Bilateral hip imaging

• Body phased array/multi-coil array/general-purpose flexible coil/body coil
  
• Immobilization pads and straps
  
• 20° wedge sponges
  
• Earplugs/headphones

Single hip imaging

• Small/large flexible coil/multi-coil array/pelvis phased array/small Helmholtz pair
  
• Immobilization pads and straps
  
• 20° wedge sponges
  
• Earplugs/headphones

Axial SE/FSE/incoherent (spoiled) GRE T1

Acts as a localizer if three-plane localization is unavailable, or as a diagnostic sequence. Thick slices/gaps are prescribed on either side of the horizontal alignment light. Both hips are included to show the location and alignment of the hips.

Axial I 25 mm to S 25 mm

Coronal FSE T2 +/– tissue suppression/STIR

Thin slices/gaps are prescribed from the posterior to the anterior margins of the musculature of the hip (from the iliacus to the anterior portion of gluteus maximus). Slices may be angled to compensate for positional rotation of the pelvis. The images should display the lateral edges of the muscles surrounding the hips (gluteus medius), and extend from the junction of the ilium and the superior acetabulum to below the lesser trochanter.

Coronal SE/FSE T1 (Figure 14.2)

Slice prescription as for coronal T2.

Axial SE/FSE T1

Thin slices/gap are prescribed from above the articular portion of the acetabulum to the superior edge of the lesser trochanter, and aligned with the superior surface of both femoral heads to correct for positional errors (Figure 14.3).

Sagittal FSE T2/coherent GRE T2* +/– tissue suppression

Thin slices/gaps are prescribed from the lateral aspect of the greater trochanter through the articular portions of the acetabulum (Figure 14.4). These images particularly demonstrate flattening of the femoral head associated with AVN. Generally, FSE is the sequence of choice, but GRE sequences provide excellent visualization of cartilage.

Axial SE/FSE/incoherent (spoiled) GRE T1

Acts as a localizer if three-plane localization is unavailable, or as a diagnostic sequence. As for bilateral examination. Use the body coil and include both hips.

Coronal SE/FSE T1

Thin slices/gap are prescribed from the posterior to the anterior margins of the femoral head and aligned parallel to the femoral neck. The area from the proximal margin of the femoral shaft (below the lesser trochanter) to the greater sciatic notch is included in the image.

Coronal coherent GRE T2*/FSE T2 +/– tissue suppression

Slice prescription as for the coronal T1.

T2* images are particularly good for identifying the labrum and loose bodies in the joint. A FSE T2 may be preferred to provide higher resolution.

Axial FSE T2 +/− tissue suppression

Thin slices/gaps are prescribed from a coronal image to include the articular components of the hip joint. Angle the slices so that they are parallel to the femoral neck.

Axial SE/FSE/incoherent (spoiled) GRE T1

Slice prescription as for axial T2.

Coronal FSE T2 +/– tissue suppression (both hips)

This may be used as an additional sequence to the unilateral examination, especially to rule out AVN of the asymptomatic hip. Use the body coil to avoid repositioning of the patient. Medium slice/gap is adequate as the contrast sensitivity of the sequence should identify pathology. Position slices to include the bony components of the hip joint and some surrounding musculature.

Technical issues

The TE influences the signal of the muscle in musculoskeletal imaging. A very long TE produces T2-weighted images in which muscle is hypo-intense. The SNR is therefore reduced but fluid detection is improved. Tissue suppression can also be used to enhance the signal from fluid even further; however, larger voxels may be required to compensate for the inherent drop in SNR. By choosing a moderate TE, muscle still retains signal (a grey-level intensity) and the images are PD weighted. The SNR is however higher and the spatial resolution can be better than a T2-weighted image. This kind of contrast is used to detect fluid and retain an anatomical image. Tissue suppression techniques are recommended with this kind of weighting because signal from fluid is reduced. Cartilage lesions can be better detected when TE is high (at least 30–40 ms) because the signal from normal cartilage decreases.

Spatial resolution is a critical parameter in joint imaging and depends on the clinical indications. For example, when examining small labral tears, resolution is paramount and utilization of thin slices/gap and a fine high matrix is necessary. However, when the lesion is large and CNR is high, resolution can be sacrificed to achieve better SNR or shorter scan times. Volume acquisitions are sometimes valuable as very thin slices with no gap are used, and joint structures may be visualized in any plane. As volume acquisitions are usually performed in order to demonstrate anatomy, an incoherent (spoiled) GRE sequence that produces predominantly T1 weighting is required.

Bilateral and unilateral examinations have been described in this section. Bilateral imaging with a large phased array coil has a number of advantages. It can provide acceptable resolution for visualizing joint structures when used with a combination of a small FOV and oversampling, but it can also be utilized with a large FOV to provide medium-resolution images of a large area of anatomy. For unilateral examinations, the requirement for fine spatial resolution is paramount. The hip joint structures are comparable in size to the shoulder, requiring high spatial resolution (pixel size less than 0.5 mm). Unfortunately, the relatively bulky musculature and the external morphology of the hip demands physically larger coils. As a result, it may be necessary to use higher NEX/NSA and longer scan times to achieve acceptable SNR.

Flexible selection of a rectangular/asymmetric FOV has greatly helped to tailor hip sequences in the axial and sagittal planes. In addition, flexible application of oversampling allows a small FOV to be used in the coronal and axial planes in conjunction with larger coils, as well as providing finer incremental changes in SNR.

Several sequences are employed in hip examinations. SE sequences usually provide better contrast and resolution in T1-weighted images than FSE. However, the development of high-performance gradients reduces blurring associated with long ETLs. Tissue suppression is an important imaging option in the musculoskeletal system, as the suppression of normal fatty marrow often enhances the visualization of bony pathology. Tissue suppression techniques are usually preferred, but STIR sequences provide very uniform fat suppression (and the degree of the fat suppression can be controlled by altering the inversion time), particularly on older systems and in large FOVs. There is a developing trend to use moderately short TE acquisitions (35–60 ms) for fat-suppressed PD-weighted FSE sequences which yield higher SNR and improve CNR between cartilage and muscle. The efficacy of this approach has not yet been established. Additional shimming may be required before tissue suppression sequences.

While reduced bandwidth sequences offer SNR increases without serious time penalties, the bandwidth should be chosen to limit the chemical shift to one or two pixels for most non-fat-suppressed sequences and particularly when producing high-resolution images. When a tissue suppression technique is used, chemical shift artefact is less problematic and a lower bandwidth can be used. However, because a reduced bandwidth improves SNR, flow sensitivity and therefore flow artefacts are more evident.

Artefact problems

The main source of artefact is from flow within the femoral and iliac vessels. Spatial pre-saturation pulses placed S and I to the FOV reduce this to a large degree. When imaging a single hip for labral tears, bowel motion may be troublesome. Placing a spatial pre-saturation pulse medial to the hip effectively reduces this. GMN further minimizes flow artefact, but as it increases the signal in vessels and the minimum TE, it is not usually beneficial in T1-weighted sequences. However, GMN can effectively increase contrast (depending on the direction GMN is applied) in synovial fluid in T2- and T2*-weighted images. For FSE imaging, using a high receive bandwidth and/or shorter ETL reduces flow artefact and motion reduction techniques, such as PROPELLER, are also beneficial. For bilateral coronal acquisitions, fat suppression may not be homogenous due to the use of a large FOV. Therefore, additional shimming may be required before tissue suppression sequences. Alternatively, STIR sequences may be used. Techniques that use the 3-point Dixon method whereby four images are acquired in one sequence (water, fat, in and out of phase), provide robust fat-suppressed images (fat–water separation).

Hip prosthesis or pins produce significant magnetic susceptibility artefact. This can sometimes, but not always, ruin an image. Do not use a GRE sequence as gradients do not compensate for magnetic field inhomogeneities, thereby increasing magnetic susceptibility artefact. To minimize the artefact, select SE or FSE sequences in conjunction with a broad receive bandwidth, a lower TE and a coarser matrix. The artefact can sometimes be shifted away from the ROI by swapping the phase and frequency encoding directions, but this, in turn, may lead to aliasing. However, aliasing can be avoided by oversampling and careful location of the hands away from the side of the body and on to the chest.

Chemical suppression techniques cannot be used when metallic implants are present. Other types of suppression or STIR sequences are recommended. Recently, protocols have been developed for imaging soft tissue in the vicinity of metallic prosthesis. These techniques significantly reduce magnetic susceptibility artefact, and therefore, inflammation can be detected close to the prosthesis.