IMAGING CARTILAGE DEGENERATION
Cartilage damage report should include
• ICRS score
• Surface affected
• Location on surface if relevant
• Maximal size of lesion
• Associated effusion, synovitis and osteochondral debris
ICRS Grading
• 1 Mild signal changes only
• 2 Defect less than 50% thickness
• 3 Defect down to subcondral plate - plate remains intact
• 4 Loss of subchondral plate extension into bone
It can also be helpful to describe the border of a lesion as this may impact decision to repair it
Acute cartilage injuries include chondral and osteochondral fracture see knee section
Imaging techniques for the diagnosis of cartilage injury principally revolve around magnetic resonance imaging.
Xray findings are late and frequently underestimate the degree of cartilage injury.
CT better than Xray but is rarely used
CT arthrography provides an exquisitely accurate image of the cartilage surface.
Ultrasound can depict the cartilage surface and intrinsic structure but only see a small portion of the articular surface
Cartilage Structure
Articular cartilage is made up of a collagen matrix with a high water content
It contains proteoglycans and chondrocytes.
Each of these is important in maintaining cartilage function and integrity.
Several distinct layers are identified histologically which are differentiated principally by their microscopic structure.
The thickest layer has organised columns of collagen perpendicular to the articular surface.
Superficial to this is a layer where the collagen fibres are arranged in arcs overriding the columnar layer.
The deeper layers comprise the transition zones between cartilage and bone and are made up of deep basal layer and tidemark.
The superficial transitional layer lies on top of these.
MRI is divided into techniques that detect morphological changes and those that detect physiological changes
Morphology MRI is focussed on the articular cartilage surface
Numerous sequences have been devised to provide high resolution images of the cartilage effusion interface
“Physiological” MRI attempts to identify earlier alterations in the collagen structure and proteoglycan content
MORPHOLOGY MR
Some sequences contrast high signal or bright articular cartilage with low signal fluid
Others try to make the fluid bright and create contrast with low signal articular cartilage.
Improved M strength and coils means many routine sequences are good at detecting cartilage lesions.
Intermediate weighted proton density or T2 weighted images in particular are efficient.
Gradient echo images particularly spoiled grass have long been used to provide three dimensional volume acquisitions
These can be reconstructed to provide volume assessments with of cartilage within the joint.
At higher field strengths isotropic imaging using spin echo can now be used in a similar way but with better resolution.
Fat suppression is frequently added to the protocol to improve resolution.
Specialised sequences continue to be devised
The holy grail is improved spacial resolution, contrast resolution and short imaging times.
Specialised sequences include
• steady state free precession (SSFP)
• water excitation true fisp
• FEMUR
• T1 or proton density sequence with a re-polarisation pulse added
• zzz
The imaging goals include detection, classification/staging followed by post operative assessments if necessary.
A variety of classifications have been proposed, the most simple is the Outerbridge although this is insensitive
The ICRS system which recognises the extent of partial thickness cartilage loss and is widely accepted.
Other classification systems allows a accurate description bur are more time consuming
Description of the borders of the cartilage lesion can help support particular surgical options.
PHYSIOLOGY MRI
Aims to identify early stages of disease prior to the development of a surface defect.
The full application and clinical implication of these techniques has yet to be fully evaluated.
Cartilage is composed of a high water content in which are regularly arranged collagen fibres.
Within this structure are contained chondrocytes and proteoglycans.
The earliest changes in cartilage degeneration include a reduction in proteoglycans content
This reduces its ability to hold water and retain turgidity increasing collagen breakdown on a microscopic level
Techniques assess both the alterations in proteoglycan content and changes in collagen content
Proteoglycan Imaging
Proteoglycans are positively charged so areas of proteoglycan loss will be relatively negatively charged
Proteoglycan imaging uses positively charged gadolinium which will seek out and become concentrated in areas of proteoglycan deficiency.
Gadolinium is given intravenously and the patient is imaged 60-90 mins later following exercise
The process is called delayed gadolinium enhanced MRI of cartilage (dGEMRIC).
A T1 mapping sequence is used.
Areas with the most gadolinium uptake will demonstrate the greatest T1 shortening.
T1 image maps will demonstrate this either numerically or as a colour map.
The T1 relaxation time of a larger region in an area known to be susceptible to articular cartilage disease can be calculated.
This can then be compared with more normal parts of the joint.
Sodium imaging
has also been used in a similar way where the positively charged ion will also concentrate in areas of relatively gadolinium depletion.
The techniques required to image differences in sodium concentration are more complex and not usable in most clinical settings.
Collagen Imaging
Collagen degeneration is anearly physiological change prior to the appearance of focal cartilage defects.
Cartilage demonstrates low signal intensity on T2 weighted images
Much of the T2 relaxation time is due to the collagen concentration.
A T2 relaxation map thus provides an assessment of cartilage content
T2 relaxation maps can be acquired in a number of ways but generally involve using different TE’s to calculate the curve.
T2 maps can be expressed either as a numerical value comparing one region of interest within the joint with another or as a colour map
T1 rho imaging
is used to identify patterns of water diffusion
This is constrained when there is a regular underlying cartilage matrix.
Disorganisation of the articular cartilage structure will lead to alterations in diffusion characteristics
Similar techniques are used in neurotract imaging.