Overall, SWI was 93% sensitive and 98% specific for the detection of AVS in BVM, with excellent interobserver agreement (κ=0.94). In the 14 previously treated arteriovenous malformations, SWI was 100% sensitive and specific for the detection of AVS. In the 28 BVM associated with intracerebral hemorrhage, SWI was 100% sensitive and 96% specific for the detection of AVS.

CONCLUSIONS

SWI is accurate for the detection of arteriovenous shunting in vascular malformations of the brain and, for some patients, SWI may offer a noninvasive alternative to angiography in screening for or follow-up of treated BVM.

The role of advanced MR imaging findings as biomarkers of traumatic brain injury

Treatment of traumatic brain injury (TBI) requires proper classification of the pathophysiology. Clinical classifiers and conventional neuroimaging are limited in TBI detection, outcome prediction, and treatment guidance. Advanced magnetic resonance imaging (MRI) techniques such as susceptibility weighted imaging, diffusion tensor imaging, and magnetic resonance spectroscopic imaging are sensitive to microhemorrhages, white matter injury, and abnormal metabolic activities, respectively, in brain injury. In this article, we reviewed these 3 advanced MRI methods and their applications in TBI and report some new findings from our research. These MRI techniques have already demonstrated their potential to improve TBI detection and outcome prediction. As such, they have demonstrated the capacity of serving as a set of biomarkers to reveal the heterogeneous and complex nature of brain injury in a regional and temporal manner. Further longitudinal studies using advanced MRI in a synergistic approach are expected to provide insight in understanding TBI and imaging implications for treatment.

Diffusion tensor imaging of mild traumatic brain injury

Mild traumatic brain injury (mTBI) remains a challenge to accurately assess with conventional neuroimaging. Recent research holds out the promise that diffusion tensor imaging (DTI) can be used to predict recovery in mTBI patients. Unlike computed tomography or conventional magnetic resonance imaging, DTI is sensitive to microstructural axonal injury, the neuropathology that is thought to be most responsible for the persistent cognitive and behavioral impairments that often occur after mTBI. Through the use of newer DTI analysis techniques such as automated region of interest analysis, tract-based voxel-wise analysis, and quantitative tractography, researchers have shown that frontal and temporal association white matter pathways are most frequently damaged in mTBI and that the microstructural integrity of these tracts correlates with behavioral and cognitive measures. Future longitudinal DTI studies are needed to elucidate how symptoms and the microstructural pathology evolve over time. Moving forward, large-scale investigations will ascertain whether DTI can serve as a predictive imaging biomarker for long-term neurocognitive deficits after mTBI that would be of value for triaging patients to clinical trials of experimental cognitive enhancement therapies and rehabilitation methods, as well as for monitoring their response to these interventions.

Imaging and clinical characteristics of children with multiple foci of microsusceptibility changes in the brain on susceptibility-weighted MRI

BACKGROUND

Microsusceptibility changes in the brain are well known to correspond with microbleeds or micrometal fragments in adults, but this phenomenon has not been explored well in children.

OBJECTIVE

To assess imaging and clinical characteristics of children with multiple foci of microsusceptibility changes using susceptibility-weighted imaging (SWI).

MATERIALS AND METHODS

Between 2006 and 2008, 12 children with multiple foci of microsusceptibility on SWI without corresponding abnormal signal on conventional MRI were identified and were retrospectively assessed.

RESULTS

The locations of foci of microsusceptibility included the cerebral white matter, basal ganglia, brainstem and cerebellar white matter, without any clear systematic anatomic distribution. CT (n=5) showed no calcification at the locations corresponding to the microsusceptibility on SWI. Conventional MR imaging showed white matter volume loss (n=5), delayed myelination (n=2), acute infarction (n=1), chronic infarction (n=1), meningitis (n=1), slight signal abnormality in the white matter (n=1) and no abnormal findings (n=1). Follow-up SWI (n=3) showed no change of the microsusceptibility foci. Interestingly, all children had a history of heart surgery under extracorporeal circulation for congenital heart disease.

CONCLUSION

Multiple foci of microsusceptibility can be seen in the brain on SWI in children with congenital heart disease who underwent heart surgery with extracorporeal circulation.

Susceptibility tensor imaging

Heterogeneity of magnetic susceptibility within brain tissues creates unique contrast between gray and white matter in magnetic resonance phase images acquired by gradient echo sequences. Detailed understanding of this contrast may provide meaningful diagnostic information. In this communication, we report an observation of extensive anisotropic magnetic susceptibility in the white matter of the central nervous system. Furthermore, we describe a susceptibility tensor imaging technique to measure and quantify this phenomenon. This technique relies on the measurement of resonance frequency offset at different orientations with respect to the main magnetic field. We propose to characterize this orientation variation using an apparent susceptibility tensor. The susceptibility tensor can be decomposed into three eigenvalues (principal susceptibilities) and associated eigenvectors that are coordinate-system independent. We show that the principal susceptibilities offer strong contrast between gray and white matter, whereas the eigenvectors provide orientation information of an underlying magnetic network. We believe that this network may further offer information of white matter fiber orientation.

Susceptibility weighted magnetic resonance imaging of cerebral cavernous malformations: prospects, drawbacks, and first experience at ultra–high field strength (7-Tesla) magnetic resonance imaging

High-resolution susceptibility weighted MR imaging at high field strength provides excellent depiction of venous structures, blood products, and iron deposits, making it a promising complementary imaging modality for cerebral cavernous malformations (CCMs). Although already introduced in 1997 and being constantly improved, susceptibility weighted imaging is not yet routine in clinical neuroimaging protocols for CCMs. In this article, the authors review the recent literature dealing with clinical and scientific susceptibility weighted imaging of CCMs to summarize its prospects and drawbacks and provide their first experience with its use in ultra–high field (7-T) MR imaging.

Pediatric central nervous system emergencies

This article summarizes current state-of-the-art techniques used in the management of pediatric neurologic emergencies. Solutions to challenges faced by the radiologist, including the selection of an appropriate modality for an individual patient, are discussed. Imaging appearances of specific entities are described with an emphasis on conditions unique to the pediatric population.