David W. Stoller - Stollers Orthopaedics and Sports Medicine: The Hip

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Contents

Guide

Practical Guide to Hip MR Imaging

Key Factors to Clinical Diagnostics

Motion Insensitivity

• Sequences and parameters that eliminate or reduce patient motion and flow-related artifacts

Uniform Fat Suppression

• Techniques insensitive to B0 and B1 inhomogeneity, preferably flexible on the amount of saturation

Contrast

• Consisting of T1, proton density, and T2* weighting

High Resolution

• Required for visualizing submillimeter structures

Isotropic Resolution

• 3D imaging techniques for postacquisition reformatting

Off-Isocenter Image Quality

• Consistent image quality for imaging away from isocenter

Reduced Distortion around Metal

• Availability of advanced techniques for imaging around total hip arthroplasty as well as fixation screws

Efficient Surface Coils

• Coils that support parallel imaging, have sufficient SNR with depth, provide stability and comfort to the patient, and are easy to set up

p. 3 p. 4

Positioning

Radiofrequency (RF) Coils

• The most common choice for hip imaging coil selection is to use flexible receive-only coils that are able to provide bilateral and unilateral anatomy coverage. Several different flexible multi-channel coils labeled as Body Array, Torso Array, or Cardiac Array are commercially available, though compatibility will depend on the specific MR system vendor, model, and field strength.
• The number of receive channels varies between 8 and 32, with a trend to higher channel counts. Higher receive channel counts will provide higher SNR closer to the surface and enable higher acceleration factors with parallel imaging. However, the drawback with higher channels is the need for intensity correction due to intense signal closer to the surface coils.
• Multi-purpose flexible receive-only multi-channel coils are another possibility in order to acquire unilateral small field of view anatomy coverage. Different coil sizes are available to accommodate patient sizes accordingly.

Immobilization

• Patients will almost always be positioned supine. The feet first orientation provides a most comfortable patient experience during the examination. Often the feet are taped or wrapped together to reduce leg movement. The pubic symphysis is used as landmark.

Basic Pulse Sequences

FSE

• The most commonly used sequence for musculoskeletal imaging is Fast Spin Echo or Turbo Spin Echo. Most vendors have versions for 2D and 3D acquisitions, but multi-planar 2D is primarily used, though there is an increasing trend towards 3D. FSE provides spin echo type contrast with faster scan times by acquiring multiple echoes in each shot. Proton density, T1, or T2 image contrast can be obtained, determined by the TR, TE, and ETL parameters. FSE protocols typically focus on controlling echo spacing (time between echoes) to avoid blurring and flow artifacts. The minimum TE is an indicator of echo spacing and shorter values are desired to provide good image quality.

  Parameter selections for decreasing echo spacing:
  Field of View (FOV)	Matrix	Slice Thickness	Bandwidth

p. 4 p. 5

FRFSE

• The Fast Recovery Fast Spin Echo sequence produces images with more T2 contribution with the same TR as FSE. It is a modified FSE sequence using additional RF pulses after the acquisition window to hasten recovery of longitudinal magnetization from signal with long T2.
• FRFSE is commonly used to reduce scan time by using a shorter TR, while maintaining T2 contrast.

  Recommended TR, TE, and ETL values for FSE and FRFSE:
  Pulse Sequence	TR	TE	ETL
  FRFSE PD	20003000ms	3060ms	610
  FRFSE T2	20003000ms	85ms	1214
  FSE T1	500900	Min.	24

GRE

• Gradient Echo sequences are used to provide T1 and T2* contrast, which can complement spin echo based acquisitions. Most practical uses in the knee are with 3D sequences, such as spoiled gradient echo for assessing physis.

  Sequence	Contrast	Flip Angle
  2D GRE/SPGR	T1 T2*	4060 2030
  3D GRE/SPGR	T1 T2*	2545 58

p. 5 p. 6

• Generally, the key parameter for managing contrast with GRE sequences is flip angle:

  Flip Angle Flip Angle

  T1 Contrast T1 Contrast

  SNR SNR

Inversion Recovery

• Inversion Recovery sequences are frequently used for uniform suppression of fat signal by acquiring data at the null point of the T1 recovery, with the key parameter being the TI (inversion time).

Scan Parameters

Spatial Resolution

• Determined by voxel size, which depends on the FOV, acquisition matrix, and slice thickness.

  Calculating Voxel Size Phase FOV# Phase matrix=Phase Dimension Frequency FOV# Frequency matrix=Frequency Dimension Phase Dimension Frequency Dimension Slice Thickness = Voxel Volume

• High resolution may add valuable information for musculoskeletal imaging.

  Parameter selections to increase resolution (i.e., reduce voxel volume):
  FOV	Matrix	Slice Thickness

Acquisition Time

• Typically, the SNR is proportional to the acquisition time. However, longer scans are more prone to motion artifacts. The primary parameters that impact acquisition times are TR, phase matrix, and the number of excitations (NEX) or averages. For 3D sequences, phase matrix includes both the in-plane and slice direction, which is typically the number of slices.

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