ECR 2019 TOPIC PACKAGE

Physics and application of MRI (in knee imaging)

Lectures

1
B. Meniscal tears: obvious and subtle

B. Meniscal tears: obvious and subtle

28:16P. Omoumi

This lecture will review the basic aspects of MRI of meniscal pathology, including some elementary technical and anatomical considerations, as well as the description of the basic semiology of meniscal tears and their classification. Common pitfalls and errors that need to be avoided will be presented.MRI is the modality of choice for the diagnosis of meniscal pathology as well as treatment planning. A certain number of technical considerations need to be understood by the radiologist. The standard sequence to image the meniscus is intermediate-weighted fast spin-echo sequences, in 2D or 3D. Certain acquisitions parameters may influence the diagnosis of meniscal tears, including the choice of the echo time. Meniscal tears can manifest either as morphological changes or intrameniscal signal changes, with specific diagnostic criteria that will be reviewed. An appropriate terminology, based on standard arthroscopic classifications, needs to be used by the radiologist in order to properly communicate the description of the tear to the referring surgeon. Secondary signs of meniscal tears, including parameniscal cyst and meniscal extrusion, should be used to increase the diagnostic performance for the presence of meniscal tears. Common pitfalls include anatomical variants (i.e. inter-meniscal meniscal and menisco-femoral ligaments, discoid meniscus), as well as other causes for false positive findings such as meniscal flounce, CPPD and meniscal ossicles. Knowledge of common mistakes and patterns of injury helps avoid unnecessary mistakes and improve diagnostic accuracy.

2
C. Looking around the corners: posteromedial and posterolateral

C. Looking around the corners: posteromedial and posterolateral

26:35U. Aydingoz

Posteromedial and posterolateral corners of the knee (PMC, PLC) can be injured along with anterior (and, less commonly, posterior) cruciate ligament tears. If an injury to the PMC and/or PLC is not properly addressed, cruciate ligament surgery may fail. Although rare, isolated injuries to the PMC and PLC may also occur. Radiologists need to be familiar with the MR imaging appearances of the PMC and PLC structures, and their injury patterns. Semimembranosus is the principal stabiliser of the PMC, while posterior oblique ligament is the main PMC structure that needs repair or reconstruction after an injury to this corner. Popliteus tendon is the key structure that helps in identifying on MR imaging the arcuate and popliteofibular ligaments at the PLC.

3
A. Basic MR: the building blocks of pulse sequences

A. Basic MR: the building blocks of pulse sequences

30:09A. Webb

This lecture will start by covering the basic principles of magnetic resonance: the net magnetisation produced within a patient, why a radiofrequency pulse has to be applied, and how the MR signal is detected. Pulse transmission and MR signal detection are then linked to the body coil and receive array coils used in a conventional MRI setup. Next, the dependence of the signal on the T1, T2 and T2* relaxation times will be explained. The sequences necessary to introduce T1 (inversion-recovery), T2 (spin-echo) and T2* contrast into the signal will be covered. Finally, the reasons why different tissues have different relaxation times will be discussed, as well as how pathology can alter these relaxation times.

4
B. MR imaging basic concepts: how to turn signals into images

B. MR imaging basic concepts: how to turn signals into images

29:32D. Lurie

To generate images in MRI, NMR signals must be “labelled” with their location. Three techniques of spatial encoding are employed, all of which use the magnetic field gradient. By sending electrical current (hundreds of amperes) through a gradient coil, a magnetic field is produced whose strength which varies linearly with the position inside the scanner bore. In frequency-encoding, the NMR signal is recorded while a field gradient is applied. Since the magnetic field varies with position along the gradient direction (e.g. X), the NMR resonant frequency (Larmor frequency) is a function of position, so the detected signal contains a range of frequencies; analysing the frequency content generates a one-dimensional projection of the water-distribution within the patient. A method called phase-encoding is employed in the second in-plane dimension (e.g. Y); here, the gradient is pulsed on and off prior to measurement of the signal, affecting the phase of the NMR signal as a function of position. Finally, the slice itself is defined using selective-excitation, in which the radiofrequency pulse used for NMR excitation is specially-shaped and is applied in the presence of a field gradient perpendicular to the slice plane (e.g. along Z). To generate data for an image of NxN pixels, the pulse sequence is usually applied N times, varying the phase-encode gradient amplitude with each repetition. A two-dimensional Fourier transform of the raw data matrix yields the MR image, which can be encoded with NMR parameters (T1, T2, diffusion etc.) information to assist diagnosis.

5
C. Practical MRI: a toolkit of standard MR pulse sequences

C. Practical MRI: a toolkit of standard MR pulse sequences

29:44I. Seimenis

The MR-sequence is an essential tool to measure MR-properties in tissues, increased contrast between these and quantify relevant data. Besides furnishing the desired tissue-derived MR signal, the sequence must allow spatial encoding to take place as efficient as possible to enable patient compliance, and high-quality MRI at the same time. Starting with a brief introduction regarding MR-tissue properties and spatial encoding in k-space, I will describe the fundamental MR toolkit, based on single echo spin-echo (SE) and gradient-echo (GE) and show how these are extended to different multiple-echo regimes. Basic image contrast and how imaging parameters: repetition time, TR; echo time, TE; flip angle, FA; inversion delay, diffusion gradients, etc.; influence contrast will be described. The possibility to further enhance imaging by adequate spin-preparation: inversion or magnetisation transfer pulses etc.; will also be discussed. Imaging speed can be achieved while remaining in the single echo regime by faster RF pulsing (shorter TR) giving valuable information linked with magnetic susceptibility like in the GE-based FLASH method, based on spoiling of unwanted echoes. Another possibility is to use multiple (spin) echoes to read out different k-space lines, and hereby speed up spatial sampling, like in the SE-based method RARE. An alternative possibility that further extends available image contrast is to retain the full magnetisation in GE-based sequences and acquire images in the different steady-state-free-precession regimes. The latter techniques have gained momentum through the advent of magnetic fingerprinting and are evolving into another fundamental part of the standard MRI-toolkit.

6
A. Sports-related injuries of the knee: what does the orthopaedic surgeon need to know?

A. Sports-related injuries of the knee: what does the orthopaedic surgeon need to know?

29:58T. Miller

The anatomy of the normal ACL and menisci will be reviewed, followed by a discussion of the appearances of various abnormalities of the ACL and menisci, as well as mechanisms of injury and injury patterns. Associated injuries will be discussed. Recommendations will be made for imaging the postoperative meniscus.

7
B. ACL reconstruction and cartilage repair

B. ACL reconstruction and cartilage repair

27:22C. Weidekamm

ACL reconstruction aims to stabilise the knee and prevent chondral and meniscal injuries, which are sequelae of anteroposterior translation and are associated with early osteoarthritis. The idea of the double-bundle ACL graft was to restore normal joint kinematics by anatomic reconstruction of the anteromedial and the posterolateral bundle of the original ACL. This was expected to improve clinical outcomes and restore anterior and rotational knee stability. The single-bundle technique, however, causes less osseous defects and is still a popular technique. Complications, such as ACL graft failure, impingement, cyclops lesion, arthrofibrosis, and patellar inferior syndrome, are discussed. The second part of this presentation will illustrate cartilage repair techniques and imaging findings. The radiologist must be familiar with the different cartilage repair procedures and characteristics in cartilage imaging to evaluate long-term progression or failure. Abnormal postoperative findings include hypertrophic filling, incomplete integration of the transplant into the surrounding cartilage, or subchondral defects, osteophytes, cysts, and persistent bone marrow oedema and joint effusion.

8
C. Interactive case discussion (Part 1)

C. Interactive case discussion (Part 1)

10:31T. Miller

9
C. Interactive case discussion (Part 2)

C. Interactive case discussion (Part 2)

16:23C. Weidekamm

Postoperative imaging after ACL or cartilage repair is indicated in patients with ongoing pain/instability or repetitive injury. Radiography remains the initial imaging modality; however, a further assessment with CT or MRI is recommended. With a clear emphasis on MRI, we will review normal postoperative findings and complications after ACL reconstructions and cartilage repair. The case discussion will cover the most significant pathologies and pitfalls, and normal postoperative findings will be illustrated.

10
A. Cruciate ligaments: what to know and what to do

A. Cruciate ligaments: what to know and what to do

27:53A. Alcalá-Galiano

Cruciate ligaments play a key role in knee kinematics and stability. Cruciate-deficient knees are predisposed to early osteoarthritis due to chronic instability. ACL is the most commonly injured major ligament of the knee. PCL lesions are far less common, but more frequently associated with multiligamentous injuries. MRI is the study of choice to evaluate acutely and chronically injured cruciate ligaments, associated injuries and for postoperative imaging following ligament reconstruction. Understanding the variations of their anatomy (double bundle configuration and insertion sites) is key to performing a tailored anatomic reconstruction. Accurate description and grading of cruciate tears and associated injuries are essential for preoperative planning of ligament reconstruction. Primary and secondary signs and indirect imaging findings of acute and chronic ligament injury will be reviewed. Atypical findings and imaging pitfalls will also be discussed. ACL reconstruction is one of the most common orthopaedic procedures, while PCL reconstruction is on the rise. Knowledge of ligament reconstruction techniques allows differentiation of normal postoperative findings from complications, which may be related to graft placement, the graft itself or the donor site. Any standard knee MRI protocol must ensure adequate detection of both soft tissue and bony injury, including the cruciate ligaments. However, protocols may be tailored for specific needs, including oblique acquisitions, 3D or metal artefact reduction sequences to better demonstrate the anatomy and facilitate detection of small tears.

PEP Subscription Required

This course is only accessible for ESR Premium Education Package subscribers.