Design of Parallel-Imaging Protocols 15

(1)

Design of Parallel-Imaging Protocols 169

S. O. Schoenberg, MD; O. Dietrich, PhD

Department of Clinical Radiology, University Hospitals – Gross hadern, Ludwig Maximilian University of Munich, Marchioni nistr. 15, 81377 Munich, Germany

C O N T E N T S

15.1 Introduction 169

15.2 Protocol Suggestions 169

Design of Parallel-Imaging Protocols 15

Stefan O. Schoenberg and Olaf Dietrich

15.1

Introduction

Parallel-imaging techniques have only recently been introduced into clinical-routine MRI, but they have already gained wide clinical acceptance in numer- ous applications. Their substantial advantages in terms of higher spatial and temporal resolution and improved image quality in single-shot applications have inﬂ uenced the design of imaging protocols for practically all applications ranging from MRI of the brain to imaging of the pedal arteries. In this chapter, general recommendations for the design of imaging protocols are listed (ordered by anatomic region) with the most important protocol requirements and their implications for parallel imaging.

More details to the speciﬁ c protocols as well as technical background information and references to publications can almost always be found in the dedicated chapters which we also refer to in the “Anatomic region” column.

15.2

Protocol Suggestions

All protocol suggestions are summarized in Table 15.1.

The most frequently found motivation to apply parallel imaging in these examples is to increase the spatial resolution of the acquired data without prolonga- tion of scan time. Other protocols in this list aim at increased temporal resolution in dynamic MRI, at improved image quality especially in single-shot applications, or at reduced scan times combined with decreased susceptibility to artefacts in moving struc- tures.

The suggested parallel-imaging parameters of all considered protocols depend on the image geometry (i.e., slice orientation, phase-encoding direction, and 2D vs 3D acquisition, etc.) in combination with the available hardware, in particular the coil systems.

Implementations on the various MR systems from different vendors may therefore vary. In several of the following examples we distinguish protocols for

“standard” coil systems and dedicated “high-end”

coil systems with improved parallel-imaging capa- bilities. The standard coil system typically consists of an 8-element head coil and of body surface-coil systems with three independent receiver channels in leftright direction (ideally positioned both ante- rior and posterior of the patient). If only coil systems with fewer elements are available, then the maximum acceleration factor, R, must be decreased, e.g., to R=2 in leftright direction. Newer coil systems with more independent elements (cf. Chaps. 14 and 44) often allow higher linear acceleration factors (e.g., R=4).

The listed protocol parameters, in particular the spatial resolution, are based on typical clinical imaging requirements. The suggested parallel-imaging parameters, such as the acceleration factor, R, are then selected as optimal compromise balancing short acquisition times, on the one hand, and sufﬁ cient image quality, particularly sufﬁ cient signal-to-noise ratio, on the other hand.

(2)

170 S. O. Schoenberg and O. Dietrich

Table 15.1. Parallel-imaging protocol parameters

Anatomic regions and spectrum of disease

Protocol requirements Requirements for parallel imaging

Brain:

Stroke

(Chaps. 18, 33, 34)

Reduction of distortion in diffusion- weighted and perfusion-weighted EPI images

Use routinely multi-element (t8) head coils, Rt2

Reduction of blurring in T2W TSE seq. Improved results with t32 coil elements, Rt4 Tumors

(Chap. 18)

Isotropic high spatial resolution 3D GRE and TSE seq. with 2D acceleration, Rt2×2; 3 T preferable

Congenital brain disease

(Chap. 18)

Isotropic high spatial resolution 3D GRE and TSE seq. with 2D acceleration, Rt2×2; 3 T preferable

Arteriovenous malformations (Chaps. 18, 47)

Time-resolved 3D CE MRA 2D acceleration, Rt2×2

Skull base and larynx:

Cranial nerves (Chap. 19)

High in-plane spatial resolution Combine head coil with dedicated surface coils; use 1024 matrix with in-plane resolution 0.5×0.5 mm²;

keep scan times <5 min Short scan times

Inner ear (Chap. 19)

Isotropic high spatial resolution

0.7×0.7×0.7 mm³

T2W 3D TSE and GRE seq. with SENSE, R=2

Laryngeal tumors (Chap. 19)

Reduction of motion artefacts Multiple averaging (NSAt2), R=2

Lungs and heart:

Inﬁ ltrates (Chap. 20)

Reduction of blurring and signal decay SSTSE images (e.g., HASTE) with R=2; use external reference (ACS) scan for short echo times

Pulmonary arterial hypertension (Chaps. 37, 43)

Combine high-resolution and time- resolved 3D CE MRA

High acceleration factors Rt3 for high temporal and spatial resolution; 2D acceleration possible for high- resolution data sets with large anteriorposterior coverage; GRAPPA-based algorithms preferable

Congenital heart disease (Chaps. 28, 31)

Comprehensive assessment of cardiac and pulmonary vascular anatomy

Dedicated multi-element coils for small infants not yet available; multi-breath-hold multi-slice (3 slices per breath-hold) cardiac cine SSFP imaging with R=2 Time-resolved MRA with scan times

8 s/frame and spatial resolution

1×1×1 mm³

Minimize FOV in leftright (phase-encoding) direction;

thus, GRAPPA preferable due to aliasing, R=2

Cardiac function (Chap. 35)

Multi-slice cine MRI with high temporal resolution (50 ms)

Single-breath-hold multi-slice cardiac cine SSFP imag- ing with high acceleration factors Rt4 and TSENSE Ischemic heart disease

(Chap. 36)

Increase anatomic coverage (t5 short- axis slices) and spatial resolution (t192×192 matrix) for perfusion scans

Good results for saturation-recovery GRE (turbo- FLASH) with GRAPPA and R=2

Reduce concentric rim-like susceptibility artefact

Segmented EPI techniques with TSENSE under clinical investigation

Liver:

Hepatocellular carcinoma (Chap. 21)

Multi-breath-hold T2*W GRE seq. (SPIO- based CM)

Use GRAPPA (axial scans R=2, coronal Rt3) for large patients and small FOVs <35 cm

Free-breathing T2W TSE seq. R=2, scan time 4 min with navigator gating Dynamic volume-interpolated (e.g.,

VIBE) 3D GRE seq. with thin slices

3 mm and scan times 15 s for arterial, portalvenous, and equilibrium phase

R=2 for axial scans, Rt3 for coronal scans with isotro- pic spatial resolution 2×2×2 mm³, 3 T preferable

Metastasis (Chap. 21)

Isotropic spatial resolution <2×2×2 mm³ for 3D VIBE with positive liver-speciﬁ c contrast agents (e.g., Gd-EOB-DTPA)

3 T preferable, coronal acquisition with Rt3

Use diffusion-weighted black-blood EPI sequences for improved lesion detection

R=2, requires large FOV (50 cm) with homogeneous

magnetic ﬁ eld 컄컄

(3)

Design of Parallel-Imaging Protocols 171

MRCP (Chap. 22)

Combine RARE and HASTE seq. to account for uncooperative patients and detection of tumor morphology

R=2, higher acceleration factors problematic due to SNR constraints at 1.5 T

Use 3D VFL-TSE for detection of small stones and evaluation of high-order branches (PSC) with isotropic spatial resolution <1×1×1 mm³

Rt3 feasible, 3 T preferable

Use diffusion-weighted black-blood EPI sequences for assessment of cholestasis

R=2, requires large FOV (50 cm) with homogeneous magnetic ﬁ eld

Pancreas (Chap. 22)

Coronal acquisitions helpful for FS T1W and T2W HASTE

R=3 routinely feasible

High diagnostic value of late arterial- phase in dynamic T1W 3D VIBE for detection of pancreatic tumors

R=2 for axial scans, Rt3 for coronal scans with isotro- pic spatial resolution 2×2×2 mm³

3D VFL-TSE preferable for evaluation of pancreatic duct

Rt3 feasible

Musculoskeletal system:

Knee (Chap. 24)

Reduce scan time <3 min per seq. particularly in patients with pain

R=2 for standard coil systems at 1.5 T; dedicated multi- element coils and 3 T preferable; Rt3 possible for 2D sequences, 2D acceleration with R=2×2 feasible for 3D sequences

Shoulder (Chap. 24)

Reduce scan time <3 min per sequence particularly in patients with pain

Avoid R>2, prone to severe aliasing artefacts in coronal scans with standard coils; dedicated multi-element coils and 3 T preferable

Angiography:

Intracranial (Chaps. 18, 26)

Time-of-ﬂ ight MRA: reduce scan time <5 min, isotropic resolution

0.7×0.7×0.7 mm³

R=2 or R=3, preferred phase-encoding direction is leftright since small volume coverage in slice direction limits the use parallel imaging in the cranio-caudal direction; 3 T preferable

3D phase-contrast MRA for detection of sinus venous thrombosis in pregnant women: scan time <8 min, spatial resolution 1.5×1.5×1.5 mm³

2D acceleration in coronal and sagittal direction, Rt2×3; dedicated multi-elements with t8 elements required

Dynamic 3D CE MRA for arteriovenous malformations, sinus venous thrombosis: temporal resolution 500 ms, spatial resolution 2×2×2 mm³

Use 2D acceleration with Rt3×2, combine with view sharing, keyhole techniques; dedicated multi-element coils with t8 elements required

Carotids (Chap. 27)

High-resolution 3D CE MRA for exact grading of carotid artery stenosis:

0.9×0.9×0.9 mm³, scan time 20 s

Standard coil systems:

R=2 at 1.5 T; R=3 at 3 T

Dedicated neurovascular coils: R=4 Dynamic 3D CE MRA for delayed inﬂ ow

with signiﬁ cant stenosis, ﬂ ow reversal with subclavian steal syndrome: temporal resolution 2 s, spatial resolution 2×2×3 mm³

Standard coil systems:

R=2 at 1.5 T; R=3 for 3 T

Renal arteries (Chap. 29, 39)

High-resolution 3D CE MRA for exact grading of renal artery stenosis on cross- sectional reformats: 0.9×0.9×0.9 mm³, scan time 20 s

R=3 in leftright direction, 2D acceleration difﬁ cult due to thin coronal slabs, limitations in coil design, and coil-sensitivity proﬁ les

Dynamic 3D CE MRA to detect increased CM transit time with signiﬁ cant stenosis, assessment of true and false lumen in aortic dissection: temporal resolution

2 s, spatial resolution 2×2×3 mm³

R=3 in leftright direction, combine with view-sharing techniques

컄컄

(4)

172 S. O. Schoenberg and O. Dietrich

Peripheral arteries (Chaps. 30, 32)

Pelvic arteries: scan time <20 s, spatial resolution 1.5×1.5×2 mm³

R=3 in leftright direction

Thighs: scan time <15 s, spatial resolution 1.5×1.5×2 mm³

R=3 in leftright direction

Calves: scan time 30 s (ellipticalcentric acquisition), spatial resolution

1.0×1.0×1.5 mm³

R=3 in leftright direction, R=4 possible with dedi- cated peripheral vascular coils

Hybrid MRA for patients with rest pain or critical limb ischemia: time-resolved MRA (temporal resolution 4 s, spatial resolution 1.5×1.5×1.5 mm³) followed by standard three-station MRA

R=3 in leftright direction, R=4 possible with dedi- cated peripheral vascular coils, 3 T, and/or CM with higher relaxivity preferable

3D CE MRA 3D contrast-enhanced magnetic resonance angiography, ACS auto-calibration signal,

CM contrast media, EPI echo-planar imaging, FOV ﬁ eld of view, FS fat saturation, GRE gradient echo,

HASTE half-Fourier-acquisition single-shot turbo spin echo, MRCP magnetic resonance cholangio-pancreaticography, NSA no. of signals averaged,

PSC primary sclerosing cholangitis, R parallel-imaging acceleration factor,

RARE rapid acquisition with relaxation enhancement, Seq. sequence,

SPIO superparamagnetic iron oxide, SNR signal-to-noise ratio,

SSFP steady-state free precession, SSTSE single-shot turbo spin echo, T Tesla,

TSE turbo spin echo (fast spin echo),

VFL-TSE turbo spin echo with variable ﬂ ip angle, VIBE volume-interpolated breath-hold examination, W weighted.