Proton Density Imaging

For Medical Students

What It Shows

A proton density image is weighted towards the number of hydrogen nuclei rather than towards either relaxation property. 5.2.12 covers the basic principles.

The result has high signal to noise and a contrast that sits between T1 and T2, which suits a structure with internal detail. 5.6.1 covers the normal meniscus.

It matters because this is the weighting on which meniscal and cruciate internal structure is best assessed, so it carries much of chapter 5.6. 5.6.2 covers meniscal signal grading.

How It Is Acquired

It is acquired with a long repetition time and a short echo time, which removes most of both relaxation weightings. 5.2.12 covers the basic principles.

The high signal to noise allows thinner slices or a finer matrix for the same study time. 5.5.1 covers the standard protocol.

Fat and fluid are both fairly bright, so this is a detail sequence rather than a conspicuity sequence. 5.2.14 covers T2 imaging.

It shows the meniscal fibre pattern and the cruciate bundle striations that a heavily weighted sequence loses. 5.7.1 covers the normal cruciate.

It is read with a fluid sensitive sequence rather than instead of one. 5.2.16 covers fat suppression.

MRCS Level

How It Is Acquired

In a viva this is the weighting to name when asked which sequence the meniscus is assessed on.

The expected points are the long repetition and short echo times, the signal to noise advantage, the intermediate contrast, the internal detail, and the pairing with a fluid sensitive sequence.

Say that meniscal surface assessment is made here. 5.6.2 covers meniscal signal grading.

Say that the detail advantage is bought with conspicuity. 5.2.14 covers T2 imaging.

Say that cruciate striations seen here are normal anatomy. 5.7.1 covers the normal cruciate.

How Good It Is

Proton density weighting gives the best internal detail of the fibrous structures and the least conspicuous pathology elsewhere.

Its best contribution is the meniscus, where surface involvement is the decision and surface detail is what this sequence gives. 5.6.2 covers meniscal signal grading.

It is limited for marrow and for oedema, where the fat suppressed sequences are needed. 5.2.16 covers fat suppression.

No figures appear for accuracy on this page, under decision 028, and no sequence timing values appear anywhere in this chapter, because those are scanner specific protocol quantities.

FRCS Level

How Good It Is

At decision level this weighting is where the meniscal and cruciate reading actually happens, which makes its presence in a protocol worth checking.

Surface involvement of a meniscal tear decides whether it is reported as a tear, and that judgement is made on this sequence. 5.6.2 covers meniscal signal grading.

Cruciate bundle assessment for a partial tear needs this detail, and a protocol without it limits that question. 5.7.2 covers partial cruciate tears.

A protocol built only from T1 and fat suppressed T2 will read menisci less well, which is a protocol criticism rather than a reporting one. 5.5.1 covers the standard protocol.

Post-operative meniscal assessment needs the detail more than the conspicuity, which argues for including it. 5.6.12 covers the post-operative meniscus.

Where It Misleads

The commonest error is reading normal internal meniscal signal as degeneration, because this sequence shows it. 5.6.2 covers meniscal signal grading.

Normal cruciate striation is read as a partial tear. 5.7.1 covers the normal cruciate.

A marrow lesion is missed because this sequence was the one reviewed. 5.2.16 covers fat suppression.

Fluid and fat are confused because both are fairly bright. 5.2.14 covers T2 imaging.

The sequence is assumed present in a protocol that does not include it. 5.5.1 covers the standard protocol.

Fellowship Level

Where It Misleads

At fellowship level proton density weighting is the clearest instance of protocol choice determining what a report can say.

The meniscal grading conventions were developed on this kind of weighting, so applying them to a study without it applies a rule to data it was not derived from. 5.21.1 covers the meniscal imaging literature.

Protocols vary between departments in whether they include it and in which planes, and the report does not say. 5.20.1 covers structured reporting.

Three dimensional isotropic sequences are replacing separate plane acquisitions and change the contrast as well as the geometry, which affects the same conventions again. 5.2.21 covers advanced magnetic resonance techniques.

That makes serial meniscal assessment across departments less comparable than it looks, which matters most where imaging is used as an outcome. 5.6.13 covers meniscal repair imaging.

The useful conclusion is that a meniscal report's authority depends on a protocol detail the report omits, and asking for the protocol is reasonable. 5.5.1 covers the standard protocol.