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Neuroradiology |
1 From the Department of Medical Imaging, Division of Neuroradiology, Toronto Western Hospital, University of Toronto, Fell Pavilion 3-404, 399 Bathurst St, Toronto, Ontario, Canada M5T 2S8 (R.I.F., J.N.S., R.A.W., W.J.M., K.G.t.B.); and Department of Imaging Research, Sunnybrook and Women’s College Health Science Centre, Toronto, Ontario, Canada (G.A.W.). Received June 5, 2002; revision requested July 3; revision received July 15; accepted July 24. Supported in part by grant A3048 from the Heart and Stroke Foundation of Ontario. Address correspondence to R.I.F. (e-mail: richard.farb@utoronto.ca).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: ATECO MR venography was performed in 23 patients, eight of whom also underwent two-dimensional time-of-flight (TOF) MR venography for imaging of the intracranial venous system. Seventeen predefined venous structures were evaluated on all venograms by two neuroradiologists. Visualization of venous structures was defined as completely visible (including clearly pathologic), partially visible, or not visible. Readers were also asked to compare the visibility of these predefined structures on ATECO and TOF MR venograms, where available.
RESULTS: Of the 23 patients, six had dural venous sinus disease. Of the remaining 17 healthy patients, five underwent both ATECO and TOF MR venography and 12 underwent ATECO MR venography alone. On ATECO MR venograms obtained in the healthy patients, visibility of the 17 predefined venous structures was complete in 92% (531 of 578) of evaluations. For the five normal TOF MR venograms, the rate of complete visibility of the same venous structures was 61% (104 of 170). The rate of complete visibility of the large dural venous sinuses was 99% for ATECO MR venograms and 75% for TOF MR venograms.
CONCLUSION: ATECO MR venography provides high-quality images of the intracranial venous anatomy and was superior to TOF MR venography for consistent complete visibility of venous structures.
© RSNA, 2002
Index terms: Cerebral blood vessels, MR, 176.12142, 176.91, 177.12142, 177.91 • Magnetic resonance (MR),technology, 17.12142 • Magnetic resonance (MR), vascular studies, 176.12142, 177.12142
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MR Examinations
All examinations were performed with a superconducting 1.5-T MR system (Signa EchoSpeed, version 8.2.3 software; GE Medical Systems, Milwaukee, Wis) with a standard head coil. Eight examinations composed TOF MR venography with coronal acquisition from occiput to the coronal suture with the following parameters: 50/4.9 (repetition time msec/echo time msec); flip angle, 60°; fractional echo acquisition; field of view, 24 cm; matrix, 256 x 128; bandwidth, 16.0 kHz; section thickness, 1.5 mm; and aquisition of approximately 70–95 sections, yielding a total imaging time of approximately 7–11 minutes. All examinations also included ATECO MR venography. Details of the fully automated detection and triggering system have been published previously for intracranial (3) and spinal (4) vascular MR imaging.
ATECO MR venography examinations included the following four integral parts: (a) initiation of a two-dimensional single-section bolus-detection sequence oriented in the transverse plane and located at the level of the cavernous carotid arteries; (b) intravenous power injection of contrast material; (c) automated detection of the arrival of intraarterial contrast material at the cavernous carotid level, which resulted in automatic termination of the detection sequence and triggering of (d) a fast three-dimensional gradient-echo MR angiography sequence with ATECO phase encoding.
Two-dimensional Detection and Triggering Sequence
Coordination of the intravenous injection and three-dimensional centrically encoded imaging was performed by using a software-based triggering tool developed at our institution. All ATECO MR venography examinations were performed by one of five MR technologists and required no input from a radiologist. The two-dimensional bolus-detection sequence was initiated and controlled in real time by using an interactive user-friendly computer interface that operated in a C-shell application in the background on the MR imager host computer. The previous version of the software was installed on a separate workstation, which was connected to the MR imager subsystems (3,4).
The detection sequence consisted of a continuously refreshed two-dimensional gradient-echo acquisition of a transverse section positioned at the level of the cavernous carotid arteries. The detection sequence parameters were as follows: 11.5/9.5; flip angle, 30°; field of view, 20 x 20 cm; matrix, 320 x 128; bandwidth, 62.5 kHz; section thickness, 10 mm. The resulting image update rate was approximately one image every 1.5 seconds.
As a first step in implementation of the detection sequence, interactive section positioning was performed to obtain a transverse image at the level of the skull base for placement of regions of interest over the carotid and basilar arteries. In all cases, the region of interest was placed interactively by the MR technologist and was drawn as a circular tracing that was approximately 1.0–1.5 cm in diameter to encircle the entire vessel. Inferior and superior saturation bands were then applied sequentially. The saturation bands were kept active during the detection sequence, which extinguished signal due to flow-related enhancement in the arteries and, thus, optimized visibility of the expected inflow of gadolinium-enhanced arterial blood. A signal intensity triggering threshold was then determined automatically.
Data from the region of interest were collected and evaluated in an automated fashion so that the mean signal intensity of the brightest 20% of pixels in the region of interest was averaged for 10 consecutive images. A triggering threshold was then set at 5 SDs higher than the mean of the averaged data. After the operator was notified that the triggering threshold had been determined, he or she potentiated, or armed, the system. Subsequent intravenous injection of a bolus of contrast material resulted in a rapid increase in signal intensity in the regions of interest that exceeded the set threshold. After an additional preprogrammed delay of 8 seconds, the detection sequence was terminated automatically and, simultaneously, the three-dimensional centric MR angiographic sequence was initiated. The 8-second delay was determined empirically to ensure sufficient perfusion of contrast material through the circle of Willis, the physiologic cerebral circuit, and well into the intracranial venous system before initiation of the centric filling of k space. With imaging and processing delays, triggering occurred 8–9 seconds after arrival of the leading edge of the bolus of contrast material. This triggering tool was presented previously in detail (3,4,7).
Contrast Material Injection
As part of the preparation for MR imaging, a commercially available two-cylinder MR-compatible injector (Spectris; Medrad, Indianola, Pa) was loaded with 30 mL of gadolinium-based contrast material (gadodiamide, Omniscan; Nycomed Amersham, Buckinghamshire, England) in one syringe (syringe 1) and 60 mL of normal saline in a second syringe (syringe 2). Intravenous access was obtained with a 20-gauge intravenous catheter located in the right antecubital vein. A slow infusion (0.5 mL/min) of saline was initiated at the time of the intravenous connection and was continued during preliminary imaging until the bolus of contrast material was injected. After the third MR angiographic sequence was prescribed and loaded into the pulse sequence controller and after the detection sequence was armed, the technologist in the control room manually initiated the injection. Thirty milliliters of the contrast material was injected at a rate of 3 mL/sec and was followed immediately with a 30-mL bolus of normal saline that was also injected at a rate of 3 mL/sec. This injection protocol was followed for all patients.
ATECO MR Angiographic Sequence
The view order for this three-dimensional MR angiographic sequence was in ascending order of radial k-space distance from the k-space origin, similar to that reported previously (3,8,9). A fast spoiled gradient-echo sequence was oriented in the sagittal plane with coverage from ear to ear by using the following parameters: 7/1.6; flip angle, 35°; fractional echo acquisition; field of view, 25 cm; matrix, 320 x 320; bandwidth, 62.5 kHz; section thickness, 1.3 mm; 124 sections, resulting in a 16-cm-thick volume; and an image time of 4 minutes 38 seconds. Resulting voxel dimensions were 0.78 x 0.78 x 1.3 mm. This resolution was obtained without zero-filling techniques.
Image Processing and Review
Source image data obtained with two-dimensional TOF and three-dimensional ATECO MR venography were transferred to a commercially available three-dimensional workstation (Advantage Windows, version 4.0; GE Medical Systems) for image manipulation. Maximum intensity projections were created for each TOF and ATECO MR venography data set. Maximum intensity projections included both an unsegmented and a segmented anteroposterior projection. The larger components of intracranial arterial anatomy located near the central skull base were easily excluded on segmented maximum intensity projections (Fig 1). This projection was rotated in an incremental fashion to provide 12 images (one maximum intensity projection every 15° through 180° of rotation).
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Two neuroradiologists (W.J.M., R.A.W.) experienced in neurovascular imaging and MR interpretation assessed independently the visibility of 17 predefined intracranial venous structures at each MR venography examination: superior and inferior sagittal sinuses, straight sinus, right and left transverse and sigmoid sinuses, junctions of the transverse and sigmoid sinuses, the torcula herophili, vein of Galen, right and left basal veins of Rosenthal, and thalamostriate and internal cerebral veins. Results of visibility of each structure by both readers were pooled, that is, if a structure was seen, it might be seen once (by one reader) or twice (by both readers).
Visualization of these structures was reported as not visible, partially visible, or completely visible (including clearly pathologic). A completely visible structure implied that the reader was highly confident that he was able to visualize the lumen of the venous structure in its entirety, whether it was pathologically thrombosed or invaded, truly hypoplastic, or clearly normal. With these criteria, the readers were not permitted to designate a dural venous sinus as simply hypoplastic as a justification for a report of partially visible or not visible.
The readers were asked to assess the overall quality of the MR venograms as diagnostic (useful for rendering an opinion regarding the presence or absence of disease) or nondiagnostic. In cases in which both ATECO and TOF MR venograms were available, the readers were asked to compare visualization of each structure by using the following scores: 1, TOF MR venogram superior to ATECO MR venogram; 2, ATECO and TOF equivalent; and 3, ATECO superior to TOF.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Of the six patients with dural sinus disease, five had dural sinus thrombosis (Figs 2, 3) and one had tumor invasion of the superior sagittal sinus (Fig 4). In three of these cases, both ATECO and TOF MR venography were performed; in the remaining three cases, only ATECO MR venography was performed. In the six patients, the large dural venous sinuses were completely visible on ATECO MR venograms in 91% (98 of 108) of cases and was partially visible in 9% (10 of 108). There were no cases in which the large dural venous sinuses were not visible on ATECO MR venograms. With TOF MR venography in the same patients, large dural venous sinuses were completely visible (or clearly pathologic) in 54% (29 of 54) of cases, were partially visible in 39% (21 of 54), and were not visible in 7% (four of 54).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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In the present study, a delay of 8 seconds was chosen on the basis of the mean normal cerebrovascular arterial to venous transit time at angiography (12) with 1 SD added. A system triggered with an 8-second delay after arterial arrival of contrast material results in images in which both the arteries and veins are depicted. Owing to the ongoing use of ATECO protocols for intracranial imaging at our institution, the unsupervised MR technologists have become adept at positioning the transverse section of the detection sequence and drawing the regions of interest over the arteries. An alternative triggering strategy that is worthy of future evaluation would include use of regions of interest drawn over the jugular veins and direct triggering off of those major venous structures without insertion of a delay.
Two-dimensional TOF MR venography has been widely accepted for imaging of the intracranial venous system despite the well-documented pitfalls associated with the technique. A major pitfall of TOF MR venography is the artifactual intravascular signal loss that occurs at predictable points in the intracranial venous anatomy, namely, the posterior superior sagittal sinus, transverse sinus, transverse-sigmoid junction, and sigmoid sinuses (Figs 4, 5). These artifacts relate to either in-plane saturation of spins or tortuosity and turbulent flow (1) and can be troublesome. They commonly necessitate that the reader correlate the TOF MR venograms to the conventional T1- and T2-weighted MR images to determine the presence of a patent sinus instead of confirming its presence on the basis of complete visibility. Nowhere is this pitfall of TOF MR venography more problematic than it is with the task of discriminating a hypoplastic from a thrombosed transverse sinus (1).
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The superiority of ATECO over TOF MR venography is a result of several factors. First, the former is flow insensitive, and depiction of a vessel requires satisfaction of two criteria: (a) The lumen of the vessel must contain a sufficient concentration of contrast material at the moment the centric sequence is initiated, and (b) the size of the vessel must be greater than the spatial resolution threshold of the image acquisition. As a result, common artifacts associated with TOF MR angiography are not encountered with ATECO MR angiography because of its flow insensitivity. Second, owing to the short repetition time of the sequence, signal from background tissue (including fat and methemoglobin) is suppressed to a greater degree than that at TOF MR angiography. Third, the relatively shorter repetition time of the ATECO sequence allows acquisition of a larger volume with higher spatial resolution in less overall image time (4 minutes 8 seconds) than that with the TOF MR sequence (approximately 7 minutes).
The elliptic centric ordering of phase encoding ensures that low-frequency k-space data (which dictate major structural signal intensity in the final image) are collected early in imaging. Raw data that correspond to the center of k space are collected at a time when a high concentration of contrast material is restricted to the intravascular compartment. In this way, centric ordering allows a further increase in the overall image time compared with that with other first-pass techniques. This gain in time is reinvested in increased spatial resolution, while a high ratio of vessel-to-background signal intensity is maintained in the final images.
The size of our sample of patients who underwent TOF MR angiography is small (n = 5); however, the visibility of venous structures with the TOF technique is similar to that reported by other authors. Ayanzen et al (1) observed flow gaps in the transverse sinuses in 31 of 100 healthy patients who underwent coronal two-dimensional TOF MR angiography; they cautioned that these flow gaps could potentially lead to diagnostic difficulties "when the issue of venous sinus thrombosis is in question." Liauw et al (2) compared phase-contrast and TOF MR venography techniques in 12 healthy patients and observed rates of complete visibility of the large dural sinuses (superior sagittal sinus, straight sinus, right and left transverse sinuses, and sigmoid sinuses) of 92% and 85%, respectively. They concluded that coronal two-dimensional TOF MR venography and three-dimensional phase-contrast MR venography were equivalent in their ability to depict the intracranial venous anatomy. In their study, other forms of TOF and phase-contrast MR venography were evaluated and found to be inferior.
Techniques of bolus injection of contrast material with subsequent acquisition with a magnetization-prepared rapid gradient-echo sequence have been described previously for the imaging of intracranial venous anatomy (13,14). In the more recent study (13), a power injector was not used, the contrast material was injected at a rate of 1–2 mL/sec, there was no attempted coordination of the injection of contrast material and initiation of the angiographic sequence, and the sequence was not centrically encoded. Nonetheless, the authors showed visibility of venous structures with gadolinium-enhanced MR venography that was better than that with two-dimensional TOF MR angiography. In their phantom experiments, the authors also showed a dramatic increase in the signal-to-noise ratio as the intravascular concentration of contrast material was increased.
The technique used in the present study benefits from an ATECO acquisition, a rapid bolus injection of contrast material, a triggering mechanism to optimize capture of venous contrast, and a high-spatial-resolution acquisition. The T1-weighted short repetition time sequence with elliptic centric ordering of k space yields a final image in which the signal intensity of the structures is determined on the basis of the distribution of gadolinium enhancement in the first several seconds after sequence initiation (8). Optimally timed application of this type of sequence provides maximum vessel-to-background contrast (15).
While the advantages of superior vessel depiction, greater suppression of background signal, and substantially shortened image time of ATECO MR venography are clear over those with TOF MR venography, perceived disadvantages of the former may include the expense of the contrast agent, as well as the expense and patient discomfort of obtaining antecubital venous access. In the case of dural sinus thrombosis, however, confident early diagnosis of this common and treatable disease can dramatically reduce patient morbidity.
In this initial comparison, ATECO MR venography provided high-quality images of the intracranial venous anatomy and was shown to be superior to TOF MR venography for visibility of venous structures. Our clinical practice has changed considerably since the addition of the ATECO technique. At our institution, auto-triggered ATECO MR venography is now the preferred method for imaging of the intracranial venous anatomy. TOF MR venography has served us well in the past, but it is now omitted from the MR venography protocols in lieu of the more informative and faster ATECO technique.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, R.I.F.; study concepts and design, R.I.F., J.N.S., G.A.W.; literature research, R.I.F., J.N.S.; clinical studies, R.I.F., J.N.S., R.A.W., W.J.M.; data acquisition, R.I.F., J.N.S., R.A.W., W.J.M.; data analysis/interpretation, R.I.F., J.N.S.; statistical analysis, R.I.F.; manuscript preparation, R.I.F.; manuscript definition of intellectual content, R.I.F., K.G.t.B.; manuscript editing, R.I.F., G.A.W., K.G.t.B.; manuscript revision/review, R.I.F., J.N.S., G.A.W., K.G.t.B.; manuscript final version approval, all authors.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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