Improved cerebral time-of-flight magnetic resonance angiography at 7 Tesla--feasibility study and preliminary results using optimized venous saturation pulses

Abstract

Purpose: Conventional saturation pulses cannot be used for 7 Tesla ultra-high-resolution time-of-flight magnetic resonance angiography (TOF MRA) due to specific absorption rate (SAR) limitations. We overcome these limitations by utilizing low flip angle, variable rate selective excitation (VERSE) algorithm saturation pulses.

Material and methods: Twenty-five neurosurgical patients (male n = 8, female n = 17; average age 49.64 years; range 26-70 years) with different intracranial vascular pathologies were enrolled in this trial. All patients were examined with a 7 Tesla (Magnetom 7 T, Siemens) whole body scanner system utilizing a dedicated 32-channel head coil. For venous saturation pulses a 35° flip angle was applied. Two neuroradiologists evaluated the delineation of arterial vessels in the Circle of Willis, delineation of vascular pathologies, presence of artifacts, vessel-tissue contrast and overall image quality of TOF MRA scans in consensus on a five-point scale. Normalized signal intensities in the confluence of venous sinuses, M1 segment of left middle cerebral artery and adjacent gray matter were measured and vessel-tissue contrasts were calculated.

Results: Ratings for the majority of patients ranged between good and excellent for most of the evaluated features. Venous saturation was sufficient for all cases with minor artifacts in arteriovenous malformations and arteriovenous fistulas. Quantitative signal intensity measurements showed high vessel-tissue contrast for confluence of venous sinuses, M1 segment of left middle cerebral artery and adjacent gray matter.

Conclusion: The use of novel low flip angle VERSE algorithm pulses for saturation of venous vessels can overcome SAR limitations in 7 Tesla ultra-high-resolution TOF MRA. Our protocol is suitable for clinical application with excellent image quality for delineation of various intracranial vascular pathologies.

Figure 1. Shown is the localizer scan (a) of a 51-year-old male patient (subject 24) with a large arteriovenous malformation in the left hemisphere with thickness of TOF MRA imaging slab (IS) and saturation pulse (SP) illustrated.

In a 29-year-old healthy subject a TOF MRA slab with identical coverage and spatial resolution was acquired with the non-optimized sequence in 17∶48 min (b: sagittal view, d: transverse view). With the optimized sequence the identical slab was acquired in 6∶22 min (c: sagittal view, e: transverse view). The same representative slice is shown for both sequences for direct comparison. In the optimized sequence TE could be reduced from 6.62 ms to 4.34 ms (due to a different asymmetric echo factor) and TR could be reduced from 56 ms to 20 ms (mainly due to reduced SAR) leading to better image contrast.

Figure 2. This figure shows the boxplots for all pathologies (aneurysms, AVM, AVF and Moyamoya angiopathy) and all rated categories.

Delineation of Circle of Willis was rated excellent except for Moyamoya angiopathy, mainly due to the disease specific early ramifications of the middle cerebral artery branches. Delineation of vascular pathologies was rated excellent except for AVF. The complex angioarchitecture and close relationship to the skull bone complicate depiction in this pathology. Vessel tissue contrast was excellent in aneurysms and Moyamoya angiopathy, but poor to excellent in AVM and AVF. Arterialized hyperintense venous blood alters vessel tissue contrast in these patients. Presence of artifacts was rated mainly good to excellent for all pathologies and only pulsation artifacts led to lower ratings. VERSE specific artifacts did not significantly alter the images. Overall image quality was good to excellent for all pathologies.

Figure 3. This 40-year-old male patient (a–c) (subject 23) suffered from a right hemispherical AVM (Spetzler-Martin grade IV).

Arrows are in transversal (a) and sagittal (b) views are indicating the saturated superior sagittal sinus. Arrow in coronal view (c) is marking a partially saturated draining vein. This 40-year-old male patient (d–f) (subject 4) suffered of a right parietal parasagittal fistulous AVM. Signal intensity in the upper part of the sagittal sinus next to the outlet of the draining vein was equal to signal intensity of left middle cerebral artery M1 segment (f): arrow in this transverse view is marking unsaturated superior sagittal sinus). In the further distal part of the superior sagittal sinus close to the confluence of sinuses, signal intensity is gradually decreasing until it is lower than in the neighboring occipital gray matter (d: arrow in this sagittal view is marking gradually saturated superior sagittal sinus). Arrow marks a partially saturated draining vein in coronal view (e). Depending on the individual cerebrovascular anatomy and venous drainage of the AVM, venous saturation can be altered significantly in parts of the TOF MRA imaging slab. Regardless of lower venous saturation in some cases overall image quality was nevertheless rated good to excellent.

Figure 4. These are the transversal (a), sagittal (b) and coronal (c) views of a 42-year-old female Moyamoya patient (subject 20) after bilateral superficial temporal artery to middle cerebral artery bypass surgery (asterisks next to bypass, black arrows pointing on saturated superior sagittal sinus).

The Moyamoya typical anomalously early ramification of middle cerebral artery branches are best depicted in axial and coronal views (white arrows in a and b).

Figure 5. This figure shows the scan of a 58-year-old male patient (subject 11) with a complex occipital dural AVF (Cognard type IIb) fed by branches of both external carotid and vertebral arteries.

It highlights the advantages of venous saturation pulses in 7 Tesla TOF MRA. The exact location of the fistulous point (arrow in magnified image section) is depicted by mixing hyperintense unsaturated arterial blood with hypointense saturated venous blood in the superior sagittal sinus.