Bones

For Medical Students

What It Shows

Magnetic resonance imaging shows bone mostly through its marrow, because cortical bone has almost no signal and appears as a dark line. 5.2.13 covers T1 imaging.

Normal adult marrow is fat, so it is bright on the unsuppressed sequences and dark on the suppressed ones. 5.2.16 covers fat suppression.

It matters because this inverts the radiographic relationship: a radiograph shows the cortex best and the marrow not at all. 5.2.2 covers the image formation.

How It Is Acquired

Assess the marrow on a T1 weighted sequence, where replacement is most conspicuous. 5.2.13 covers T1 imaging.

Assess for oedema on a fat suppressed fluid sensitive sequence. 5.2.16 covers fat suppression.

Assess the cortex as a dark line and accept that a fine cortical detail is a radiographic or computed tomography question. 5.2.22 covers the computed tomography acquisition.

Assess the subchondral plate separately, since it is where most knee marrow pathology sits. 5.11.7 covers bone marrow lesions.

Know the expected red marrow distribution for the patient's age. 5.5.4 covers the paediatric protocol.

What Normal Looks Like

Bright fatty marrow on T1 weighted sequences throughout the adult metaphyses and epiphyses. 5.2.13 covers T1 imaging.

Dark marrow on fat suppressed sequences, with no focal brightness. 5.2.16 covers fat suppression.

A thin continuous dark cortical line.

A thin dark subchondral plate beneath bright cartilage. 5.5.7 covers the cartilage.

MRCS Level

How It Is Acquired

In a viva the expected answer explains why magnetic resonance imaging is better for marrow and worse for cortex than a radiograph.

The expected points are the T1 marrow assessment, the fat suppressed oedema sequence, the cortex as a dark line, the subchondral plate, and the age dependent marrow.

Say that fine cortical detail belongs to computed tomography. 5.2.22 covers the computed tomography acquisition.

Say that marrow replacement is a T1 finding. 5.13.7 covers bone tumours.

Say that marrow oedema is sensitive and non-specific. 5.13.1 covers bone marrow oedema.

What Normal Looks Like

The reasoning expected is that signal comes from hydrogen in a mobile environment, so dense mineralised cortex with little free water returns almost nothing while fatty marrow returns a great deal. 5.2.12 covers the magnetic resonance basic principles.

That makes cortical assessment an assessment of an absence, which is why a thin undisplaced cortical fracture can be invisible while its marrow oedema is obvious. 5.13.3 covers stress fractures.

Red marrow is cellular rather than fatty, so it darkens T1 signal in a pattern that resembles infiltration, and its distribution is the discriminator. 5.2.13 covers T1 imaging.

The subchondral plate is thin and carries most of the joint's load, which is why it is the site of the marrow lesions that accompany cartilage loss. 5.11.7 covers bone marrow lesions.

How Good It Is

Magnetic resonance imaging is the best modality for marrow and the worst of the three for cortical detail.

Its best contribution is marrow replacement and marrow oedema, neither of which radiography shows. 5.13.1 covers bone marrow oedema.

It is limited for cortical architecture, for which radiography and computed tomography are better. 5.2.22 covers the computed tomography acquisition.

No figures appear for accuracy on this page, under decision 028, and no marrow signal intensity or plate thickness values appear either: those are published sequence dependent quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level marrow is where magnetic resonance imaging adds most to a bony question.

An occult fracture with normal radiographs is found through its marrow oedema, which changes management. 5.13.3 covers stress fractures.

Subchondral marrow change accompanying cartilage loss describes the compartment's state. 5.11.7 covers bone marrow lesions.

Marrow replacement raises a systemic question that a knee protocol cannot answer. 5.5.5 covers the tumour protocol.

Cortical and bony geometry questions go to radiography and computed tomography rather than here. 5.16.1 covers the fracture assessment.

Where It Misleads

The commonest error is reading normal red marrow as infiltration. 5.2.13 covers T1 imaging.

Marrow oedema is reported as a diagnosis rather than a finding. 5.13.1 covers bone marrow oedema.

A cortical question is asked of the wrong modality. 5.2.22 covers the computed tomography acquisition.

Failed fat suppression is read as marrow oedema. 5.2.16 covers fat suppression.

A marrow lesion is characterised from a knee protocol that did not cover the bone. 5.5.5 covers the tumour protocol.

Fellowship Level

Where It Misleads

At fellowship level marrow is the tissue where magnetic resonance imaging's sensitivity does the most harm.

Marrow oedema is detected in asymptomatic knees and in nearly every symptomatic one, so its presence moves a probability very little while reading as a significant finding. 5.13.1 covers bone marrow oedema.

Its prognostic associations in osteoarthritis are population level and are routinely applied to individuals, which is the same error the osteophyte and joint space pages describe. 5.11.7 covers bone marrow lesions.

The normal red marrow boundary is wide and moves with age, training and haematological state, so the distinction from infiltration rests on distribution and symmetry rather than on signal. 5.2.13 covers T1 imaging.

Quantitative marrow assessment would address both and is not in routine knee practice, so a qualitative finding is used quantitatively. 5.18.1 covers quantitative magnetic resonance imaging.

The disciplined report describes the pattern, states whether it is within the expected distribution, and says when it names nothing. 5.20.1 covers structured reporting.