Atypical Scan Angles in Musculoskeletal MRI

Unknown: 

What kind of an image is this?  What is the purpose of this view?  How does one acquire this image?

Answer

This is an obliquely angled (axial/coronal) proton density weighted fat-suppressed image of the ankle.  The purpose is to better display the obliquely angled peroneal tendons as they extend around the distal fibula to better visualize partial tears of the tendons.  The view is obtained by angling the acquisition plane of the scan along a 45-degree angle to the ankle mortise (discussed below).

Introduction

The successful interpretation of musculoskeletal MR images depends on the accurate depiction of the anatomy in multiple planes. Although conventional multiplanar MR allows most pathology to be visualized, certain joints and pathological processes can be better assessed by using oblique planes. Some of these planes require specific, non-conventional positioning of the patient, while others can be obtained with the patient in standard position. The purpose of this article is to introduce the reader to various views of different joints, how to acquire the images, and examples of how these views can be utilized to help visualize and diagnose specific pathological processes.

 

Ankle oblique axial

Indication:

MRI is a useful modality for evaluating persistent lateral ankle pain following inversion injury.  Pathology involving the peroneus brevis and peroneus longus tendons may be a source for lateral ankle pain.¹ Given their oblique course, conventional axial and coronal images may result in volume averaging that can make evaluation for subtle pathology difficult. Oblique images obtained perpendicular to the course of the peroneal tendons allows both tendons to be visualized in full cross-section. This provides excellent resolution of tendon pathology, such as peroneus brevis partial tears.

 

Protocol/Setup:

With the patient lying supine on the MRI table, the foot is placed in neutral position within the appropriate foot coil and secured to limit motion (Figure 3a, b).

 

Using conventional sagittal localizer images, an oblique axial plane is prescribed that is perpendicular to the peroneal tendons as they course between the lateral malleolus and 5th metatarsal base. This allows for the full cross-sectional thickness of the tendons to be viewed (Figures 4-7).

 

Anatomy

The peroneal tendons course posterior to the lateral malleolus along the retromalleolar groove. The peroneus brevis tendon inserts onto the lateral aspect of the fifth metatarsal tuberosity (Figure 6).² The peroneus longus tendon courses inferior to the cuboid bone within the cuboid tunnel and inserts onto the plantar aspect of the first cuneiform and the proximal first metatarsal. The peroneus brevis tendon is generally positioned anteromedial to the peroneus longus tendon at the level of the retromalleolar groove¹. The peroneus brevis’ medial position relative to the longus tendon is demonstrated on the oblique axial images (Figure 7)¹. The peroneal tendons have a round-ovoid morphology when viewed in cross section on oblique axial images (Figure 7).

 

Pathology

The oblique axial images are excellent to evaluate the peroneal tendons. In the provided example of a peroneus brevis tendon split tear, the peroneus brevis tendon has a C-shape morphology that wraps around the upper aspect of the peroneus longus tendon (Figure 8).

 

Elbow FABS

Indication:

The FABS (Flexed elbow, ABducted Shoulder, Supinated wrist/forearm) view is used to visualize the distal biceps tendon in a single plane.³ Conventional axial and coronal images can result in volume averaging due to the tendon’s oblique insertion onto the radius. This can occasionally obscure pathology involving the distal bicep’s radial insertion. By supinating the forearm/wrist, the distal tendon is “unwound” from the radial bicipital tuberosity and its distal insertion can be seen in its full width.³

 

Protocol/Setup:

To perform the FABS view, the patient is placed on the MR table in the prone position. The elbow is then flexed 90 degrees in front of the patient’s head and the forearm is supinated so that the thumb is pointed towards the ceiling (Figure 8). The elbow is placed within an appropriately sized shoulder coil.

Conventional coronal localizing images are obtained. Next, planes that are parallel to the long axis of the humeral diaphysis are prescribed (Figure 10), which allows for the entire length of the distal biceps tendon to be viewed on a single image (Figure 11 and 12).³

 

Anatomy:

In the majority of cases, the distal biceps tendon is composed of two macroscopically distinct tendons with varying degrees of decussating tendon fibers arising from the long head and short head of the biceps muscle. The distal tendon arising from the long head of the biceps muscle inserts proximally onto the radial bicipital tuberosity, while the distal tendon arising from the short head attaches distally onto the bicipital tuberosity.⁴ The normal tendon is low signal on all pulse sequences (Figure 11 and 12).

 

Pathology:

As mentioned above, the FABS position unwinds the distal biceps tendon, allowing a splayed-out view of the distal tendon width.  The following is an example of a subtle partial tear of the biceps tendon (Figure 13).

 

Athletic Pubalgia

Indication:

A spectrum of musculotendinous injuries can lead to symptoms of groin pain that fall under the broad category of athletic pubalgia. These injuries are common in athletes that require rapid change of direction, forceful twisting of the trunk, kicking, and repetitive side-to side steps.⁵ These injuries generally involve the anatomy at or arising from the pubic symphysis, which consists of the complex integration of multiple tendinous and ligamentous insertions. A dedicated MRI plane allows for the improved visualization of these insertions and aids in the accurate diagnosis of the injury.

 

Protocol/Set-up:

The patient is placed supine, on the MR table and a body coil is placed over the pelvis (Figure 14). An oblique axial plane is prescribed parallel to the arcuate line⁵ using conventional sagittal images (Figure 15). The superior margin of the FOV is set at the L4-L5 level and the inferior margin is set along the inferior margin of the gluteal cleft.

 

Anatomy:

The pubic symphysis and the surrounding musculotendinous structures make up a complex joint that helps stabilize the anterior pelvis. The complete anatomical analysis is beyond the scope of this article and attention will be focused on the structures primarily evaluated on the oblique plane. The pubic symphysis is the attachment site for multiple musculotendinous structures, which include the external and internal obliques, transversus abdominis, rectus abdominis, pectineus, gracilis, adductor longus, adductor brevis, and adductor magnus⁵. The most important muscles for providing stability to the anterior pelvis are the rectus abdominis and the adductor longus (Figures 16 and 17).⁵

 

Pathology:

The following is an example of an avulsion of the adductor longus pubic insertion (Figures 18-20).

 

Hip Oblique Axial

Indication:

Hip pain is a common indication that leads to MR imaging.  Acetabular labral pathology is a common cause of hip pain and frequently occurs with bony morphologic changes contributing to femoroacetabular impingement. The spherical morphology of the hip can lead to volume averaging, which can obscure underlying impingement pathology.⁶ Oblique axial images splay out the femoral neck and can nicely demonstrate aspherical morphology which may contribute to CAM-type impingement.

 

Protocol/Setup:

The patient is placed in the supine position and a body coil is used to acquire images of the pelvis. Alternatively, a surface coil can be used to image the hip (Figure 21a,b). The imaging plane is prescribed parallel to the axis of the femoral neck, which results in oblique axial images (Figure 21c).

 

Anatomy:

Figure 22 below demonstrates typical oblique anatomy on an oblique axial fat-suppressed T2-weighted image, with normal spherical morphology at the femoral head/neck junction.

 

Pathology:

Figure 23 below is an example of an oblique axial fat-suppressed T2-weighted image of the hip demonstrating abnormal aspherical morphology of the femoral neck with an abnormal convex “bump” (arrow), which can contribute to cam-type femoroacetabular impingement.

 

Knee Oblique “ACL” view

Indication:

The anterior cruciate ligament of the knee normally extends along an oblique coronal course, from posterosuperior to anteroinferior.  Therefore, oblique imaging along an oblique coronal course will splay out the ligament, and can be useful in evaluating for more subtle or partial tears of the ligament.⁷ Although often described as an “ACL” view, this oblique plane is also useful in evaluating the menisci and collateral ligaments.

 

Protocol/Set up:

The patient is placed in the routine knee supine position on the MRI table typically using a knee coil (Figure 24a,b).

 

The oblique coronal “ACL” view is obtained by prescribing a plane parallel to Blumensaat’s line using a conventional sagittal localizer and should extend to include the distal femur superiorly and the lower PCL tibial attachment inferiorly (Figure 25).⁷ The anterior to posterior extent of the anatomy required to be imaged is depicted in Figure 26; extending through the entire posterior menisci is useful in detecting tears of the posterior roots.

 

Anatomy:

The oblique coronal images allow for visualization of the ACL along its entire length (Figure 27). This is often quite useful visualizing partial tears of the ACL.⁷  Additionally, the oblique angle also provides an additional perspective of the menisci, useful in confirming subtle tears more questionably visualized on conventional planes (Figure 28).

 

Pathology:

Figures 29-31 demonstrate the oblique coronal views depicting injuries involving the ACL, MCL, and menisci, respectively.

 

Pectoralis

Indications:

Rupture of the pectoralis muscle and/or tendon is often suspected clinically. However, assessment of the exact location and extent of the injury can be limited in the acute setting due to the patient’s symptoms limiting a complete physical evaluation of the shoulder and pectoralis muscle. The pectoralis major may be injured at its distal tendon attachment or more proximally at its musculotendinous junction.⁸ MRI provides the anatomic detail required to localize the injury, determine extent, chronicity, and muscle atrophy/fatty infiltration.

 

Protocol/Setup:

The patient is placed in the supine position on the MRI table and a body coil is placed over the upper chest (Figure 32). Using conventional axial images, a plane similar to the sagittal images of the shoulder are used, which are perpendicular to the scapula. The anatomy covered by these coronal images should include the entire pectoralis muscle and axilla (Figure 33a). The sagittal plane is prescribed perpendicular to the coronal plane (Figure 33b).

 

Anatomy:

The pectoralis muscle consists of 3 heads originating from clavicular, sternal, and abdominal origins and has a fan shaped appearance. The muscle is the most superficial muscle along the superior chest well. The tendon length is variable ranging from 5-15 mm and the fibers can insert over a cephalocaudal distance of 4-6 cm⁸. The tendon inserts onto the humerus along the lateral ridge of the bicipital groove. The superior most inserting fibers can be seen at the level of the quadrilateral space curving anterior to the biceps tendon. The inferior extent of the tendon insertion is usually seen just proximal to the deltoid tubercle.⁸

 

Pathology:

 

Shoulder ABER

Indication:

The ABER (Abduction External Rotation) view primarily is used for uncovering subtle tears of the anteroinferior labrum (such as the Perthes lesion), but can also be used to evaluate for partial articular surface rotator cuff tears.⁹ By placing the shoulder in the ABER position, traction is applied to the anteroinferior labrum, which allows joint fluid/intra-articular contrast to seep into labral pathology, which may be more closely approximated and subtle on neutral positioning.

 

Protocol/Setup:

The patient is placed supine on the MRI table and shoulder is both abducted and externally rotated so that the arm can be placed above the patient’s head, as shown in Figure 36. A surface coil is placed in the axilla of the shoulder to be imaged.

 

Using conventional coronal images, a plane that is parallel to the humerus’ diaphysis and perpendicular to mid glenohumeral joint is prescribed (Figure 37). This results in oblique axial images through the glenohumeral joint.

 

Anatomy:

Interpreting ABER images may be initially confusing due to the unconventional position and plane.  Using the PACS, it is helpful to rotate the image so that the axilla faces right or left, and the humeral shaft extends superiorly.  The anteroinferior labrum can be seen adjacent to the taut inferior glenohumeral ligament (anterior band), deep to the axillary fold (Figure 38).

 

Pathology:

As mentioned above, the ABER view is best used to evaluate subtle non-displaced anteroinferior labral injuries such as a Perthes lesion.   In the ABER view (Figure 39), a clear contrast filled tear can be seen involving the anteroinferior labrum, assisted by the traction placed on the tissue. Figure 40 is a conventional axial image of the same shoulder in neutral position without traction. The tear is less conspicuous on this image.

 

Thumb

Indications:

The 2^nd-5^th digits of the hand can be easily imaged in conventional MR planes.¹⁰ The thumb, however, is rotated towards the palm and has an oblique orientation relative to the other digits (Figure 42). Therefore, conventional hand imaging planes are typically inadequate for the thumb as they result in oblique images of the thumb joint anatomy.¹¹ Dedicated images with the planes aligned to the thumb orientation should be performed, particularly if there is concern about the metacarpal-phalangeal joint (MCP) joint collateral ligaments (e.g. ski-pole thumb, gamekeeper’s thumb, Stener lesion).

 

Protocol/Setup:

The patient is typically placed in the prone “superman” position on the MRI table, with the wrist in the isocenter of the bore. The pronated hand is placed into a small high channel coil, such as a dedicated wrist or HD knee coil. Using ample amount of padding helps ensure that the patient is comfortable and limits movement of the hand during the scan (Figure 41).

 

Small field-of-view short axis axial images are first obtained through thumb, focusing on the MCP joint.  The coronal and sagittal images are then obtained by prescribing planes that are perpendicular and parallel to the line that bisects the sesamoid bones and metacarpal head (Figure 42a and 42b).  Adequate coronal imaging planes are key in the evaluation of thumb ligament injuries.

 

 

Anatomy:

Dedicated coronal and sagittal images of the thumb better depict the tendons and ligaments of the thumb (Figures 43 and 44).

 

Pathology:

The following images demonstrate injury to the ulnar collateral ligament at the thumb metacarpal-phalangeal joint (Figures 45-47).

 

Conclusion

One of the great advantages of MRI is the ability to scan in any desired plane. There are multiple anatomical regions and pathological processes within the musculoskeletal system that can be better visualized by using special oblique planes. Understanding how the planes are prescribed and the anatomy depicted on the oblique views can allow for more accurate assessment of these regions.

References