MRI of the wrist: normal anatomy
The radiocarpal joint, more commonly known as the wrist, is the articulation between the distal forearm and the hand. It is formed by the apposition of the radius and three proximal carpal bones: scaphoid, lunate and triquetrum. The radiocarpal joint is reinforced by several ligaments and provides the passage for many soft tissues and neurovascular structures on their way towards the hand. Therefore, this compact region contains many small and detailed anatomical structures that can be quite challenging to distinguish radiologically.
The imaging method that best deals with such complexity is the magnetic resonance imaging (MRI). This technique uses magnetic fields and radio waves to distinguish between the nuclear magnetic properties of various tissues. As a result, MRI is safe (no ionizing radiation), has the best soft tissue contrast resolution and image quality is not degraded by the presence of bone or air. These make it a perfect investigational tool for radiocarpal joint anatomy and pathology.
|MRI basis||Creation of 2D and 3D images by exploiting the proton density (hydrogen ions) of various tissues|
|T1 weighted images||High signal (hyperintensity): fat, contrast (gadolinium), bone marrow
Low signal (hypointensity): fluid, cartilage, cortical bone
Intermediate signal: muscles, ligaments, blood vessels, nerves
|Contrast improving techniques||Fat saturation, contrast, proton density MRI|
|Axial views||Proximally: distal radioulnar joint level
Distally: palmar radiocarpal ligament level
This article will describe the radiological anatomy observed on a wrist MRI.
MRI takes advantage of the proton (hydrogen ion) density of various tissues to create images with a high resolution and contrast. The density is proportional to the signal magnitude.
Basically, anatomical structures with more protons appear brighter and lighter (hyperintense), while those with fewer protons appear darker (hypointense). Structures with an average amount of protons have an intermediate signal intensity and appear grey.
Exposure of any anatomical structure to magnetic fields and radio waves in an MRI machine excites protons. The excited protons release their energy and return back to their initial energy levels after a while, in a very tissue specific process called relaxation. Relaxation happens in two steps (T1, T2), which can produce T1 and T2 weighted images according to specific tissue excitation parameters set by the MRI machine operator. Broadly speaking, T1 images are superior to visualize the normal anatomy of structures, while T2 images are better at highlighting pathological changes. Our aim is to understand the normal radiological anatomy of the radiocarpal joint, therefore the focus will be on T1 weighted images.
T1 weighted MRI images have several characteristics:
- Fluid and cartilage have low signal, hence they are hypointense. This gives physiological and pathological fluid collections, hyaline cartilage and tendons (fibrocartilage) a black aspect. Highlighting and differentiating liquid based content requires T2 weighted images, which makes it hyperintense.
- Fat has high signal (hyperintense), hence it appears white. This easily reveals both physiological structures (articular fat pads, subcutaneous tissue) and pathological changes such as fat droplet accumulation in the synovial fluid resulting from joint trauma.
- Paramagnetic substances like MRI contrast agents also appear hyperintense and very bright. The most commonly used agent is gadolinium. Contrast agents are especially exploited in T1 MRI images to highlight and improve tissue differentiation. Other common alternatives include fat saturation (suppression or attenuation) and proton density (PD) MRI technique.
- Bones are both hyper and hypointense on T1 weighted MRI images. The fatty bone marrow appears bright white and the surrounding cortical bone looks black.
- Muscles, ligaments and neurovascular structures highlight in grey due to their intermediate signal intensity. As a result, it can be quite difficult to distinguish between them. The only way to pinpoint them precisely is to know their locations and overall anatomy of the radiocarpal joint. MRI contrast agents and fat saturation can also help.
For a proper radiological interpretation, wrist MRI images must be obtained in all three planes; coronal, axial and sagittal. Axial views are especially good to visualize tendons, blood vessels, nerves and the two passageways of the radiocarpal joint (carpal tunnel, ulnar canal). The bones and ligaments are also visible in axial views, but they are best analyzed in coronal views.
Sagittal views best highlight the alignment of the carpal bones, so they have a limited advantage compared to other views when understanding the normal radiological anatomy of the radiocarpal joint. Therefore, they are mostly optional.
Proximal joint structures
The radiocarpal joint represents the articulation between the radius and three proximal carpal bones: scaphoid, lunate and triquetrum. Let’s begin by understanding the distal end of the radius, which represents the proximal limit of the radiocarpal joint. Here’s how an axial MRI (T1 weighted) of this region looks.
When faced with such an image, the first step is to get orientated. The acquisition of a MRI image can take up to 20 minutes, so patient comfort is of utmost importance. Regardless of the patient’s position, the standard position of the hand and radiocarpal joint during image acquisition is always in pronation. Imagine you are taking a cross-section of the pronated wrist and looking perpendicular to it in the distal direction of the upper extremity. In the final MRI image, the radial aspect of the wrist will be on the right hand side of the image. The ulnar aspect will be located on the left. In turn, the dorsal aspect will face superiorly and the palmar aspect inferiorly. The larger distal radius occupying the right hand side of the image can help with orientation. In addition, the irregularly shaped dorsal radial tubercle points superiorly on the dorsal aspect.
Before diving straight into MRI interpretations, ease your learning by taking a look how a cadaveric cross section through the radiocarpal joint looks like.
Now that you’ve got your bearings, let’s start identifying the bones making up the proximal limit of the radiocarpal joint. The most obvious one is the hyperintense articular surface of the radius located on the right side of the image. It is directly involved in forming the radiocarpal joint. Only the tip of the ulnar styloid process is visible on the left side because the rest is covered by the hypointense articular disc of the distal radioulnar joint. This joint connects the radius and ulna via the ulnar notch of radius. You can easily locate it as a hypointense structure located between the two bones. The distal radioulnar joint does not take part in forming the radiocarpal joint.
Tendons and muscles
No ligaments are visible in this section, so let’s radiate outwards and look at the numerous tendons surrounding the two bones. There are six extensor tendon compartments located superiorly, along the dorsal aspect of the radiocarpal joint. They appear as hypointense circles or ovals following the outlines of the radius and ulna, so it’s easy to spot them. Starting on the radial aspect, you can see the first two compartments. Each one contains two tendons surrounded by their respective grey (intermediate signal) tendinous sheath, so it’s easy to remember them using this association (‘first two compartments-two muscles’). The first compartment contains the abductor pollicis longus and extensor pollicis brevis tendons, while the second compartment contains the extensor carpi radialis longus and brevis tendons. The distinction between the two compartments is provided by an oblique plane passing through the cephalic vein.
Continuing along the dorsal aspect of the radius you can see the evident dorsal radial tubercle. This is an important landmark that separates the second and third extensor tendon compartments. The third compartment contains only the extensor pollicis longus tendon and its surrounding tendinous sheath. Continuing towards the ulnar aspect, the fourth compartment contains the extensor digitorum and indicis tendons, both enveloped within the same tendinous sheath. A tip to easily locate the third and fourth compartments is that they stop approximately at the level of the distal radioulnar joint, so they only overlie the radius. The last two compartments each contain one tendon and follow the outline of the ulna and its articular disc, so you can pinpoint them easily. The fifth compartment contains the extensor digiti minimi tendon while the sixth compartment contains the extensor carpi ulnaris tendon.
To better understand the anatomy of the forearm extensors, take a look at the following study unit:
Now that we’ve finished with the extensor tendons, let’s move on to the palmar aspect and see the flexor tendons. Only two are visible on the radial aspect as hypointense structures; the deeper flexor pollicis longus tendon and the overlying flexor carpi radialis tendon. Each one has its respective grey labelled tendinous sheath. Moving medially, you can see the most superficial tendon, that of the palmaris longus muscle. If you forcefully oppose your thumb and little finger, you can see the tendon popping subcutaneously on the palmar aspect of the wrist. Therefore, it is easy to remember it as the most superficial one.
Continuing towards the ulnar aspect, you can see two muscles rather than tendons; the flexor digitorum profundus and superficialis muscles. As their names imply, the former is located deeper (profound) compared to the latter (superficial). They appear as a congregation of hypointense ovals because they begin to divide into their numerous tendons. The carpal tunnel is not yet visible at this particular axial level. These flexor muscles are enveloped by the common flexor tendon sheath of hand which is represented by the grey, thin interface outlining the deeper aspect of the subcutaneous tissue. To the left of the sheath you can see the flexor carpi ulnaris muscle and its tendon. The muscle has an intermediate signal (grey).
Master the anatomy of forearm flexors using the videos, quizzes, illustrations and articles in the following study unit:
The bones and soft tissues are finished, so let’s examine the next surrounding layer containing the neurovasculature. Seven major vessels and nerves are present in this axial view at the level of the distal radioulnar joint. Moving from the radial to the ulnar aspect, these are the cephalic vein, radial artery, median nerve, ulnar artery, ulnar nerve, basilic vein and dorsal venous network of the hand.
The veins are easily identified because they are superficial. Therefore, they appear as grey structures surrounded by hyperintense (fatty) subcutaneous tissue. The cephalic vein is found on the radial side and the basilic vein on the ulnar side. If you follow the bright subcutaneous tissue inferiorly, you can meet the radial artery on the radial side and the ulnar artery and nerve on the ulnar side. They also appear grey and are located superficially. You can easily palpate the arteries underneath the skin, so it’s easy to remember them. The median nerve is the most central neurovascular structure, being located close to the midline of the MRI axial view. It travels close to the flexor digitorum profundus and superficialis muscles, preparing to enter the carpal tunnel.
Distal joint structures
So far, you’ve seen all the structures visible at the proximal limit of the radiocarpal joint. Let’s take another axial slice a few millimeters distally and see what happens at the distal limit of the joint. This is represented by the articular surfaces of three proximal carpal bones; scaphoid, lunate and triquetrum. Here’s how an axial MRI (T1 weighted) of this region looks.
The orientation of the image remains identical to the previous axial section. However, only the radial styloid process is visible at this level on the right side. The ulna is no longer visible and has been replaced by other bones which will be described next. A new anatomical structure is now obvious, the carpal tunnel. It consists of many congregated hypointense ovals representing all the structures passing through it. The carpal tunnel can be used as the new inferior landmark instead of the previous dorsal radial tubercle, which is no longer visible superiorly.
We’ll follow a similar approach to the previous axial MRI to describe the visible structures. We’ll start with the skeletal framework i.e. the hyperintense bones. Quite a lot has changed at this level. Only the radial styloid process is visible on the extreme right hand side of the image and three carpal bones have become visible. Moving from right to left, you can see the scaphoid, lunate and triquetrum. If you know the anatomy of the proximal row of carpal bones, the order and location are quite obvious. The shape of the bones can guide you as well. The scaphoid resembles a boat, the lunate has a crescent (moon) shape and the triquetrum resembles a pyramid.
Find out more about the anatomy of the carpal bones using the following study unit:
In contrast to the previous MRI image, there are several ligaments apparent in this axial view. You can see two thick, grey structures (intermediate intensity) spanning the superior and inferior margins of the radius, scaphoid and lunate bones. These are two extrinsic ligaments of the radiocarpal joint that connect the radius to each carpal bone; the dorsal and palmar radiocarpal ligaments. As their names imply, the dorsal radiocarpal ligament is located superiorly on the dorsal aspect. Its palmar counterpart is found inferiorly on the palmar aspect. Between the scaphoid and lunate bones you can see a thick, grey, interconnecting band. This is an intrinsic ligament of the radiocarpal joint which interconnects adjacent carpal bones. It is called the scapholunate interosseous ligament.
Continuing towards the left side (ulnar aspect), you can see two more grey thickenings overlying the lunate and triquetrum. These represent two extrinsic ligaments that connect the ulna to each carpal bone; the dorsal and palmar ulnocarpal ligaments. The former is located superiorly while the latter is inferior. It’s important to note that the radiocarpal and ulnocarpal ligaments are composed of several smaller ligaments, each named according to the carpal bone it connects to. However, they cannot be distinguished on this axial image. Last but not least, you can see the ulnar collateral ligament of the wrist joint on the far, extreme left. It connects the ulna to the triquetrum, so you can locate it very easily.
Tendons and carpal tunnel
If you compare the MRI images of the proximal and distal limits of the radiocarpal joint, you will see two major differences; the latter contains no actual muscles but showcases the important carpal tunnel and ulnar canal. The arrangement of the tendons at this axial level stays almost identical to the previous one. The only exception is the extensor pollicis longus tendon which is now located on the radial aspect of the extensor carpi radialis brevis tendon. This is because the tendon of extensor pollicis longus has a more pronounced trajectory towards the thumb compared to its neighbour.
The carpal tunnel is a passageway between the distal forearm and hand. It consists of a base, two walls and a roof. The base and walls are formed by the distal row of carpal bones while the roof is represented by the flexor retinaculum of the wrist. The carpal tunnel contains the median nerve and nine tendons; one of flexor pollicis longus, four of flexor digitorum profundus and four of flexor digitorum superficialis. The carpal tunnel is located on the palmar aspect of the wrist, in the midline. All ten structures passing through it are visible at this MRI level. They appear as aggregated hypointense circles surrounded by grey soft tissue. The tendons are layered identically to their muscular counterparts observed in the first axial MRI image.
Last but not least, let’s see how the neurovasculature changes distally along the radiocarpal joint. Luckily for you, it stays almost the same with two exceptions. The ulnar artery and nerve travel within a hyperintense ulnar canal (Guyon’s canal). This passageway is located superficially to the common flexor tendon sheath of hand, sharing a border with the latter. The last remaining difference is the appearance of the superficial palmar branch of the radial artery. This is also located within the hyperintense subcutaneous tissue on the radial aspect of the radiocarpal joint (right side of image), but more superficial than its parent blood vessel.