Intracerebral arteriovenous malformations: evaluation with selective MR angiography and venography

Target Delineation in Radiosurgery for Cerebral Arteriovenous Malformations

A study of 6 selected arteriovenous malformation (AVM) patients was performed to investigate the feasibility of delineating an AVM on MR images and to compare the AVM volume outlined on different images. Conventional stereotaxic angiograms, stereotaxic MR images and MR angiograms using several different pulse sequences were obtained prior to radiosurgery. Treatment plans were made from the conventional stereotaxic angiograms. These plans were then transferred to a separate dose planning computer which displayed the MR images with the superimposed isodose lines. The radiated volumes of AVM and brain tissue were measured from these MR images. Last, an assessment was made of the radiation volume needed for an appropriate treatment of the AVM if the treatment plan was made from the MR images rather than from the conventional stereotaxic angiogram. It was possible to delineate medium and large size AVM nidi on stereotaxic MR images based on an integration of information obtained from various pulse sequences. The estimated volumes of the AVM nidi were found to be larger on the conventional stereotaxic angiograms than on the stereotaxic MR images. Consequently, a dose plan based on a conventional stereotaxic angiogram would result in a higher integral dose to the brain with the same target dose. By using reliable MR information it is expected that the volume of brain exposed to radiation could be decreased and the adverse effects of stereotactic radiosurgery for AVM thereby minimized.

Obliteration dynamics in cerebral arteriovenous malformations after cyberknife radiosurgery: quantification with sequential nidus volumetry and 3-tesla 3-dimensional time-of-flight magnetic resonance angiography

OBJECTIVE

To investigate the time-dependent obliteration of cerebral arteriovenous malformations (cAVM) after CyberKnife radiosurgery (CKRS) (Accuray, Inc., Sunnyvale, CA) by means of sequential 3-T, 3-dimensional (3D), time-of-flight (TOF) magnetic resonance angiography (MRA), and volumetry of the arteriovenous malformation (AVM) nidus.

METHODS

In this prospective study, 3D TOF MRA was performed on 20 patients with cAVMs treated by single-fraction CKRS. Three-dimensional TOF MRA was performed on a 3-T, 32-channel magnetic resonance scanner (Magnetom TIM Trio; Siemens Medical Solutions, Erlangen, Germany) with isotropic voxel size at a spatial resolution of 0.6 x 0.6 x 0.6 mm3. The time-dependent relative decay of the transnidal blood flow evidenced by 3D TOF MRA was referred to as "obliteration dynamics." Volumetry of the nidus size was performed with OsiriX imaging software (OsiriX Foundation, Geneva, Switzerland). All patients had 3 to 4 follow-up examinations at 3- to 6-month intervals over a minimum follow-up period of 9 months. Subtotal obliteration was determined if the residual nidus volume was 5% or less of the initial nidus volume. Stata/IC software (Version 10.0; Stata Corp., College Station, TX) was used for statistical analysis and to identify potential factors of AVM obliteration.

RESULTS

Regarding their clinical status, case history, and pretreatments, the participants of this study represent difficult-to-treat cAVM patients. The median nidus volume was 1.8 mL (range, 0.4-12.5 mL); the median minimum dose prescribed to the nidus was 22 Gy (range, 16-24 Gy) delivered to the 67% isodose line (range, 55-80%). CKRS was well tolerated, with complications in 2 patients. No further hemorrhages occurred after RS, except 1 small and clinically inapparent incident. The median follow-up period after RS was 25.0 months (range, 11.7-36.8 months). After RS, a statistically significant obliteration was observed in all patients. However, the obliteration dynamics of the cAVMs showed a pronounced variability, with 2 types of post-therapeutic behavior identified. cAVMs of Group A showed a faster reduction of transnidal blood flow than cAVMs in Group B. The median time to subtotal obliteration was 23.8 months for all patients, 11.6 months for patients in Group A, and 27.8 months for patients in Group B (P = 0.05). Logistic regression analysis revealed dose homogeneity and the circumscribed isodose to be the only variables (P < 0.01) associated with the obliteration dynamics in this study. The cumulative complete angiographic obliteration rate was 67% (95% confidence interval, 32-95%) 2 years after RS.

CONCLUSION

The use of sequential 3D TOF MRA at 3 T and nidus volumetry enables a noninvasive quantitative assessment of the dynamic obliteration process induced by CKRS in cAVMs. This method may be helpful to identify factors related to AVM obliteration after RS when larger patient cohorts become available.

Intracranial contrast-enhanced magnetic resonance venography with 6.4-fold sensitivity encoding at 1.5 and 3.0 Tesla

PURPOSE

To prospectively compare vessel conspicuity and diagnostic image quality between three-dimensional intracranial contrast-enhanced MR venography acquired at 1.5 Tesla (T) and 3.0T, with 6.4-fold sensitivity encoding.

MATERIALS AND METHODS

Ten healthy volunteers were imaged on 1.5T and 3.0T MR scanners using eight-element head coil arrays. The intracranial venous vasculature was divided into five groups for evaluation based on vessel size and anatomical location. Two radiologists independently assessed vessel conspicuity, level of artifacts, and diagnostic image quality. Informed consent was obtained, and the study was approved by the institutional review board.

RESULTS

With the exception of large cerebral sinuses where 1.5T and 3.0T results were rated as equivalent, 3.0T images demonstrated superior vessel continuity, sharpness, and signal contrast to background tissue than 1.5T for all other intracranial venous vasculature (P < 0.01). No statistical significance in overall image quality was found between 1.5T and 3.0T venograms, and all data sets were deemed sufficient for diagnostic interpretation.

CONCLUSION

Whole brain contrast-enhanced venography with 6.4-fold sensitivity encoding is robust and has the potential to become the method of choice for fast visualization of the intracranial venous vasculature. At 3.0T, demonstration of small cerebral vessels is superior to 1.5T.

Multimodale MR-Diagnostik arteriovenöser Malformationen

Arteriovenous malformations (AVM) are complex inborn malformations of the vascular system. In the brain they can lead to severe complications, such as parenchymal haemorrhaging, epileptic seizures or neurologic failure, which necessitates a safe diagnosis and therapy. In addition to conventional angiography, magnetic resonance imaging (MRI) and MR angiography (MRA) have now been established as the gold standard diagnostic procedures. Using MRA the angioarchitecture of malformations can be captured and with a parallel imaging of the parenchyme the method is very well suited for therapy planning and monitoring. This review summarizes the present day possibilities of multimodal MRT diagnosis of AVM and describes the purely morphologic as well as the physiologic and pathophysiologic characteristics. The various MR angiographic techniques will be firstly described with respect to the basic technical principles and the results obtained so far will be summarized. The principle of functional MRT will be described and the diagnostic possibilities with respect to AVM will be discussed.

High-spatial-resolution contrast-enhanced MR angiography of the intracranial venous system with fourfold accelerated two-dimensional sensitivity encoding

Informed consent was obtained; the study was HIPAA compliant and institutional review board approved. Fourfold accelerated (FFA) two-dimensional (2D) sensitivity encoding (SENSE) (65 seconds) was prospectively compared with its nonaccelerated counterpart (4 minutes 20 seconds) for diagnostic image quality and sharpness of visualization of blood vessels at 1.5 T with three-dimensional (3D) intracranial contrast-enhanced magnetic resonance venography in 18 consecutive volunteers (10 men, eight women; mean age, 48.4 years) and two patients (55-year-old man, 30-year-old woman). Two readers compared FFA 2D SENSE results with results from its nonaccelerated counterpart; they rated visualization of large and medium sinuses as equivalent (P>.1) and that of small deep cerebral veins (P<.01) and superficial cerebral veins (P<.001) as superior. Overall diagnostic image quality ratings were excellent for 62% and 80% of nonaccelerated and FFA 2D SENSE results, respectively (P<.05). FFA 2D SENSE may become the method of choice for fast visualization of intracranial venous vasculature in clinical practice.

Angiorressonância magnética do crânio: revisão de 100 casos

Neste trabalho foram analisados, retrospectivamente, os exames de 100 pacientes que se submeteram a angiografia por ressonância magnética (angio-RM) do crânio com diferentes indicações clínicas. Os objetivos deste trabalho foram discutir as vantagens e desvantagens da técnica 3D TOF em exames de angio-RM cerebral e verificar as principais indicações de angio-RM na avaliação das lesões vasculares intracranianas e a freqüência dos principais achados nestes exames. As principais indicações para a realização de angio-RM neste trabalho foram cefaléia (n = 29), acidente vascular encefálico (n = 14) e ataque isquêmico transitório (n = 11). Em 11 casos não havia indicação clínica e os demais tiveram indicações diversas. Cinqüenta e dois por cento dos exames apresentaram alguma alteração. Os principais achados foram estenose vascular (n = 30), aneurisma (n = 10), variações anatômicas (n = 6), malformações vasculares (n = 4) e trombose dos seios durais (n = 2). A técnica 3D TOF, única utilizada neste trabalho, mostrou-se eficiente na maioria dos casos, com um tempo relativamente curto para sua aquisição. Como desvantagem, não se mostra eficaz para cobrir grandes volumes.

Cerebral MR venography

This paper illustrates the technique and clinical use of cerebral MR venography (MRV). Dural sinus thrombosis is detected by both two-dimensional time-of-flight (TOF) and three-dimensional phase-contrast (PC) techniques. Venous angiomas are demonstrated by the three-dimensional PC method. The arterial feeders, nidus and draining veins of cerebral arteriovenous malformation (AVM) can be visualized on contrast-enhanced TOF three-dimensional MR angiogram. The high-flow draining veins of cerebral and dural AVMs, vein of Galen malformation and carotid cavernous fistula are better seen on contrast-enhanced three-dimensional gradient-echo MRV.

Contrast-enhanced MRA for investigation of cerebral arteriovenous malformations

We evaluated contrast-enhanced MRA (enhanced 3-D fast gradient-echo [efgre3d] with spectral inversion recovery) for identification of 15 intracranial arteriovenous malformations (AVMs) in 14 patients. Demonstration of the feeding arteries was classified as good for 16 examinations on maximum-intensity projections and multiprojection volume reconstruction images. The nidus was seen well in all patients. Definition of the draining veins was good or fair except for one poor result. Therapeutic effects were clearly demonstrated in three follow-up series. Contrast-enhanced MRA using efgre3d is useful for delineation of AVMs and for follow-up after treatment.

Magnetic resonance imaging with lateralized arterial spin labeling

We report the development of a new MRI technique which allows spins from right-sided arteries to be labeled separately from spins from left-sided arteries. This method uses two spatially-selective adiabatic inversion pulses to alternate the labeling of the right carotid and vertebral artery separate from the left carotid and vertebral artery. Normal volunteers were scanned on a clinical 1.5 T system and the resultant brain images correlated with the T2 anatomic images. Arterial anatomy was depicted using the new sequence and corresponded to the labeling scheme employed by the sequence. It was demonstrated that spatially selective inversion pulses permit the encoding of the spins within specific vascular origins and the observation of their run-off territory.

High-resolution MR venography of cerebral arteriovenous malformations

The purpose of this study was to evaluate the diagnostic potential of a high-resolution magnetic resonance (MR) venography technique in patients with cerebral arteriovenous malformations (AVMs). A high-resolution 3D gradient echo sequence was used with a long echo time TE to obtain venous information down to sub-pixel sized vessel diameters of several hundred microns. The method is based on the paramagnetic property of deoxyhemoglobin, and the resulting developing phase difference between veins and brain parenchyma at long echo times which leads to signal cancellation. The reconstructed venograms were compared with time-of-flight (TOF)-MR angiography using qualitative and quantitative criteria with the conventional digital subtraction angiography serving as the reference gold standard. In 17 patients with angiographically proven cerebral AVMs, the method indicates its potential in clinical applications. Venography was able to detect all AVMs whereas TOF-MRA failed in three patients. In the delineation of venous drainage patterns MR venography was superior to TOF-MRA, however, the method failed in the detection of about half of the main feeding arteries, as expected. Due to susceptibility artifacts at air/tissue boundaries and interference with paramagnetic hemosiderin, venography was limited with respect to the delineation of the exact nidus sizes and shapes in ten patients with AVMs located close to the skull base or having suffered from previous bleeding. Although the visualization of draining veins represents an important prerequisite in the surgical and radiosurgical treatment planning of cerebral AVMs, application of high resolution MR venography may be limited in the diagnostic work-up in some of these patients. On the other hand, it may be of special importance in the early detection and assessment of small AVMs that are difficult to diagnose with other MR methods.

Magnetic resonance angiography in patients with brain infarction

This study evaluated the role of magnetic resonance angiography (MRA) in detecting extra- or intracranial vascular disease in 118 patients with brain infarction and the accuracy of MRA diagnosis when compared with conventional angiography in patients who had both investigations. Magnetic resonance angiography ruled out extra- and intracranial large vessel disease in 36% of the patients. MRA also demonstrated extra- or intracranial disease in 56% (probably symptomatic in 31, possibly symptomatic in 18, and asymptomatic in 17 patients), and provided no information in 8% of the 118 patients. Among the 176 major vessels visualized by both MRA and conventional angiography, conventional angiography confirmed the presence of 9/10 extracranial and 32/40 intracranial large vessel abnormalities detected on MRA. There were two false-negative findings on MRA: occlusion of a distal branch of middle cerebral artery, and an asymptomatic posterior cerebral artery stenosis. Magnetic resonance angiography is a clinically useful method for screening extra- and intracranial disease in patients with brain infarction and selecting high-yield patients for conventional angiography.

Magnetically labeled water perfusion imaging of the uterine arteries and of normal and malignant cervical tissue: initial experiences

PURPOSE

The aim of this pilot study was to evaluate a magnetically labeled water perfusion imaging technique as a non-contrast-enhanced approach to demonstrate the uterine artery, its branches, and to assess the cervical uterine blood flow in healthy volunteers and in patients with advanced uterine cervical carcinoma (FIGO IIB-IVA).

METHODS AND MATERIALS

Seven healthy volunteers (mean age, 29 years) and twenty-two patients (mean age, 52 years) with advanced cancer of the uterine cervix (FIGO IIB-IVA) were prospectively examined by magnetically labeled water perfusion imaging at different inversion delay times (300-900 ms). The magnetic resonance imaging (MRI) findings of all patients were matched to the findings of contrast-enhanced dynamic MRI and multiple biopsies (n = 5) and/or surgical whole mount specimens (n = 17), which were available in all patients.

RESULTS

The uterine artery was well visualized with short inversion delay times of 300-500 ms. It was characterized as single or multiple helical loops before dividing into its intracervical branches. The intracervical branching was observed at inversion delay times of 500-700 ms. With longer inversion delay times, arterial signal enhancement disappeared and cervical tissue enhancement was noted. Enhancement of benign tissue was observed at inversion delay times of 1100-1700 ms and in malignant tissue at shorter inversion delay times of 900-1300 ms. The maximum of this diffuse signal enhancement of benign tissue was seen at inversion delay times of 1500 ms (1100-1700 ms) in malignant tissue at significantly (p < 0.5) shorter inversion delay times of 1100 ms (900-1300 ms).

CONCLUSION

Our preliminary results show that the vascular supply and blood flow of the normal uterine cervix and of advanced cervical cancer can be assessed by magnetically labeled water perfusion imaging and that malignant cervical tissue is earlier and stronger perfused than normal cervical tissue.

Influence of imaging parameters, flow velocity, and pulsatile flow on three-dimensional time-of-flight MR angiography: experimental studies

OBJECTIVE

The purpose of this study is to evaluate the influence of imaging parameters (TR and flip angle), velocity and pulsatility of flow on three-dimensional time-of-flight magnetic resonance (3D TOF MR) angiography by using a flow phantom.

METHODS

The 3D TOF MR angiography of a flow phantom was performed at 1.5-T with various TRs, flip angles and flow velocity. The influence of pulsatile flow was also evaluated.

RESULTS

The imaging parameters and flow velocity affected the saturation effects of inflowing spins. Signal loss due to saturation of inflowing spins was more pronounced with a shorter TR, larger flip angle, or lower flow velocity. However signal intensity of stationary tissue were more suppressed with a shorter TR resulting in a larger signal difference to noise ratio of the flowing fluid. Velocity of the flow also affected the signal loss due to intravoxel phase dispersion, which was apparently observed with a stenotic tube. Pulsatility of flow resulted in decreased intensity of flowing fluid and in ghost artifacts along the phase-encoding and slice-select directions. The spacing between these ghosts and intensity of them were dependent on TR, flip angle and pulse rates.

CONCLUSIONS

Understanding the influence of imaging parameters and flow characteristics is essential for evaluating the MR angiograms or for improving their quality.

Usefulness of three-dimensional phase contrast MR angiography on arteriovenous malformations

Prospective three-dimensional phase contrast (3D-PC) MR angiography was obtained in 34 patients with arteriovenous malformations (AVM) and comparison was made between digital substraction angiography (DSA) and three-dimensional time-of-flight (3D-TOF) methods. Velocity encoding (VENC) for 3D-PC was adjusted to 60 and 10 cm/sec., and was changed only when adequate information was not obtained. VENC 60 cm/sec, demonstrated the main feeders in 100% of cases and the nidus in 86% of cases whereas VENC 10 cm/sec. showed the draining vein in 78% of cases. The detection rate of feeder, nidus and drainer was 60%, 40% and 13% respectively by the TOF technique. The mean size of the nidus as compared with DSA as standard was 130% with MRI, 108% with 3D-PC and 92% with the TOF technique and this difference was not statistically significant. 3D-PC was clearly superior in detecting AVM in the presence of hemosiderin, hematoma or surgical clips. It also showed gradual disappearance of the lesion after radiosurgery. We found 3D-PC superior to 3D-TOF in the diagnosis, therapeutic planning and follow-up of AVM.

STAR MR angiography for rapid detection of vascular abnormalities in patients with acute cerebrovascular disease

BACKGROUND AND PURPOSE

We undertook to investigate the usefulness of signal targeting with alternating radiofrequency magnetic resonance angiography (STAR MRA) in the diagnosis of acute cerebrovascular disease. The potential advantage of the technique is that angiographic images can be acquired in less than 1 minute.

METHODS

We studied 19 patients (11 men and 8 women, ranging in age from 36 to 84 years [mean age, 66 years]) presenting with signs and symptoms of acute stroke. Patients underwent STAR MRA and three-dimensional fast imaging with steady-state precession (3D FISP) MRA. The MRAs were analyzed as to image quality and vascular abnormalities in the vascular territory of stroke as defined by diffusion-weighted imaging abnormalities and compared using a Wilcoxon signed-rank test.

RESULTS

STAR MRAs had slightly inferior image quality compared with 3D FISP MRA (P < .05). STAR MRA and 3D FISP MRA agreed in 18 of 19 cases regarding vascular abnormalities in the territory of the infarct (occlusion, n = 8; stenosis, n = 4; no abnormality, n = 6). In one patient, the techniques disagreed, when 3D FISP MRA was normal and STAR MRA demonstrated a vessel occlusion in the vascular territory of a stroke as defined by diffusion-weighted imaging abnormalities (P > .05).

CONCLUSIONS

Despite slightly inferior image quality compared with 3D FISP MRA, STAR MRA is comparable with 3D FISP MRA in depicting abnormalities in the proximal parts of the cerebral arteries corresponding to ischemic regions on diffusion-weighted imaging, in a strikingly shorter acquisition time. Further studies are necessary to confirm that the smaller branches are better shown by using longer inversion times.

MR angiography of the portal venous system

MR angiography (MRA) has become an increasingly important and practical clinical tool for the noninvasive assessment of abdominal vessels. Both two-dimensional time-of-flight and phase contrast techniques allow accurate evaluation of the portal venous system. This article reviews these two MRA techniques and discusses their impact on the diagnosis of vascular abnormalities of the portal venous system.

Uterine cirsoid aneurysm: MRI and MRA

Uterine cirsoid aneurysm is uncommon. It is important to make a diagnosis of this disease preoperatively, because repeated curettages may induce life-threatening massive genital bleeding. We present a case of a 51-year-old woman with uterine cirsoid aneurysm in whom MRI and MRA were very useful for the preoperative diagnosis. The radiologic appearances on ultrasonography, CT, conventional SE MRI, MRA, dynamic MRI, and pelvic angiography are presented. Conventional SE T1-weighted and T2-weighted images demonstrated multiple flow voids in the uterus and bilateral adnexal regions. MRA demonstrated a cluster of distinct, tortuous, and coiled vascular channels in the pelvis. MRA could obtain images almost equal to angiography and was considered to be an excellent noninvasive imaging technique for the diagnosis of uterine cirsoid aneurysm.

Radiosurgery for cerebral arteriovenous malformations: assessment of early phase magnetic resonance imaging and significance of gadolinium-DTPA enhancement

PURPOSE

To evaluate the initial changes within the nidus of arteriovenous malformations (AVMs) and to assess the reaction to the brain tissue surrounding AVMs after radiosurgery by serial magnetic resonance (MR) imaging.

METHODS AND MATERIALS

Twenty-one patients, treated using 60Co gamma knife unit with cerebral AVMs, were retrospectively evaluated. Forty-seven follow-up MR images of the 21 patients were performed including 10 patients with two or more serial gadolinium enhanced studies (Gd-MR). Two or more sequential MR angiographies (MRA) were obtained in 13 patients. Three-dimensional (3D) time-of-flight MRA and two-dimensional (2D) phase contrast MRA were used in 13 patients for evaluating the flow changes of AVMs. The follow-up period after radiosurgery ranged from 3 to 30 months (average 10.8 months) and the interval time of MRI ranged from 34 days to 13 months (average 4.9 months).

RESULTS

Reduction of nidus size was observed in 14 of 21 patients (67%) between 4 to 13 months on spin echo (SE) images. Complete obliteration was observed on SE images in 4 of these 14 patients; three were confirmed by conventional angiography. New hyperintense areas surrounding the nidus on T2s-weighted images (T2WI) developed in 9 of the 14 patients who showed nidus reduction between 5 to 17 months after radiosurgery; in three patients, size of the hyperintense area started to decrease between 6 to 7 months after its appearance. Probable radiation necrosis of pons developed in one patient 26 months after radiosurgery. The irradiated area within the AVM nidus was significantly enhanced in 8 of the 10 patients who underwent Gd-MR. The degrees of enhancement within the nidus increased with time in 7 of the 10 patients. Overall, total enhancement of irradiated areas was observed in four of the 10 patients; in three of the four, the enhancement decreased in size and degree, indicating nidus reduction. In three patients who had a partial volume irradiation within the nidus, the enhancing areas corresponded with the exact portions of irradiated volume. The nidus reduction was observed in 7 of the 13 patients on MRA during 5 to 13 months after radiosurgery. MRA was more useful compared to SE images in four of the seven patients in evaluating the size change of AVM nidus, feeding arteries, and draining veins.

CONCLUSION

Magnetic resonance imaging and MRA were useful in assessing the progress of nidus reduction. T2-weighted imaging was sensitive to radiation-induced reaction in and around the AVM nidus. The enhancement within the AVM nidus on Gd-MR may represent the initial sign of nidus reduction and demonstrates the exact location of irradiation in the nidus. The changes of the enhancement pattern are presumed to represent the processes of nidus reduction and irradiated reaction within the AVM nidus.

Improved target volume definition in radiosurgery of arteriovenous malformations by stereotactic correlation of MRA, MRI, blood bolus tagging, and functional MRI

In this methodological paper the authors report the stereotactic correlation of different magnetic resonance imaging (MRI) techniques [MR angiography (MRA), MRI, blood bolus tagging (STAR), and functional MRI] in 10 patients with cerebral arteriovenous malformations (AVM) and its application in precision radiotherapy planning. The patient's head was fixed in a stereotactic localization system that is usable at the MR and the linear accelerator installations. By phantom measurements different materials (steel, aluminium, titanium, plastic, wood, ceramics) used for the stereotactic system were tested for mechanical stability and geometrical MR image distortion. All metallic stereotactic rings (closed rings made of massive metal) led to a more or less dramatic geometrical distortion and signal cancellation in the MR images. The best properties-nearly no distortion and high mechanical stability-are provided by a ceramic ring. If necessary, the remaining geometrical MR image distortion can be "corrected" (reducing displacements to the size of a pixel) by calculations based on modeling the distortion as a fourth-order two-dimensional polynomial. Using this method multimodality matching can be performed automatically as long as all images are acquired in the same examination and the patient is sufficiently immobilized. Precise definition of the target volume could be performed by the radiotherapist either directly in MR images or in calculated projection MR angiograms obtained by a maximum-intensity projection algorithm. As a result, information about the hemodynamics of the AVM was provided by a three-dimensional (3D) phase-contrast flow measurement and a dynamic MRA with the STAR technique leading to an improved definition of the size of the nidus, the origin of the feeding arteries, and the pattern of the venous drainage. In addition, functional MRI was performed in patients with lesions close to the primary motor cortex area leading to an improved definition of structures at risk for high-dose application in radiosurgery. The different imaging techniques of MR provide a sensitive, noninvasive, 3D method for defining target volume, critical structures, and for calculating dose distributions for radiosurgery of cerebral arteriovenous malformations, because dose calculation of radiosurgery at sufficient accuracy can be based on 3D MR data of the geometrical conformation of the patient's head.

Cerebral arteriovenous malformations: improved nidus demarcation by means of dynamic tagging MR-angiography

Our purpose was to further improve the target volume definition for radiosurgical treatment of cerebral arteriovenous malformations (AVMs) by means of dynamic MRA (dMRA) using a blood bolus tagging sequence. We therefore compare this technique with 3D-TOF-MRA and transfemoral high resolution angiography in plain film technique. Twenty patients with angiographically proven cerebral AVMs were investigated by dMRA, TOF-MRA, and conventional angiography during the MR-assisted radiosurgical planning protocol. The patient's head was fixed in an MR-compatible stereotactic device. The different angiography techniques were evaluated by consensus of two radiologists. AVMs were characterized by the number and origin of feeding arteries, the maximum diameter of the AVM nidus, and the venous drainage pattern. Dynamic MRA was able to demonstrate the complete AVM characteristics and hemodynamics in 12 out of 20 patients. In three patients with an AVM nidus smaller than 1 cm in diameter the technique could not reliably depict the malformation. Technical problems due to steel screws and pins in the initially used stereotactic frame occurred in five patients. Due to reduced vessel overlap and the lack of disturbances caused by formations with short T1 time, dMRA was superior to TOF-MRA in the detection and the exact localization of the AVM nidus in four patients. We conclude that dMRA is able to demonstrate reliably AVM characteristics and hemodynamics in AVMs with a nidus larger than 1 cm in diameter. Because of the improved demarcation of the AVM nidus, this technique may be a valuable adjunct to radiosurgery planning of cerebral AVMs.

The role of magnetic resonance angiography in head and neck surgery

Magnetic resonance angiography (MRA) is a recently developed, noninvasive vascular imaging technique. The authors of this investigation assessed the diagnostic value of MRA, along with its influence on therapeutic decisions, in 11 patients with a variety of head and neck disorders. In 5 patients, MRA diagnosed or ruled out an intrinsic vascular lesion. MRA was used to evaluate 5 of 8 patients with cancer for evidence of direct tumor involvement of vascular structures. Other uses of MRA included preoperative determination of tumor vascularity and delineation of anatomic relationships between normal vessels and head and neck pathology. Overall, MRA results guided management in 10 patients, and in some cases it determined the extent of surgical intervention. Because MRA is safer and more practical than traditional angiography, the authors recommend more frequent use of this imaging technique in the practice of head and neck surgery.

Phase-contrast MRA in the evaluation of EC-IC bypass patency

We have assessed three-dimensional (3-D) phase-contrast (PC) MR angiography (MRA) in the evaluation of extracranial (EC)-intracranial (IC) bypasses. This technique was performed in 22 patients (26 examinations) to evaluate the patency of their EC-IC bypasses (a total of 37 operated sites). Three sets of phase images for antero-posterior, right-left, and supero-inferior flow velocity components were also obtained in eight patients to produce directional flow images. Directional 2-D PC flow imaging was also performed in three other patients. The patency of bypasses was confirmed in 57% and suggested in 24% of operated sites using 3-D PC MRA alone. The diagnostic value of PC MRA proved superior to that of time-of-flight MRA, because the former emphasizes the signal intensity of vessels without interference from stationary tissues and is less affected by progressive saturation of inflowing spins. 2-D or 3-D directional flow images aided in understanding the flow dynamics in 69%. In patients who had been examined pre-operatively changes in intracranial flow of dynamics could be estimated on post-operative PC MRA. Although PC MRA has several limitations, it proved to be a useful imaging modality for following up patients who had undergone EC-IC bypass operations.

Intracranial vascular malformations

Intracranial vascular malformations are nonneoplastic developmental anomalies that present a variety of clinical patterns, ranging from asymptomatic to fatal intracranial hemorrhage. A practicing radiologist can expect to encounter these abnormalities, which include capillary telangiectasias, venous angiomas (also called developmental venous anomalies), cavernous angiomas, and arteriovenous malformations. The imaging findings that characterize these lesions are reviewed in this article, along with their pathological and clinical features.

Acute subarachnoid haemorrhage: 3D time-of-flight MR angiography versus intra-arterial digital angiography

To evaluate the efficacy and reliability of 3D time-of-flight MR angiography (TOF MRA) as a noninvasive procedure, 27 patients with acute subarachnoid haemorrhage (SAH) were studied with MRA immediately before or after intra-arterial digital subtraction angiography (DSA).3DTOF MRA was performed with an axial slab of 60 mm centered on the circle of Willis and isotropic voxels. DSA showed 22 aneurysms and 1 dural arteriovenous fistula in 21 patients; the aneurysms ranged in size from 2 to 8mm. MRA failed to show 2 small aneurysms, at the origin of the posterior and anterior communicating arteries. The 3D display of the intracranial vessels obtained with maximum intensity projection (MIP) or targetted MIP sometimes rendered the aneurysms better than DSA. However, due to its high spatial resolution, DSA more clearly defined the overall anatomy of the walls of the normal and abnormal vessels.

The role of magnetic resonance angiography in the diagnosis of subclavian steal

PURPOSE

To compare magnetic resonance angiography (MRA) with digital angiography for diagnosis of subclavian steal syndrome.

METHODS

A comparison study between MRA and digital contrast arteriography was carried out in 10 patients with suspected subclavian steal syndrome. Two of these patients were studied by MRA before and after percutaneous transluminal angioplasty (PTA). MRA was obtained on a 1.5 tesla superconductive magnet with linear head coil using a fast low angle shot (FLASH 2D) sequence in the axial plane as well as a fast imaging with steady state precession (FISP 3D) with velocity compensation gradient echo sequence in the coronal plane. The coronal images were used as source data for the construction of projection images with the use of a maximum-intensity pixel algorithm. The images were rotated from -45 degrees to 45 degrees in 15 degrees steps.

RESULTS

All 10 patients had evidence of proximal subclavian artery obstruction and flow reversal in the ipsilateral vertebral artery. On MRA, consistent visualization of the affected vertebral artery in the FLASH 2D sequences and nonvisualization in the FISP 3D sequences was interpreted as an indirect sign of subclavian steal. The subclavian artery obstruction could not be assessed due to field size limits of MRA.

CONCLUSION

MRA allows determination of flow reversal in the diagnosis of subclavian steal.

MP-RAGE subtraction venography: a new technique

Preliminary evaluation of a new magnetic resonance (MR) venography technique was performed with data sets from five patients undergoing MR imaging of the brain before and after intravenous administration of gadopentetate dimeglumine. Before contrast agent injection, the patients were imaged with MP-RAGE (magnetization-prepared rapid gradient-echo) and axial turbo T2-weighted sequences. After contrast agent injection, the MP-RAGE sequence was repeated. Images were post-processed with an algorithm that calculates, on a pixel-by-pixel basis, the absolute value of signal intensity of each postcontrast MP-RAGE partition minus that of each precontrast MP-RAGE partition. These subtracted partitions were then subjected to a standard maximum-intensity-projection algorithm to obtain the venogram. In all cases, the new method afforded a high-resolution venogram with clear depiction of venous sinus anatomy. Cortical venous anatomy was also clearly depicted.

Transcranial color-coded duplex sonography, magnetic resonance angiography, and computed tomography angiography: methods, applications, advantages, and limitations

Transcranial color-coded duplex sonography (TCCD), magnetic resonance angiography (MRA), and computed tomography angiography (CTA) are novel noninvasive or minimally invasive techniques for the study of the intracranial circulation. TCCD is relatively inexpensive and permits bedside examination. It improves the accuracy and reliability of conventional transcranial Doppler studies. The main limitation of TCCD are the ultrasonic windows. They restrict the area of insonation to the major cerebral arteries and the proximal part of its branches, lower the spatial resolution, and may prevent transtemporal insonation. Using MRA, both large and small intracranial arteries and veins can be imaged by selecting the appropriate imaging parameters. MRA provides morphologic information about the cerebral vessels, relying on blood flow as the physical basis for generating contrast between stationary tissues and moving spins. MRA is highly sensitive for the detection of occlusive disease in large intracranial arteries. However, with bright blood techniques the degree of stenosis tends to be exaggerated. Flow direction, eg, in collaterals, can be determined by selective or phase-contrast MRA. Perfusion imaging techniques provide information about blood flow at the capillary level. Diffusion imaging depicts molecular motion. TCCD and MRA used in combination or alone may eliminate the need for intra-arterial digital subtraction angiography (DSA) in most patients studied for occlusive cerebrovascular disease. DSA may be reserved for those patients where there is disagreement among the noninvasive techniques, and for the diagnosis of cerebral aneurysms and arteriovenous malformations. CTA relies on spiral CT technology and intravenous contrast injection. To date, intracranial use has been predominantly for the diagnosis of aneurysms. The role of CTA for the detection of nonaneurysmal intracranial vascular disease has yet to be established.

Magnetic resonance angiography of supratentorial tumours: comparison with selective digital subtraction angiography

We compared magnetic resonance angiography (MRA) and intra-arterial digital subtraction angiography (IADSA) in the study of brain tumours and assessed the utility of gadolinium-enhanced MRA. We studied 17 patients with supratentorial brain tumors. The entire brain was imaged with multiple overlapping thin volume acquisitions. After IV injection of gadolinium-DTPA, a single thick-slab MRA acquisition was performed. Standard three-dimensional (3D-TOF) acquisitions (in six patients) and 3D-TOF with magnetization transfer prepulse and tilted optimisation nonsaturing radiofrequency excitation pulses (in 11 patients) were used. Displacement of the anterior cerebral artery, main stem and insular branches of the middle cerebral artery was seen well on unenhanced and contrast-enhanced MRA. Displacement of the lenticulostriate and anterior choroidal arteries was seen only once, after Gadolinium. Tumour encasement of the middle cerebral artery was demonstrated in one patient. Tumour vessels were seen in 2 of 8 cases before and 3 of 8 after gadolinium; Tumour hypervascularity was seen only after gadolinium, in 3 of 8 cases. Study of the veins was possible only on gadolinium-enhanced MRA. Displacement of the venous angle was seen in 4 of 7 patients in the frontal, and in all of 8 patients on the lateral projections. Early venous drainage was not seen. Patency of the dural venous sinuses was demonstrated in all patients, but in one neoplastic occlusion of a cortical vein was recognized.

Venous angiomas: plain and contrast-enhanced MRI and MR angiography

We studied 17 patients with venous angiomas, 4 of whom had associated cavernous angiomas. All underwent MRI with spin-echo T1- and T2-weighted images and T1-weighted images after Gd-DTPA; MR angiography (MRA) was also performed with 3D and 2D time-of-flight technique; 5 patients underwent conventional angiography. Contrast-enhanced MRI demonstrated all the lesions, showing the peripheral medullary veins, the collector and the type of drainage. Both 3D and 2D MRA provided diagnostic information identical to that obtained after infusion of Gd-DTPA. Contrast-enhanced were T1-weighted images and MRA superior in all the cases to images without gadolinium. The possible association with cavernous angiomas (24% in our study) indicates T2-weighted imaging.

Magnetic resonance angiography and selective angiography following extra-intracranial bypass operations

Magnetic resonance angiography (MRA) was compared with conventional angiography in 14 patients following extra-intracranial arterial anastomosis. In 13 patients the bypass was shown by MRA and confirmed by conventional angiography. In five of these, the anastomosed vessels, in particular the superficial temporal artery, was of the same calibre or smaller than the same vessels on the contralateral, healthy side. In one patient no anastomosis could be shown by MRA, and occlusion of the bypass was confirmed by conventional angiography. Absence of dilatation of the anastomotic vessels may indicate insufficient bypass function due to stenosis or an incorrect indication for surgery.

MR angiography of meningioma

The clinical evaluation of three-dimensional time-of-flight MR angiography was prospectively performed in 16 patients with intracranial meningioma. MR angiographic techniques used in this study both with and without gadopentetate dimeglumine made it possible to demonstrate the topographic relationship between the tumors and the neighboring vascular structures. MR angiograms were compared with available conventional angiograms. The three-dimensional enhanced MR angiograms were successful in depicting sinus involvement in greater detail in six patients than conventional angiography. In six patients, the feeding arteries of meningiomas were demonstrated with the unenhanced MR angiograms. However, MR angiograms failed to show small feeding arteries in three patients as delineated with conventional angiography. Arterial displacement was shown in nine patients. Venous drainage and collateral circulations were also visualized in four patients. Our results indicate that multiview MR angiograms noninvasively provide useful information for planning the surgical treatment of patients with meningioma.

Imaging of the central nervous system in pediatrics and adolescence

Magnetic resonance imaging (MRI) has taken its place beside computed tomography (CT) and ultrasonography in effective imaging of the central nervous system in infants and children. The major initial impact of MRI in pediatric neuroradiology has been the displacement or replacement of the more expensive and invasive procedures such as angiography, myelography, cisternography, and ventriculography. Ultrasonography remains the screening procedure for fetal and infant central nervous system abnormalities, whereas CT continues to be an effective brain screening technique after loss of the ultrasonography windows with growth of the child. The better the clinical information transmitted to the radiologist in advance of imaging of the patient, the more likely the clinician will receive valuable information in return.

MRA review

Magnetic resonance (MR) angiography is a rapidly evolving field in MR imaging, the main goal of which is a noninvasive tool for visualizing blood vessels. The acquisition of continuous images of the blood vessels, comparable to the conventional DSA images, as well as the functional evaluation of blood-flow velocities and flow patterns are the subject of continuing developments. The basic principles of the various techniques and their clinical applications are reviewed and the practical limitations in specific anatomic regions discussed. The review concludes with a survey of future developments.

Angiographic magnetic resonance color composites of the normal brain and its vasculature

Vascular magnetic resonance imaging (MRI) has become an important noninvasive adjunct to conventional cerebral MRI studies. To detect parenchymal changes associated with vascular anomalies, optimal diagnostic evaluation requires the comparison of both spin-echo and angiographic gradient-echo MRIs. To compress image data into a single 24-bit color image possessing the combined tissue contrast characteristics of both conventional spin-echo "black-blood" images and flow-sensitive gradient-echo "bright-blood" images, the red-green-blue color model and computer-based image-processing software were used to generate composites of MRI sets in which blood appears bright red while many stationary tissues possess near-natural colors. This technique may have potential applicability to human cerebrovascular MRI.

MR angiography of lower-extremity arterial disease: preliminary experience

Nineteen patients underwent magnetic resonance (MR) angiography for evaluation of lower-extremity arterial disease. The underlying conditions included atherosclerotic occlusive disease in 12 patients, femoral or popliteal aneurysms in four, and bypass graft stenoses or occlusion in four. In the patients with occlusive disease, the iliac and femoropopliteal vessels were classified as patent, moderately stenotic, severely stenotic, or occluded. Fifteen of 16 occlusions (accuracy = 94%) were correctly classified. In the one missed case, there was a long delay between MR angiography and x-ray angiography and it is likely that the occlusion occurred during the interval. Three of five severe stenoses were correctly classified with MR angiography. In two cases of iliac artery stenosis, there was a signal void at the point of maximal stenosis, which on the basis of anatomic features could be recognized as severe stenosis rather than an occlusion. Three of four moderate stenoses were correctly classified. Correlation with x-ray angiography or surgery demonstrated the ability of MR angiography to accurately depict the status of runoff vessels.

MRA-guided stereotactic radiation treatment planning for cerebral angiomas

Stereotactic neurosurgical interventions are characterized by a high risk of rupturing intracranial vessels or damaging vital brain structures. By MRI and MRA, the anatomic information necessary for stereotactic treatment planning may be assessed with a single modality as a high-resolution digital image volume. The adequacy of MR as a modality for stereotactic guidance is demonstrated by the example of the radiosurgical treatment of cerebral angiomas. An optimized acquisition protocol, a stereotactic apparatus, a distortion correction and minimization method, and a 3D treatment planning workstation are elements of a proposed preoperative approach which in a clinical study met with good acceptance.

Selective magnetic resonance angiography of the head

Selective magnetic resonance angiography (MRA) is a combination of two- or three-dimensional MRA with radiofrequency presaturation pulses. Presaturation pulses can be applied over user-defined regions. They reduce or eliminate signals within the presaturation slab. As a result, blood flowing within or through the presaturation slab turns dark whereas blood flowing outside and not through the presaturation slab still appears bright. Using this technique, flow direction and origin of flow can be determined and flowing blood can be differentiated from stationary bright tissues such as thrombus. Arteries and veins can be imaged selectively due to their usually opposite flow direction. The intracranial collateral circulation about the circle of Willis can be shown, and vessels feeding or draining arteriovenous malformations can be identified more accurately. In conclusion, selective MRA can provide functional information about the cerebrovascular system, which cannot be obtained from conventional MR imaging (MRI) or MRA alone.

Correction of spatial distortion in magnetic resonance angiography for radiosurgical treatment planning of cerebral arteriovenous malformations

A treatment planning system based on magnetic resonance (MR) angiographic imaging data for the radiosurgery of inoperable cerebral arteriovenous malformations is reported. MR angiography was performed using a three-dimensional (3D) velocity-compensated fast imaging with steady-state precession (FISP) sequence. Depending on the individual MR system, inhomogeneities and nonlinearities induced by eddy currents during the pulse sequence can distort the images and produce spurious displacements of the stereotactic coordinates in both the x-y plane and the z axis. If necessary, these errors in position can be assessed by means of two phantoms placed within the stereotactic guidance system--a "2D-phantom" displaying "pincushion" distortion in the image, and a "3D-phantom" displaying displacement, warp, and tilt of the image plane itself. The pincushion distortion can be "corrected" (reducing displacements from 2-3 mm to 1 mm) by calculations based on modeling the distortion as a fourth order 2D polynomial. Displacement, warp, and tilt of the image plane may be corrected by adjustment of the gradient shimming currents. After correction, the accuracy of the geometric information is limited only by the pixel resolution of the image (= 1 mm). Precise definition of the target volume could be performed by the therapist either directly in the MR images or in calculated projection MR angiograms obtained by a maximum intensity projection algorithm. MR angiography provides a sensitive, noninvasive 3D method for defining target volume and critical structures, and for calculating precise dose distributions for radiosurgery of cerebral arteriovenous malformations.

Intracranial magnetic resonance angiography

Intraarterial angiography remains the "gold standard" for the evaluation of the intracranial vasculature, but it carries with it the risks of local vascular damage, systemic reactions, transient neurologic deficits, permanent neurologic compromise, and even death. To date, magnetic resonance angiography (MRA) has been applied to a variety of different manifestations of intracranial vascular disease. Presently, MRA studies and flow measurement techniques serve to compliment the more traditional spin-echo evaluation of patients with small aneurysms, arterial and venous occlusions, vascular malformations, and in some cases of neoplastic vascular invasion. Practically, the limitations of these techniques must be taken into account such that the appropriate method is applied to answer a specific clinical question and the acquisition parameters are chosen to maximize the sensitivity and specificity of the study. The intent of this review is to summarize the technical approaches, clinical role, pitfalls, and potential improvements in the MRA techniques as they apply to the intracranial circulation.