Magnetic Resonance Imaging of the Knee

Imaging Protocol

The proper imaging protocol is essential for a high diagnostic accuracy rate. A sagittal T1-weighted (or proton density) sequence is essential for examining the menisci. Four- or five-millimeter-thick slices with a small (12–14 cm) field of view and at least a 192 matrix are recommended. The knee should be imaged using a dedicated knee coil and externally rotated about 5 to 10 degrees (do not exceed 10 degrees) to put the anterior cruciate ligament (ACL) in the plane of imaging. T2 spin-echo or T2∗ gradient-echo sagittal images are obtained primarily to examine the cruciate ligaments and cartilage. With T2 spin-echo images, meniscal tears may be difficult to see; however, they will be picked up on the proton density images. Thus the menisci and the cruciate ligaments are examined primarily on the sagittal images. Although the menisci and the cruciate ligaments can be seen on the coronal and axial images, it is uncommon for those images to show an abnormality that is not seen on the sagittal images.

Coronal images are obtained to examine the collateral ligaments and to look for meniscocapsular separations. These abnormalities can most often only be seen with T2-weighted images. T1 coronal images are therefore a waste of time, as there is nothing to be seen on these images that cannot be equally well seen on the sagittal images. T2∗ gradient-echo coronals or fast spin-echo (FSE) T2 sequences in the coronal plane are imperative. FSE (also called turbo spin-echo, or TSE) sequences should have fat suppression applied or fluid cannot be distinguished from fat.

A few centers continue to use T1-weighted coronal images without realizing that they give no additional information and in fact can hide significant abnormalities. Why would they do this? T1-weighted coronal images were part of everyone’s protocol when we first began to use MR imaging in the knee. Articles, book chapters, and speakers (including me) would give the standard protocol, not thinking enough about what each sequence was showing us. I remember reading out a knee study in the late 1980s with one of the radiology residents, and I asked him to hedge on the diagnosis of a partial tear of the medial collateral ligament (MCL) and request a repeat exam with T2-weighted coronals to rule out a meniscocapsular separation. This was the second or third time that week that the resident had been so instructed, so he asked, “Why don’t we just routinely do a T2 coronal—we don’t see anything on the T1 coronal and we often have to hedge or repeat the exam with a T2 coronal?” I just said for him to worry about dictating the cases; I and others would take care of setting up the protocols. Well, 2 weeks later, he rotated to another service, and we began doing T2 coronal images as part of our standard knee protocol. Uppity residents!

Axial images were initially used by the technicians as a scout view. They were then found to be useful for viewing the patellofemoral cartilage and for identifying and characterizing fluid collections. As in the coronal images, to afford an opportunity to see any pathology, T2 images must be obtained.

We have found that fat-suppressing the sagittal T1 or proton-density meniscus-sensitive sequence increases the dynamic range of signal in the meniscus and makes meniscal pathology more conspicuous. It gets rid of all the distracting high signal in the marrow, making it easier to visualize the meniscus.

The use of FSE sequences with a short TE (FSE proton density) has been shown in several reports to be useful for meniscal tears, and yet others have reported a decreased sensitivity. What is the truth? The truth is that every published report I can find shows a sensitivity of around 80% for FSE proton density sequences, whereas conventional spin-echo sequences have a sensitivity closer to 95%. It is very controversial, but basically everyone’s results are the same (80% sensitivity); only the conclusions differ. If you’re willing to decrease your sensitivity for meniscal tears from 95% to 80% to save 3 minutes (the only advantage of FSE), then your time is more valuable than your diagnostic accuracy. Get another job!

The protocol I currently recommend consists of a sagittal proton density–weighted spin-echo series with fat suppression and sagittal, coronal, and axial FSE T2 with fat suppression ( Table 9.1 ). Many acceptable variations of this protocol exist. Many centers, for various reasons, prefer not to use FSE images and instead use gradient-echo.

Menisci

The normal meniscus is a fibrocartilaginous, C-shaped structure that is uniformly low in signal on both T1- and T2-weighted sequences ( Fig. 9.1 ). With T2∗ sequences, the menisci will usually demonstrate some internal signal. With T1-weighted images, any signal within the meniscus is abnormal, except in children, where some signal is normal and represents normal vascularity. Meniscal signal that does not disrupt an articular surface is representative of intrasubstance degeneration ( Fig. 9.2 ), which is myxoid degeneration of the fibrocartilage. It most likely represents aging and normal wear and tear. It is not felt to be symptomatic and cannot be diagnosed clinically or with arthroscopy. Some choose, therefore not to mention intrasubstance degeneration in the radiology interpretation.

When high signal in a meniscus disrupts the superior or inferior articular surface, a meniscal tear is diagnosed ( Figs. 9.3 and 9.4 ). Care must be taken to be sure that the signal actually disrupts the articular surface of the meniscus before calling a tear. When high signal approaches the articular surface of the meniscus, it seems many radiologists tend to overcall it whether or not it disrupts the surface. This is evidenced not only from my experience of watching residents and fellows, but also by noting that most published series on accuracy of knee MR imaging have a lower specificity than sensitivity (i.e., there are more false positives than false negatives). One way to aid in avoiding false positive calls is to cover up the meniscus with a card, or your thumbnail, leaving only a thin margin of the articular surface of the meniscus visible. If this margin of articular surface of the meniscus is seen as a straight, uninterrupted line, no tear of the meniscus is present. If the thin margin is interrupted, a meniscal tear is present.

Meniscal tears have many different configurations and locations, with an oblique tear extending to the inferior surface of the posterior horn of the medial meniscus the most common type. In a small but significant percentage of cases (around 10%), it can be virtually impossible to be certain if meniscal high signal disrupts an articular surface. In these cases, it is recommended that the surgeon be advised that it is too close to call. The surgeon can then rely on his clinical expertise to decide if arthroscopy is warranted, and, if it is, the MR will guide him to where the questionable tear is located. DeSmet showed that signal that disrupts the surface of the meniscus on only one sagittal image should be considered a sign of an equivocal tear. He found only 56% of the medial menisci and 30% of the lateral menisci were torn if only one sagittal image showed the “tear.” If these equivocal cases are excluded, the remaining cases will have an extremely high accuracy rate.

Another very common meniscus tear, one that is frequently missed by radiologists, is a bucket handle tear. This is a vertical longitudinal tear that can result in the inner free edge of the meniscus becoming displaced into the intercondylar notch ( Fig. 9.5 ). It is most easily recognized by observing on the sagittal images that only one image is present, which has the bowtie appearance of the body segment of the meniscus ( Fig. 9.6 ). Normally, two contiguous sagittal images with a bowtie shape are seen, since the normal meniscus is 9 to 12 mm in width, and the sagittal images are 4 to 5 mm in thickness. On the coronal images, a bucket handle tear may reveal the meniscus to be shortened and truncated; however, often the torn meniscus remodels and truncation cannot be appreciated ( Fig. 9.7 ). The displaced inner edge of the meniscus (the “handle” of the bucket) should be seen in the intercondylar notch on sagittal or coronal views ( Fig. 9.8 ).

Another meniscus tear that is diagnosed by having too few bowtie segments present in the sagittal plane is a radial tear or parrot beak tear. This is a tear of the free edge of the meniscus ( Fig. 9.9 ), which is a common tear. It should be suspected when only one bowtie segment is present, and the adjacent sagittal image shows a small gap (a bucket handle tear will have a large gap) in the expected bowtie ( Fig. 9.10 ). The apparent anterior and/or posterior horn triangles will often be rounded or truncated instead of pointed.

Use of the “bowtie sign,” that is, having two consecutive sagittal images that demonstrate a bowtie configuration, is one of the most useful signs I can give beginners in evaluating a knee MR image. It will allow a bucket handle tear to virtually never be overlooked and, as the next section describes, can be used to diagnose a discoid meniscus. There are four pitfalls to be aware of in applying the bowtie sign ( Table 9.2 ). First, if the knee and the menisci are very small, as in a child’s knee, only one bowtie may be observed without a bucket handle tear being present. However, there will be only two or three sagittal images that demonstrate the anterior and posterior horns. A normal-sized knee will have two bowties and three or four images that show the anterior and posterior horns. Also, in a small knee, both the medial and lateral menisci will have only one bowtie image, and bucket handle tears involving both the medial and lateral menisci are very rare.

The second pitfall in the bowtie sign is seen in older patients—over the age of 60. Patients older than 60 often have worn down the inner free edges of their menisci so that they will only have one sagittal image of the body seen, followed by four or five images that show the anterior and posterior horns—usually a good sign for a bucket handle tear. This, unlike the pitfall described in children or small knees, does not necessarily occur in both menisci. How do I differentiate this from a real bucket handle tear? First, degenerative joint disease is always present, and, second, no displaced meniscal fragment can be found.

A third pitfall to be aware of in using the bowtie sign is that the sign only works if the entire meniscus is covered with sagittal images. If the tech doesn’t begin the sagittal images at the far medial or lateral aspects of the knee, the meniscus will not be imaged in its entirety. One will quickly learn to appreciate whether or not the entire meniscus is covered.

The fourth pitfall is seen in knees with prior surgery in which a partial meniscectomy has been performed. This can be differentiated from a bucket handle tear by the lack of a displaced fragment (no handle of the bucket). In fact, all four of the pitfalls mentioned can be recognized by the inability to see a displaced meniscal fragment.

A discoid meniscus is a large disc-like meniscus that can have many different shapes—lens shaped, wedged, flat, and others. It is not known if it is congenital or acquired, but most are found in children or young adults. It is seen laterally in up to 3% of the population, with a discoid medial meniscus being rare. A discoid meniscus is felt to be more prone to tear than a normal meniscus and can be symptomatic even without being torn. Although they are easily identified on coronal images by noting meniscal tissue extending into the tibial spines at the intercondylar notch ( Fig. 9.11 ), they are most reliably diagnosed by noting more than two consecutive sagittal images that show the meniscus with a bowtie appearance ( Fig. 9.12 ). Hence the bowtie sign can be used to diagnose a bucket handle tear (fewer than two bowties) or a discoid meniscus (more than two bowties). If thinner slices than 4 or 5 mm are used, the bowtie sign can be adjusted to whatever slice thickness is employed.

The lateral meniscus often has what appears to be a tear on the anterior horn near its upper margin, which is a pseudotear from the insertion of the transverse ligament ( Fig. 9.13 ). This can easily be differentiated from a real tear by following it medially across the knee in Hoffa fat pad to where it inserts onto the anterior horn of the medial meniscus. Although less common than on the lateral meniscus, a pseudotear from the insertion of the transverse ligament onto the anterior horn of the medial meniscus can be seen.

Meniscal cysts occur in about 5% of cases and can cause pain, even if the meniscus is not torn. The etiology is unknown, but they occur more frequently in discoid menisci. If the meniscus is not torn, the surgical approach used by some is percutaneous with decompression and packing performed, whereas if a meniscus tear is associated with the cyst, it is approached intraarticularly. Hence accurate diagnosis of a tear is imperative. The intrameniscal portion of the cyst typically does not get fluid-bright in signal on T2 sequences ( Fig. 9.14 ), which has misled many radiologists into discounting the presence of a cyst. A meniscal cyst will enlarge the meniscus and give it a swollen appearance unless it decompresses into the soft tissues (called a parameniscal cyst) or into the joint via a meniscus tear. Decompression into a parameniscal cyst does not indicate a meniscus tear. A meniscus tear, by definition, has to disrupt the articular surface of the meniscus. Although a meniscus with a meniscal cyst is more likely to tear than an otherwise normal meniscus, up to 40% are not torn. Many reports cite that menisci with cysts are torn up to 98% of the time—this is simply not true.