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Hi, I'm Chris servant and I'm a knee surgeon in Ipswich. I'm sorry, I can't be with you live, but I hope you find this prerecorded presentation instructive. You need to know about knee, MRI for two main reasons. Firstly, to be able to answer a question on MRI in the FRC S exam. Secondly, so you can make an accurate diagnosis of a knee injury. Despite the impressive technology of MRI, it usually only helps a little in diagnosing knee injuries. Most experienced knee surgeons rely on a history and examination to make a diagnosis with imaging only helping to confirm the diagnosis. It's imperative to correlate the MRI findings with the clinical findings. So do not rely on just reading the report even better is if you can read the scan yourself and only look at the report to see if your opinion matches that of the radiologist. Before I run through reading a scan, you need to know some basic physics of MRI for the exam, unlike X ray and CT, which use ionizing radiation, MRI uses a magnetic force. Mr uses magnetic fields and radio waves to measure how much water there is in different tissues and then uses this information to generate a detailed image. A water molecule is made up of two hydrogen atoms and one oxygen atom. The hydrogen atoms are key to the physics of MRI. The nucleus of a hydrant atom contains a single proton with a positive charge protons constantly spin. And the moving electrical charge induces a small magnetic force so that the protons act like tiny bar magnets. When a strong magnetic field is introduced, as is the case in an MRI scanner protons align with that field. The MRI scanner then introduced a radio frequency pulse that disrupts the proton and forced it either into a 90 degree or 180 degree realignment with a static magnetic field. As soon as the radio frequency pulse is switched off, the protons settle back into their original position releasing energy. An MRI scanner is essentially a giant magnet. The strength of the magnet is measured in a unit called tesla. Most MRI scanners used in hospitals are 1.5 or three tesla, putting that into perspective. A three tesla MRI scanner is around 60,000 times stronger than the earth's magnetic field. The first stage in the manufacture of a scanner is to seal the main magnet in an insulated casing liquid helium keeps the temperature of the magnet close to absolute zero so that it loses all electrical resistance and becomes a superconductor capable of generating very powerful magnetic fields. The gradient coils and radio frequency coils are assembled next before the radio frequency coils are inserted into the gradient coils which in turn I inserted into the bore of the main magnet, the housing is then completed. So the main components of an MRI system are the primary magnet gradient coils, radio frequency coils and computer system. The three grading coils generate secondary magnetic fields which allow the strength of the primary field to be altered in all three planes, producing sagittal coronal and axial images. The coils are switched on and off rapidly causing them to contract and expand by tiny amounts. Producing the characteristic loud bangs of a scanner radio frequency coils transmit the radio frequency pulse and receive the resultant signals. They were designed for each body part such as this knee coil and aimed to improve signal to noise ratio and improve image quality. The coils measure proton relaxation longitudinally called T one and transversely called T two. The T one and T two times are unique to each tissue type as of all T one is sharper and better accessing anatomy. Whereas T two shows fluid brightly, particularly if fat suppression is used. And so it's better for detecting abnormalities. So first, I'll run through how I look at an MRI scan using a study of a normal left knee. Usually I start with T two satchel images and I've placed at two fat suppressed image on the right and at one image on the left. For comparison, we are starting laterally and the head of the fibula confirms this as we scroll across the lateral compartment. You can see the body of the lateral meniscus as a single structure before it forms the two dark triangles of the anterior and posterior horns. The bone is dark gray with the articular cartilage being light gray. The patient is 16 years old and he can still see the feces faintly as we move towards the center of the knee. You can see the patella tendon, patella and quadriceps tendon at the front in the center is the notch with the ACL which should be uniform and dark slightly further medially is the P cell. The medial meniscus should form two uniformly dark angles before becoming a single structure. The hamstring tendons can be seen on the far medial slices. I'll keep the T two saddle image on the right for reference and move to the coronal images. I've placed at two sequence on the left starting a anteriorly first, the patella will come into view followed by the quadriceps and patellar tendons. Once into the uh tibiofemoral joint, you'll be able to see the two menisci. The ACL should be seen to be attached to the lateral wall of the notch. The M CL should be a thin dark band medially with the it band attached to the tibia laterally a little more posteriorly. The Popliteus tendon can be seen in its groove on the lateral femoral condyle. The PCL attaches to the middle wall of the notch as you scroll further backwards. Have a good look at the posterior horns and posterior attachments of both menisci. As most meniscal tears involve the posterior horn poster laterally. You can see the uh lateral collateral ligament, pop fibular ligament and the biceps femoris tendon attached to the head of the fibula. Finally, the axial images I start distally at the tibial tuberosity and trace the patella tendon approximately. You should see the popliteal neurovascular structures posteriorly at the level of the joint. You may be able to make out the menisci. You should also be able to see the A cell and the P cell within the notch anteriorly. The trochlear should be concave with a good even layer of articular cartilage visible. The patella should be well centered with the medial and lateral retinaculum on either side, moving further proximately, the trochlea will merge into the femoral shaft. Now, I will show you some examples of the various pathologies that can be seen on MRI starting with meniscal tears. First longitudinal vertical tears, they can be seen as a vertical line of high signal, usually starting the posterior horn of the meniscus and best seen on sanit images. Sometimes you may be able to see the tear on axial images, bucket handle tears are displaced longitudinal tears that cause locked knees. They are com common media on a coronal image. The meniscus appear smaller than normal because the bucket handle fragment has flipped into the notch on a central sagittal image. The fragment can be seen lying in front of the PC forming what is called a double PCL sign. Radial tears are often more difficult to spot with the disruption of meniscal tissue often being subtle on a sat image. There may be a short gap in the body of the meniscus. And on a coronal image, the meniscus may appear truncated. A root tear is a particular type of radial tear whose importance has been increasingly recognized. The root is the posterior attachment of the meniscus to the tibia. And a complete tear has major adverse effects on the coronal images. You may see high signal in the femoral condyle, which is often termed either so spontaneous osteonecrosis of the knee or an insufficiency fracture as well as an extrusion of the meniscus. More posteriorly. You may see a rounded torn posterior horn that is no longer attached to the tibia on the sagittal images. As you scroll towards the notch, the posterior horn may be very small or nonexistent being replaced by the high signal of joint fluid known as a ghost sign. On the axial images. You may also be able to see the rounded torn posterior horn. Horizontal meniscal tears are the most common type of tear and result from meniscal degeneration in patients about 35 years and older. They are often asymptomatic and not the cause of the patient's knee pain. The horizontal cleavage may allow joint fluid to escape but not return easily due to a flap valve effect. And this causes a para meniscal cyst to form as well as often being asymptomatic horizontal tears are over reported. A radiologist will often be cautious and interpret high signal as a horizontal tear. When in fact, the meniscus is degenerate but not torn to warrant an arthroscopy. A line of high signal should extend to the superior or inferior articular surface of the meniscus on at least two adjacent MRI slices and the tear should correlate with the clinical findings. Occasionally, the attachment of the intermeniscal ligament to the front of the lateral meniscus is misdiagnosed as an anterior horn tear of the lateral meniscus. In younger patients, the meniscus is more vascular than in older patients and a nutrient blood vessel may be misdiagnosed as a tear. The commonest congenital abnormality is a discoid lateral meniscus with more meniscal tissue filling the lateral compartment than normal. Now, moving on to ligament, tears, ACL tears are common and are commonly missed starting naturally in acute injuries. You may see the secondary sign of bone bruising of the lateral femoral condyle and the posterolateral tibial plateau caused by the two areas impacting together as the knee sublux is at the time of injury. Bone bruising takes about three months to recover on MRI moving into the notch. The ACL appears edematous having lost its dark low signal appearance and is flatter than normal there's a double PCL sign as this patient has an associated bucket handle tear of the medial meniscus. The medial meniscal remnant is small on the coronal view. The ACL is indistinct and there is a bucket handle fragment lying majorly in the notch. Sometimes the tibial stump of the torn. A cell forms a ball of tissue known as a cyclops lesion that can block full extension further back of the notch. The PCL is clearly seen attaching to the medial wall but the lateral wall is empty, which is characteristic of an ACL rupture with a PCL tear here. The ACL is clearly seen as a linear dark structure but the PCL has a midsubstance tear with some rounding of the torn ends on the axial view. The P cell appears indistinct in its mid portion. Most M CL tears are partial tears. The M cell in this patient is no longer thin dark band. It is thickened and edematous mainly at the femoral end appears continuous and this is typical of a grade one or grade two injury. This patient on the other hand has a grade three injury with complete disruption of the deep and superficial M cl and peripheral detachment from the medial meniscus. The anatomy of the post or natural corner is complex. This patient has an avulsion of the ITB from the tibia and disruption of the structures that normally attach to the fibular head. The biceps, femoris tendon, the popliteofibular ligament and a rather wavy lateral collateral ligament, osteochondritis dissecans is a partial or complete separation of a piece of articular cartilage and subchondral bone. During adolescence, the lateral part of the medial femoral condyle is usually affected. MRI can help judge size and stability with high signal surrounding the lesions, suggestive of instability. Finally, patella fetal malalignment. The sagittal view allows the measurement of an index of patella height. I use the KON index which is the ratio of the distance from the anterior corner of the tibial plateau to the inferior margin of the patellar articular cartilage divided by the length of the patellar articular cartilage. Our PAX does not have a ratio tool so I have to calculate the ratio myself. The upper limit of normal is 1.2. So this patient has patella alta. You can also er judge the degree of overlap of the patella and trochal articular cartilage. As long as the knee is extended fully on the axial view, I start distally and trace the patella tendon approximately. This patient's patella tendon impinges on the lateral trochlea which is known as patella tendon conflict and indicates excessively lateral tibial tuberosity. The trochlea is shallow but is a good layer of articular cartilage. The patella is in a position of marked lateral subluxation and there was an ossicle adjacent to the medial border, indicative of an old MPFL avulsion to confirm if the tibial tuberosity is excessively lateral. The tt tg distance can be measured some units will superimpose two actual slices to make the measurement easier. But what I have to do is place my cursor on the center of the tibial Rosti scroll up to the most proximal slice on which the concavity of the trochlear is still visible. Click to anchor the distance tool, move to the center of the trach on a line parallel to the posterior condyles and click to obtain the TT TG distance. Thank you very much for listening. I hope you can now read a knee MRI scan more confidently, I'll leave you with this. You do. I know, can you? No?