MRA of Brain Vessels 26

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R. M. Hoogeveen, PhD

Philips Medical Systems, Clinical Scientist, QR0113 – PO Box 10000, 5680 DA Best, The Netherlands

C O N T E N T S

26.1 Introduction 285

26.2 Time-of-Flight MRA Using SENSE 285 26.3 Phase-Contrast MRA Using SENSE 286 26.4 Contrast-Enhanced MRA Using SENSE 286 26.4.1 Anatomical Imaging: CENTRA and SENSE 287 26.4.2 Functional, Time-Resolved Imaging:

CENTRA Keyhole and SENSE 287 References 290

MRA of Brain Vessels 26

Romhild M. Hoogeveen

26.1

Introduction

The brain is an excellent area for parallel imaging.

One reason for this is that the head is almost cylin- drical in shape, allowing coil elements around the brain to “see” an equal share of the brain. This allows the possibility of reaching high acceleration factors.

Another reason is that the head is a relatively small object and that the number of elements in commer- cially available coils (e.g., eight) is getting close to optimal for parallel imaging. Finally, the sizes of the head do not vary as much as other body parts, thus allowing a more optimal design of the head coil.

Because of the many coil elements circularly posi- tioned around the brain, the two main directions for parallel imaging are left-right and antero-posterior.

In many 3D sequences, both directions can be used, e.g., in 2D SENSE (Weigert et al. 2002), and high speedup factors can be reached up to the number

of coil elements. This is particularly useful in large volume scans, such as 3D phase-contrast or 3D contrast-enhanced (CE) MRA of the brain.

The reasons for using parallel imaging for MRA are manifold: (1) lengthy sequences, such as 3D time-of- ﬂ ight MRA, can be shortened, (2) spatial resolution can be increased, particularly in CE-MRA sequences with abundant SNR, (3) the number of slices can be increased to gain more volume coverage, (4) higher temporal resolution can be obtained in dynamic sequences and (5) venous contamination in ﬁ rst-pass CE-MRA can be further reduced as k-space is traversed with more speed.

In most MRA methods at 1.5 T there is abundant SNR available to justify the use of parallel imaging, e.g., to decrease scan time or to increase resolution.

It may be clear that with the doubling of SNR in the head at 3 T, the use of parallel imaging is almost indispensable. Using parallel imaging at 3 T has the additional beneﬁ t of reducing the SAR values.

In the next sections we will address the use of SENSE parallel imaging (Pruessmann et al. 1999) for various MRA methods.

26.2

Time-of-Flight MRA Using SENSE

Currently, the most frequently used MRA technique in the brain is 3D time-of-ﬂ ight (TOF) MRA to image the intracranial arteries without the use of a contrast agent. TOF-MRA needs to be planned perpendicular to the arterial inﬂ ow, and axial slices are acquired.

Consequently, the choice of the slice-encoding direction is ﬁ xed: feet-head. The small volume coverage in slice-direction limits the use of SENSE in this direction. The preferred phase-encoding SENSE direction is therefore the left-right phase-encoding direction. In this setup, typically a SENSE speedup factor of two to three can be obtained, which is used for shortening the sequence and improving the spatial

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286 R. M. Hoogeveen

resolution at the same time. A TOF example can be found in Fig. 26.1 using a SENSE factor of R=2.5. This protocol took less than 5 min (in the past, without SENSE this would have been 12 min) for a resolution of 0.4×0.7×0.5 mm³, 512 scan matrix and 150 slices.

Images were acquired on a Philips Achieva 3.0-T system equipped with an eight-element head coil.

SENSE in TOF-MRA enables the possibility to further increase the acquisition matrix. Traditionally, 512 matrices were used, but now routinely matrices between 512 and 1,024 can be applied (both on 1.5 T and 3 T). The extra gain in SNR at 3 T even permits to go beyond 1,024-matrix scanning. In all these cases, SENSE or other strategies for parallel imaging are crucial to keep the imaging time within clinically acceptable limits.

26.3

Phase-Contrast MRA Using SENSE

In the early 1990s, there was a debate about whether TOF-MRA was preferred above phase-contrast MRA

(PCA) in the brain. PCA has a number of advantages over TOF-MRA: better suppression of the back- ground, no severe in-plane saturation and no limits in the choice of the positioning of the imaging volume.

The inherently longer scan times of PCA, however, prevented the technique to become used routinely.

The only main application where PCA is still used in the brain is venography, e.g., for evaluating the sagittal sinus and its branches to exclude sinus venous thrombosis in pregnant women.

With the availability of parallel imaging techniques, the main disadvantage of PCA can now be overcome, and this might cause a renaissance for PCA. Since PCA can be planned in any direction, the favorable choice of the phase/slice-encoding directions is left-right and anterio-posterior (see Sect. 26.1). With coronal and sagittal imaging, SENSE can be applied in these two directions together, and much higher speedup factors can be obtained than in TOF imaging. In contrast to TOF- MRA, with PCA whole-brain angiography or whole- brain venography can be performed. With SENSE acceleration factors of 6-8 (combination of, e.g., 2×3 and 2×4), a whole-brain PCA acquisition with a spatial resolution of about 1×1×1 mm³ can be obtained in 4-7 min (Hoogeveen et al. 2003); see Fig. 26.2.

Without SENSE, the acquisition time would have been clinically unacceptable (roughly 40 min). The availability of high acceleration factors opens doors to sequences that would just take too long to be practical.

SENSE and PCA are a perfect match, not only because PCA exhibits inherently long scan times that could never be signiﬁ cantly shortened by stronger gradients, but also because PCA has a very high SNR and CNR. The future will tell whether PCA will revive again.

26.4

Contrast-Enhanced MRA Using SENSE

Although contrast-enhanced (CE) MRA has replaced TOF-MRA in many parts of the body, this is not the case for the brain. The arterio-venous circulation time in the brain is so fast (3–4 s) that a venous- free contrast-enhanced image of the brain arteries cannot be acquired with a sufﬁ ciently high spatial resolution. The contrast-enhanced timing-robust angiography (CENTRA) technique (Philips Medi-

Fig. 26.1. Infl ow time-of-fl ight MRA at 3.0 T using SENSE and an 8-channel head-coil. Protocol parameters: 3D T1-weighted gradient-echo sequence with TR/TE/fl ip =25/3.45/20ô; matrix 512; reconstruction matrix 1,024; resolution: 0.4×0.7×0.9 mm³;

interpolated to 0.2×0.35×0.45 mm³; 150 slices in four chunks/

subvolumes; SENSE acceleration R=2.5; scan time 4:48 min–

12 min without SENSE.

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cal Systems) (Willinek et al. 2002) – a version of elliptical centric imaging (Wilman et al. 1997) with improved robustness – can be used in CE-MRA to extend the imaging time beyond the arterio-venous window to prevent venous opaciﬁ cation. The addi- tion of SENSE to the CENTRA technique for CE- MRA in the brain has two main advantages: the spatial resolution can be further improved and at the same time, k-space is traversed much faster out- wards from the center of k-space, thereby improving venous suppression. With the availability of CENTRA with parallel imaging, CE-MRA of the brain is starting to be used routinely for anatomical imaging.

26.4.1

Anatomical Imaging: CENTRA and SENSE

The main reason to use CE-MRA instead of TOF-MRA in the brain is to image regions where TOF-MRA fails: in areas with complex ﬂ ow, around stenoses and in aneurysms, and in areas where TOF-MRA suf- fers from saturation artefacts, such as in AVMs. Also patients with coiled aneurysms are eligible for CE- MRA rather than the more artefact-prone TOF-MRA technique. Typically, a coronal or transverse volume is planned around the circle of Willis, and SENSE is applied in a left-right direction with acceleration factors of 2–3. An example is found in Fig. 26.3. For timing of the contrast agent, either a test-bolus can

be used or ﬂ uoroscopic triggering (e.g., BolusTrak) can be applied.

Similar to the PCA technique (Sect. 26.3), higher acceleration factors can be used when the volume is larger and two phase-encoding directions are used.

Recently, this has led to a 40-s high-resolution arterial whole-brain protocol with SENSE acceleration factor R=8 (2×4); see Fig. 26.4. By extending this scan with a second acquisition, a steady-state volume can be obtained with detailed high-resolution venous information (Fig. 26.4, right image). Without the high acceleration of parallel imaging, this protocol would not have been possible.

In summary, parallel imaging in the brain in combination with CENTRA or similar techniques is an essential ingredient for contrast-enhanced high-resolution MRA of the brain.

26.4.2

Functional, Time-Resolved Imaging:

CENTRA Keyhole and SENSE

Dynamic imaging of a contrast bolus injection is clinically relevant in patients with AVMs or highly vascularized tumors such as glomus tumors or juvenile nasoangioﬁ bromas. In both cases, information about the feeding and draining vessels and enhancement of the AVM or tumor is needed. So far, DSA is the gold standard for these applications because the temporal resolution of MR does not

Fig. 26.2. Whole-brain phase-contrast venography at 1.5 T using an 8-element SENSE head coil. A SENSE reduction factor of R=8 was used, thereby reducing the scan time from impractical 40 min to about 5 min. Parameters: 3D T1-weighted PCA; three ﬂ ow directions; V_enc=10 cm/s; TR/TE/

ﬂ ip: 15 ms/4 ms/15^o; FOV 256×192 mm²; matrix 256; 365 coronal slices; acquisition resolution:

1.0×1.0×1.2 mm³; reconstructed to 0.5×0.5×0.6 mm³

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Fig. 26.3. High-resolution arterial 3D CE-MRA of the circle of Willis using CENTRA and SENSE. Imaging was performed using an 8-channel head coil at 3.0 T. CENTRA was used to obtain high-resolution images with excellent suppression of venous signal. For timing of the contrast arrival, Bolus- Trak ﬂ uoroscopic triggering was used. Spatial resolution:

0.52×0.69×0.45 mm³ (oc); 130 slices. Protocol parameters:

TR/TE/ﬂ ip =6.5 ms/1.92 ms/30^o; FOV 210×180 mm²; matrix 400/512r. SENSE with acceleration factor R=2.5 was used. Scan time was 45 s, and no venous enhancement was seen. Cour- tesy: University Medical Center Utrecht, The Netherlands

Fig. 26.4. MIP projections of a dynamic 3D arterial and venous high-resolution whole brain CE-MRA acquisition using an 8- channel head coil at 3.0 T. Spatial resolution: 0.63×0.63×0.5 mm³ (oc). The volumes of each phase (arterial/venous) contained 365 slices and were acquired in just 40 s using a SENSE acceleration of R=8. Further protocol parameters: TR/TE/ﬂ ip =5.0 ms/

1.5 ms/30^o. FOV 250×212; matrix 400/512r. Fluoroscopic triggering was obtained using BolusTrak and a CENTRA proﬁ le order of the 3D acquisition to suppress veins in the arterial phase. Courtesy: ASAN Medical Center, Seoul, Korea

meet the required speed: roughly 1 s/volume or better. To meet this requirement, ﬁ rst, spatial resolution can be somewhat lowered (1–2 mm) and high acceleration SENSE factors should be used. With this, frame rates in the order of 5 s/volume can be obtained. Additionally, CENTRA keyhole imaging (Hoogeveen et al. 2004; Willinek et al. 2005) can be used to further speed up the acquisition. The combination of SENSE and CENTRA with keyhole imaging has now routinely led to the following 4D protocol: SENSE acceleration: 6–8, CENTRA keyhole acceleration: 6×, whole-brain coverage with 140 slices of 1.1 mm (in-plane resolution 1.1×1.4 mm²) and 29 volumes in 20–30 s.

An example is shown in Fig. 26.5: a complete high- resolution 3D volume can be dynamically scanned with a temporal resolution of less than 1 s (0.66 s/

volume). Zooming in on a few dynamic frames, the arterial feeders and venous drainage of an AVM can be recognized (Fig. 26.6).

Speciﬁ cally, in dynamic imaging, it is important to acquire calibration data for the sensitivity of the coils before the actual acquisition. Most SENSE imple- mentations usually do this. Techniques with auto- calibration of the coil systems (e.g., GRAPPA with calibration during the scan) will provide lower frame rates in these applications because time is lost for the calibration during the scan (cf. Chaps. 8 to 10). This limitation could be overcome with the implementa- tion of more recent algorithms for dynamic imaging

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Fig. 26.5. MIP projections of a 4D CE-MRA brain protocol using CENTRA keyhole and SENSE. An 8-channel head-coil was used on an Achieva 3.0-T MR system. Protocol parameters: 29 consecutive 3D volumes with 140 slices. Spatial resolution of 1.1×1.4×1.1 mm³ (oc). SENSE acceleration R=6; CENTRA keyhole acceleration factor: 6; total acceleration factor: 36. Temporal resolution: 0.66–1.1 s/volume. Courtesy: Bonn University Hospital, Bonn, Germany

Fig. 26.6. Comparison of a few projections of the 4D CE-MRA dataset from Fig. 26.5 with DSA. Left MR image shows the arterial feeder of an AVM, right MR image shows the venous drainage of the same AVM. The MR images show a very good correlation with DSA (see insets). Courtesy:

Bonn University Hospital, Bonn, Germany

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like TSENSE (Kellman et al. 2001) or k-t BLAST/k-t SENSE (Tsao et al. 2003) (cf. Chaps. 7 and 12).

In conclusion, the combination of parallel imaging with other speedup techniques (viz. SENSE with CENTRA keyhole) can lead to acceleration factors up to 50×, and this opens new possibilities for dynamic MR imaging.

References

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Hoogeveen RM, Visser F, van den Brink J (2003) High-resolution isotropic whole-brain angiography at 1.5 T and 3.0 T with PCA using highly accelerated SENSE imaging [abstract]. Proc Int Soc Mag Reson Med 11:329

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