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

Use of a Commercial 7-T MRI Scanner for Clinical Brain Imaging: Indications, Protocols, Challenges, and Solutions-A Single-Center Experience

The first commercially available 7-T MRI scanner (Magnetom Terra) was approved by the FDA in 2017 for clinical imaging of the brain and knee. After initial protocol development and sequence optimization efforts in volunteers, the 7-T system, in combination with an FDA-approved 1-channel transmit/32-channel receive array head coil, can now be routinely used for clinical brain MRI examinations. The ultrahigh field strength of 7-T MRI has the advantages of improved spatial resolution, increased SNR, and increased CNR but also introduces an array of new technical challenges. The purpose of this article is to describe an institutional experience with the use of the commercially available 7-T MRI scanner for routine clinical brain imaging. Specific clinical indications for which 7-T MRI may be useful for brain imaging include brain tumor evaluation with possible perfusion imaging and/or spectroscopy, radiotherapy planning; evaluation of multiple sclerosis and other demyelinating diseases, evaluation of Parkinson disease and guidance of deep brain stimulator placement, high-detail intracranial MRA and vessel wall imaging, evaluation of pituitary pathology, and evaluation of epilepsy. Detailed protocols, including sequence parameters, for these various indications are presented, and implementation challenges (including artifacts, safety, and side effects) and potential solutions are explored.

Semiology, EEG, and neuroimaging findings in temporal lobe epilepsies

Temporal lobe epilepsy (TLE) is the most common type of focal epilepsy. First descriptions of TLE date back in time and detailed portraits of epileptic seizures of temporal origin can be found in early medical reports as well as in the works of various artists and dramatists. Depending on the seizure onset zone, several subtypes of TLE have been identified, each one associated with peculiar ictal semiology. TLE can result from multiple etiological causes, ranging from genetic to lesional ones. While the diagnosis of TLE relies on detailed analysis of clinical as well as electroencephalographic (EEG) features, the lesions responsible for seizure generation can be highlighted by multiple brain imaging modalities or, in selected cases, by genetic investigations. TLE is the most common cause of refractory epilepsy and despite the great advances in diagnostic tools, no lesion is found in around one-third of patients. Surgical treatment is a safe and effective option, requiring presurgical investigations to accurately identify the seizure onset zone (SOZ). In selected cases, presurgical investigations need intracerebral investigations (such as stereoelectroencephalography) or dedicated metabolic imaging techniques (interictal PET and ictal SPECT) to correctly identify the brain structures to be removed.

7 Tesla and Beyond: Advanced Methods and Clinical Applications in Magnetic Resonance Imaging

ABSTRACT

Ultrahigh magnetic fields offer significantly higher signal-to-noise ratio, and several magnetic resonance applications additionally benefit from a higher contrast-to-noise ratio, with static magnetic field strengths of B0 ≥ 7 T currently being referred to as ultrahigh fields (UHFs). The advantages of UHF can be used to resolve structures more precisely or to visualize physiological/pathophysiological effects that would be difficult or even impossible to detect at lower field strengths. However, with these advantages also come challenges, such as inhomogeneities applying standard radiofrequency excitation techniques, higher energy deposition in the human body, and enhanced B0 field inhomogeneities. The advantages but also the challenges of UHF as well as promising advanced methodological developments and clinical applications that particularly benefit from UHF are discussed in this review article.

Clinical vascular imaging in the brain at 7T

Stroke and related cerebrovascular diseases are a major cause of mortality and disability. Even at standard-field-strengths (1.5T), MRI is by far the most sensitive imaging technique to detect acute brain infarctions and to characterize incidental cerebrovascular lesions, such as white matter hyperintensities, lacunes and microbleeds. Arterial time-of-flight (TOF) MR angiography (MRA) can depict luminal narrowing or occlusion of the major brain feeding arteries, and this without the need for contrast administration. Compared to 1.5T MRA, the use of high-field strength (3T) and even more so ultra-high-field strengths (7T), enables the visualization of the lumen of much smaller intracranial vessels, while adding a contrast agent to TOF MRA at 7T may enable the visualization of even more distal arteries in addition to veins and venules. Moreover, with 3T and 7T, the arterial vessel walls beyond the circle of Willis become visible with high-resolution vessel wall imaging. In addition, with 7T MRI, the brain parenchyma can now be visualized on a submillimeter scale. As a result, high-resolution imaging studies of the brain and its blood supply at 7T have generated new concepts of different cerebrovascular diseases. In the current article, we will discuss emerging clinical applications and future directions of vascular imaging in the brain at 7T MRI.

7-T MRI in Cerebrovascular Diseases: Challenges to Overcome and Initial Results

Magnetic resonance imaging (MRI) plays a key role in the investigation of cerebrovascular diseases. Compared with computed tomography (CT) and digital subtraction angiography (DSA), its advantages in diagnosing cerebrovascular pathology include its superior tissue contrast, its ability to visualize blood vessels without the use of a contrast agent, and its use of magnetic fields and radiofrequency pulses instead of ionizing radiation. In recent years, ultrahigh field MRI at 7 tesla (7 T) has shown promise in the diagnosis of many cerebrovascular diseases. The increased signal-to-noise ratio (SNR; 2.3x and 4.7x increase compared with 3 and 1.5 T, respectively) and contrast-to-noise ratio (CNR) at this higher field strength can be exploited to obtain a higher spatial resolution and higher lesion conspicuousness, enabling assessment of smaller brain structures and lesions. Cerebrovascular diseases can be assessed at different tissue levels; for instance, changes of the arteries feeding the brain can be visualized to determine the cause of ischemic stroke, regional changes in brain perfusion can be mapped to predict outcome after revascularization, and tissue damage, including old and recent ischemic infarcts, can be evaluated as a marker of ischemic burden. For the purpose of this review, we will discriminate 3 levels of assessment of cerebrovascular diseases using MRI: Pipes, Perfusion, and Parenchyma (3 Ps). The term Pipes refers to the brain-feeding arteries from the heart and aortic arch, upwards to the carotid arteries, vertebral arteries, circle of Willis, and smaller intracranial arterial branches. Perfusion is the amount of blood arriving at the brain tissue level, and includes the vascular reserve and perfusion territories. Parenchyma refers to the acute and chronic burden of brain tissue damage, which includes larger infarcts, smaller microinfarcts, and small vessel disease manifestations such as white matter lesions, lacunar infarcts, and microbleeds. In this review, we will describe the key developments in the last decade of 7-T MRI of cerebrovascular diseases, subdivided for these 3 levels of assessment.

Clinical applications at ultrahigh field (7 T). Where does it make the difference?

Presently, three major MR vendors provide commercial 7-T units for clinical research under ethical permission, with the number of operating 7-T systems having increased to over 50. This rapid increase indicates the growing interest in ultrahigh-field MRI because of improved clinical results with regard to morphological as well as functional and metabolic capabilities. As the signal-to-noise ratio scales linearly with the field strength (B0 ) of the scanner, the most obvious application at 7 T is to obtain higher spatial resolution in the brain, musculoskeletal system and breast. Of specific clinical interest for neuro-applications is the cerebral cortex at 7 T, for the detection of changes in cortical structure as a sign of early dementia, as well as for the visualization of cortical microinfarcts and cortical plaques in multiple sclerosis. In the imaging of the hippocampus, even subfields of the internal hippocampal anatomy and pathology can be visualized with excellent resolution. The dynamic and static blood oxygenation level-dependent contrast increases linearly with the field strength, which significantly improves the pre-surgical evaluation of eloquent areas before tumor removal. Using susceptibility-weighted imaging, the plaque-vessel relationship and iron accumulation in multiple sclerosis can be visualized for the first time. Multi-nuclear clinical applications, such as sodium imaging for the evaluation of repair tissue quality after cartilage transplantation and (31) P spectroscopy for the differentiation between non-alcoholic benign liver disease and potentially progressive steatohepatitis, are only possible at ultrahigh fields. Although neuro- and musculoskeletal imaging have already demonstrated the clinical superiority of ultrahigh fields, whole-body clinical applications at 7 T are still limited, mainly because of the lack of suitable coils. The purpose of this article was therefore to review the clinical studies that have been performed thus far at 7 T, compared with 3 T, as well as those studies performed at 7 T that cannot be routinely performed at 3 T. Copyright © 2015 John Wiley & Sons, Ltd.

Comparison of 7.0- and 3.0-T MRI and MRA in ischemic-type moyamoya disease: preliminary experience

OBJECT

The authors compared the image quality and diagnostic sensitivity and specificity of 7.0-T and 3.0-T MRI and time-of-flight (TOF) MR angiography (MRA) in patients with moyamoya disease (MMD).

METHODS

MR images of 15 patients with ischemic-type MMD (8 males, 7 females; age 13–48 years) and 13 healthy controls (7 males, 6 females; age 19–28 years) who underwent both 7.0-T and 3.0-T MRI and MRA were studied retrospectively. The main intracranial arteries were assessed by using the modified Houkin’s grading system (MRA score). Moyamoya vessels (MMVs) were evaluated by 2 grading systems: the MMV quality score and the MMV area score. Two diagnostic criteria for MMD were used: the T2 criteria, which used flow voids in the basal ganglion on T2-weighted images, and the TOF criteria, which used the high-intensity areas in the basal ganglion on source images from TOF MRA. All data were evaluated by 2 independent readers who were blinded to the strength field and presence or absence of MMD. Using conventional angiography as the gold standard, the sensitivity and specificity of 7.0-T and 3.0-T MRI/MRA in the diagnosis of MMD were calculated. The differences between 7.0-T and 3.0-T MRI and MRA were statistically compared.

RESULTS

No significant differences were observed between 7.0-T and 3.0-T MRA in MRA score (p = 0.317) or MRA grade (p = 0.317). There was a strong correlation between the Suzuki’s stage and MRA grade in both 3.0-T (rs = 0.930; p < 0.001) and 7.0-T (rs = 0.966; p < 0.001) MRA. However, MMVs were visualized significantly better on 7.0-T than on 3.0-T MRA, suggested by both the MMV quality score (p = 0.001) and the MMV area score (p = 0.001). The correlation between the Suzuki’s stage and the MMV area score was moderate in 3.0-T MRA (rs = 0.738; p = 0.002) and strong in 7.0-T MRA (rs = 0.908; p < 0.001). Moreover, 7.0-T MR images showed a greater capacity for detecting flow voids in the basal ganglion on both T2-weighted MR images (p < 0.001) and TOF source images (p < 0.001); 7.0-T MRA also revealed the subbranches of superficial temporal arteries much better. Receiver operating characteristic curve analysis showed that, according to the T2 criteria, 7.0-T MRI/MRA was more sensitive (sensitivity 1.000; specificity 0.933) than 3.0-T MRI/MRA (sensitivity 0.692; specificity 0.933) in diagnosing MMD; based on the TOF criteria, 7.0-T MRI/MRA was more sensitive (1.000 vs 0.733, respectively) and more specific (1.000 vs 0.923, respectively) than 3.0-T MRI/MRA.

CONCLUSIONS

Compared with 3.0-T MRI/MRA, 7.0-T MRI/MRA detected and delineated MMVs more clearly and provided higher diagnostic sensitivity and specificity, although it did not show significant improvement in depicting main intracranial arteries. The authors speculate that 7.0-T MRI/MRA is a promising technique in the diagnosis of MMD because it is noninvasive compared with conventional angiography and it is more sensitive than 3.0-T MRI/MRA.

Correlation of the venous angioarchitecture of multiple cerebral cavernous malformations with familial or sporadic disease: a susceptibility-weighted imaging study with 7-Tesla MRI

OBJECTIVE Multiple cerebral cavernous malformations (CCMs) are rare lesions that occur in sporadic or familial form. Depending on the disease form, the natural history and treatment of the lesions strongly vary. Molecular analysis of an underlying germline mutation (CCM1-3) is the most sensitive screening method to distinguish between sporadic and familial cases. However, based on the different pathomechanisms that are believed to be involved in either form, significant distinctions in the CCM-associated cerebral venous angioarchitecture should be detectable. This has not been systematically studied. METHODS A consecutive series of 28 patients with multiple CCMs (681 total) diagnosed on 1.5-T MRI underwent genetic screening for CCM1-3 mutations and high-resolution susceptibility-weighted imaging (SWI) of the cerebral venous angioarchitecture with 7-T MRI. Imaging data were analyzed to examine the CCM-associated venous angioarchitecture. Results were correlated with findings of molecular analysis for CCM1-3 mutations. RESULTS Two different SWI patterns (sporadic and familial) were found. The presence of associated developmental venous anomalies correlated with negative screening for germline mutations (11 sporadic) in all cases. All patients with confirmed familial disease showed normal underlying venous angioarchitecture. Additionally, a very unusual case of a probable somatic mutation is presented. CONCLUSIONS The SWI results of the venous angioarchitecture of multiple CCMs correlate with sporadic or familial disease. These results are consistent with the theory that venous anomalies are causative for the sporadic form of multiple CCMs.

Perspectives of Ultra-High-Field MRI in Neuroradiology

PURPOSE

Magnetic resonance imaging (MRI) is one of the most important methods for the diagnosis and therapy monitoring of various diseases. Today, magnets up to 3 T are standard. This review will give an overview of the clinical perspectives of ultra-high field MRI, meaning mainly 7 T.

METHODS

Literature review with focus on clinical applications of 7 T imaging in neuroscience combined with examples of own studies and perspectives.

RESULTS

This high-resolution technique offers the potential to improve certain tissue contrasts and signal in functional (fMRI) and metabolic (MRS) imaging. This overview demonstrates already existing potentials and advantages of the ultra-high magnetic field for central nervous system (CNS) diseases.

CONCLUSIONS

Although there are still some technical challenges for brain and spine imaging at 7 T, the method has clear benefit in selected structural, functional, and metabolic imaging.

Management of cerebral cavernous malformations: from diagnosis to treatment

Cerebral cavernous malformations are the most common vascular malformations and can be found in many locations in the brain. If left untreated, cavernomas may lead to intracerebral hemorrhage, seizures, focal neurological deficits, or headaches. As they are angiographically occult, their diagnosis relies on various MR imaging techniques, which detect different characteristics of the lesions as well as aiding in planning the surgical treatment. The clinical presentation and the location of the lesion are the most important factors involved in determining the optimal course of treatment of cavernomas. We concisely review the literature and discuss the advantages and limitations of each of the three available methods of treatment—microsurgical resection, stereotactic radiosurgery, and conservative management—depending on the lesion characteristics.

Ultra-high-field MR imaging in polymicrogyria and epilepsy

BACKGROUND AND PURPOSE

Polymicrogyria is a malformation of cortical development that is often identified in children with epilepsy or delayed development. We investigated in vivo the potential of 7T imaging in characterizing polymicrogyria to determine whether additional features could be identified.

MATERIALS AND METHODS

Ten adult patients with polymicrogyria previously diagnosed by using 3T MR imaging underwent additional imaging at 7T. We assessed polymicrogyria according to topographic pattern, extent, symmetry, and morphology. Additional imaging sequences at 7T included 3D T2* susceptibility-weighted angiography and 2D tissue border enhancement FSE inversion recovery. Minimum intensity projections were used to assess the potential of the susceptibility-weighted angiography sequence for depiction of cerebral veins.

RESULTS

At 7T, we observed perisylvian polymicrogyria that was bilateral in 6 patients, unilateral in 3, and diffuse in 1. Four of the 6 bilateral abnormalities had been considered unilateral at 3T. While 3T imaging revealed 2 morphologic categories (coarse, delicate), 7T susceptibility-weighted angiography images disclosed a uniform ribbonlike pattern. Susceptibility-weighted angiography revealed numerous dilated superficial veins in all polymicrogyric areas. Tissue border enhancement imaging depicted a hypointense line corresponding to the gray-white interface, providing a high definition of the borders and, thereby, improving detection of the polymicrogyric cortex.

CONCLUSIONS

7T imaging reveals more anatomic details of polymicrogyria compared with 3T conventional sequences, with potential implications for diagnosis, genetic studies, and surgical treatment of associated epilepsy. Abnormalities of cortical veins may suggest a role for vascular dysgenesis in pathogenesis.

Susceptibility-weighted MR imaging: a better technique in the detection of capillary telangiectasia compared with T2* gradient-echo

BACKGROUND AND PURPOSE

Enhancing lesions on brain MR imaging can present a diagnostic quandary as both benign lesions such as brain capillary telangiectasia and pathologic lesions such as demyelination may appear similar. Stagnation of blood in low-flow venous channels of brain capillary telangiectasias results in susceptibility effect secondary to the increased local deoxyhemoglobin. Both T2* gradient-echo imaging and SWI were demonstrated as valuable in the diagnosis of brain capillary telangiectasia. Because SWI is more sensitive to susceptibility changes than gradient-echo, we aim to demonstrate increased diagnostic value of SWI compared with gradient-echo in making the diagnosis of brain capillary telangiectasia.

MATERIALS AND METHODS

We retrospectively reviewed the MR images of 17 patients with a presumed diagnosis of brain capillary telangiectasia and who were examined from June 2010 to September 2012. All patients underwent MR imaging at 1.5T with T1, T2, FLAIR, gradient-echo, SWI, and gadolinium-enhanced T1 sequences. Lesions were evaluated for the presence or absence of signal abnormality on each particular sequence.

RESULTS

All 17 brain capillary telangiectasias demonstrated distinct signal-intensity loss on SWI compared with 7 of 17 (41%) who showed signal-intensity loss on gradient-echo. The increased frequency of detection using SWI versus gradient-echo is statistically significant (z = 2.85, P < .01; χ(2) = 8.10, P < .01). Six of the lesions showed signal-intensity changes on T1 and/or T2 whereas the remaining lesions were isointense to normal brain.

CONCLUSIONS

Brain capillary telangiectasias are more conspicuous on SWI than gradient-echo imaging and other precontrast MR imaging. SWI is a valuable tool in diagnosing these benign lesions and should serve to increase diagnostic confidence.

New design concept of monopole antenna array for UHF 7T MRI

PURPOSE

We have developed and evaluated a monopole antenna array that can increase sensitivity at the center of the brain for 7T MRI applications.

METHODS

We have developed a monopole antenna array that has half the length of a conventional dipole antenna with eight channels for brain imaging with a 7T MRI. The eight-channel monopole antenna array and conventional eight-channel transceiver surface coil array were evaluated and compared in terms of transmit properties, specific absorption ratio (SAR), and sensitivity. The sensitivity maps were generated by dividing the SNR map by the flip angle distribution.

RESULTS

A single surface coil provides asymmetric sensitivity resulting in reduced sensitivity at the center of the brain. In contrast, a single monopole antenna provides higher sensitivity at the center of the brain. Moreover, the monopole antenna array provides uniform sensitivity over the entire brain, and the sensitivity gain was 1.5 times higher at the center of the brain compared with the surface coil array.

CONCLUSION

The monopole antenna array is a promising candidate for MRI applications, especially for brain imaging in a 7T MRI because it provides increased sensitivity at the center of the brain.

Surgical resection of cavernous malformations of the brainstem: evolution of a minimally invasive technique

OBJECTIVE

The purpose of this study is to provide an institutional retrospective review of surgically treated brainstem cavernous malformations.

METHODS

Between 2005 and 2010, 22 consecutive patients with brainstem cavernous malformations (15 female and 7 male) with a mean age of 43 years underwent surgical treatment. Mean volume of the resected cavernous malformations was 0.65 cm(3). A minimally invasive resection technique was used for these cases, in conjunction with skull base approaches.

RESULTS

The mean follow-up period was 26.6 months (range, 4-68 months). Of the 22 patients, 9% did not have clear evidence of hemorrhage at the time of presentation. Of the remainder, 22% had two or more instances of hemorrhage documented by magnetic resonance imaging. After resection and during follow-up, 54% of patients had an improvement in their modified Rankin scale, whereas 14% were worse compared with their preoperative presentation; 32% were unchanged and 9% of patients were found to have residual cavernoma post-surgery.

CONCLUSION

Our longitudinal experience has guided us to emphasize minimally invasive approaches during resection of the brainstem cavernous malformations, occasionally at the expense of achieving a complete resection, to improve patient outcomes.

Susceptibility-weighted imaging at 7 T: Improved diagnosis of cerebral cavernous malformations and associated developmental venous anomalies

Background and aim

In the diagnosis of cerebral cavernous malformations (CCMs) magnetic resonance imaging is established as the gold standard. Conventional MRI techniques have their drawbacks in the diagnosis of CCMs and associated venous malformations (DVAs). The aim of our study was to evaluate susceptibility weighted imaging SWI for the detection of CCM and associated DVAs at 7 T in comparison with 3 T.

Patients and methods

24 patients (14 female, 10 male; median age: 38.3 y (21.1 y–69.1 y) were included in the study. Patients enrolled in the study received a 3 T and a 7 T MRI on the same day. The following sequences were applied on both field strengths: a T1 weighted 3D GRE sequence (MP-RAGE) and a SWI sequence. After obtaining the study MRIs, eleven patients underwent surgery and 13 patients were followed conservatively or were treated radio-surgically.

Results

Patients initially presented with haemorrhage (n = 4, 16.7%), seizures (n = 2, 8.3%) or other neurology (n = 18, 75.0%). For surgical resected lesions histopathological findings verified the diagnosis of CCMs. A significantly higher number of CCMs was diagnosed at 7 T SWI sequences compared with 3 T SWI (p < 0.05). Additionally diagnosed lesions on 7 T MRI were significantly smaller compared to the initial lesions on 3 T MRIs (p < 0.001). Further, more associated DVAs were diagnosed at 7 T MRI compared to 3 T MRI.

Conclusion

SWI sequences at ultra-high-field MRI improve the diagnosis of CCMs and associated DVAs and therefore add important pre-operative information.

Ultrahigh-field magnetic resonance imaging: the clinical potential for anatomy, pathogenesis, diagnosis, and treatment planning in brain disease

In this review, current (clinical) applications and possible future directions of ultrahigh-field (≥7 T) magnetic resonance (MR) imaging in the brain are discussed. Ultrahigh-field MR imaging can provide contrast-rich images of diverse pathologies and can be used for early diagnosis and treatment monitoring of brain disease. These images may provide increased sensitivity and specificity. Several limitations need to be overcome before worldwide clinical implementation can be commenced. Current literature regarding clinically based ultrahigh-field MR imaging is reviewed, and limitations and promises of this technique are discussed, as well as some practical considerations for the implementation in clinical practice.

Ultrahigh-field MRI in human ischemic stroke--a 7 tesla study

Introduction

Magnetic resonance imaging (MRI) using field strengths up to 3 Tesla (T) has proven to be a powerful tool for stroke diagnosis. Recently, ultrahigh-field (UHF) MRI at 7 T has shown relevant diagnostic benefits in imaging of neurological diseases, but its value for stroke imaging has not been investigated yet. We present the first evaluation of a clinically feasible stroke imaging protocol at 7 T. For comparison an established stroke imaging protocol was applied at 3 T.

Methods

In a prospective imaging study seven patients with subacute and chronic stroke were included. Imaging at 3 T was immediately followed by 7 T imaging. Both protocols included T1-weighted 3D Magnetization-Prepared Rapid-Acquired Gradient-Echo (3D-MPRAGE), T2-weighted 2D Fluid Attenuated Inversion Recovery (2D-FLAIR), T2-weighted 2D Fluid Attenuated Inversion Recovery (2D-T2-TSE), T2* weighted 2D Fast Low Angle Shot Gradient Echo (2D-HemoFLASH) and 3D Time-of-Flight angiography (3D-TOF).

Results

The diagnostic information relevant for clinical stroke imaging obtained at 3 T was equally available at 7 T. Higher spatial resolution at 7 T revealed more anatomical details precisely depicting ischemic lesions and periinfarct alterations. A clear benefit in anatomical resolution was also demonstrated for vessel imaging at 7 T. RF power deposition constraints induced scan time prolongation and reduced brain coverage for 2D-FLAIR, 2D-T2-TSE and 3D-TOF at 7 T versus 3 T.

Conclusions

The potential of 7 T MRI for human stroke imaging is shown. Our pilot study encourages a further evaluation of the diagnostic benefit of stroke imaging at 7 T in a larger study.

7-T MR--from research to clinical applications?

Over 20,000 MR systems are currently installed worldwide and, although the majority operate at magnetic fields of 1.5 T and below (i.e. about 70%), experience with 3-T (in high-field clinical diagnostic imaging and research) and 7-T (research only) human MR scanners points to a future in functional and metabolic MR diagnostics. Complementary to previous studies, this review attempts to provide an overview of ultrahigh-field MR research with special emphasis on emerging clinical applications at 7 T. We provide a short summary of the technical development and the current status of installed MR systems. The advantages and challenges of ultrahigh-field MRI and MRS are discussed with special emphasis on radiofrequency inhomogeneity, relaxation times, signal-to-noise improvements, susceptibility effects, chemical shifts, specific absorption rate and other safety issues. In terms of applications, we focus on the topics most likely to gain significantly from 7-T MR, i.e. brain imaging and spectroscopy and musculoskeletal imaging, but also body imaging, which is particularly challenging. Examples are given to demonstrate the advantages of susceptibility-weighted imaging, time-of-flight MR angiography, high-resolution functional MRI, (1)H and (31)P MRSI in the human brain, sodium and functional imaging of cartilage and the first results (and artefacts) using an eight-channel body array, suggesting future areas of research that should be intensified in order to fully explore the potential of 7-T MR systems for use in clinical diagnosis.

Natural history and imaging prevalence of cavernous malformations in children and young adults

OBJECT

This study was undertaken to define the age-related prevalence of cavernous malformations (CMs) in children and young adults undergoing intracranial imaging. In addition, the authors aim to clarify the natural history of CMs in young people, especially in those with incidentally discovered lesions.

METHODS

To identify those patients with CMs, the authors retrospectively reviewed the electronic medical records of 14,936 consecutive patients 25 years of age or younger who had undergone brain MR imaging. In patients with a CM, clinical and imaging data were collected. Patients with untreated cavernomas who had more than 6 months of clinical and MR imaging follow-up were included in a natural history analysis. The natural history analysis included 110 CMs in 56 patients with a 3.5-year mean clinical follow-up interval (199 patient-years and 361 cavernoma-years).

RESULTS

In 92 patients (0.6%), 164 CMs were identified. The imaging prevalence of cavernomas increased with advancing age (p = 0.002). Multiple CMs occurred in 28 patients (30%), and 8 patients (9%) had a family history of multiple CMs. Fifty patients (54%) presented with symptoms related to the cavernoma, of whom 30 presented with hemorrhage (33%). Of the 164 cavernomas identified, 103 (63%) were considered incidental, asymptomatic lesions. Larger size was associated with acute symptomatic presentation (p = 0.0001). During the follow-up interval, 6 patients with 8 cavernomas developed 11 symptomatic hemorrhages after initial identification. Five of the patients who had a hemorrhage during the follow-up interval had initially presented with hemorrhage, while only 1 had presented incidentally. The hemorrhage rate for all patients in the natural history group was 1.6% per patient-year and 0.9% per cavernoma-year. The hemorrhage rate was 8.0% per patient-year in the symptomatic group versus 0.2% in the incidental group. Symptomatic hemorrhage after long-term follow-up was associated with initial acute presentation (p = 0.02).

CONCLUSIONS

The imaging prevalence of CM increases with advancing age during childhood. Patients presenting without hemorrhage have a significantly lower risk of bleeding compared with those who present with acute neurological symptoms. Comparing this series of children to prior analyses of CM natural history in adults, the authors' data do not suggest a higher bleeding risk in younger patients.

Evaluation of hardware-related geometrical distortion in structural MRI at 7 Tesla for image-guided applications in neurosurgery

RATIONALE AND OBJECTIVES

Geometrical distortion is a well-known problem in structural magnetic resonance imaging (MRI), leading to pixel shifts with variations up to several millimeters. Because the main factors of geometrical distortion are proportional to B(0), MRI spatial encoding distortions tend to increase with higher magnetic field strength. With the increasing prospects of utilizing ultra-high-field MRI (B(0) ≥ 7 Tesla) for neuroimaging and subsequently for image-guided neurosurgical therapy, the evaluation and correction of geometrical distortions occurring in ultra-high-field MRI are essential preconditions for the integration of these data. Hence, we conducted a phantom study to determine hardware-related geometrical distortion in clinically relevant sequences for structural imaging at 7 T MRI and compared the findings to 1.5 T MRI.

MATERIAL AND METHODS

Hardware-related geometrical distortion was evaluated using a MRI phantom (Elekta, Sweden). Both applied scanner systems (Magnetom Avanto 1.5 T and Magnetom 7 T, Siemens Healthcare, Erlangen, Germany) were equipped with similar gradient coils capable of delivering 45 mT/m of maximum amplitude and a slew rate of 220 mT/m/ms. Distortion analysis was performed for various clinically relevant gradient echo and spin echo sequences.

RESULTS

Overall, we found very low mean geometrical distortions at both 7 T and 1.5 T, although single values of up to 1.6 mm were detected. No major differences in mean distortion between the sequences could be found, except significantly higher distortions in turbo spin-echo sequences at 7 T, mainly caused by B(1) inhomogeneities.

CONCLUSION

Hardware-related geometrical distortions at 7 T MRI are relatively small, which may be acceptable for image coregistration or for direct tissue-targeting procedures. Using a subject-specific correction of object-related distortions, an integration of 7 T MRI data into image-guided applications may be feasible.