MRI examinations involve a series of image acquisitions that can be optimised in respect of four key factors – patient, referring clinician, clinical question and the MR system capabilities.

The variation in these factors explains why protocols vary between patients and institutions and the challenge is to get the best possible diagnostic result for your patients taking these factors into account.

A good first guiding principle is to ask yourself for every series of images in your protocols – Do I need this set of images? What do they contribute to the clinical question?

A second principle is that if you don’t understand the multitude of parameters that can be changed for every image acquisition then find or employ someone who does.

A third principle is to consider the sequence protocol as a whole and whether it flows logically, does each sequence help plan or inform the next? Has the most important information been gained early on in case the patient doesn’t tolerate the full examination?

In the 21st century MRI manufacturers have eased the burden of sequence optimisation by providing “typical protocols” for most clinical situations. These have usually optimised many of the complex parameters for you – but can still be modified to suit a particular patient and clinical situation.

Over the last two decades the capability and complexity of MR exams has increased and there is a tendency to keep adding new capabilities to generic MR protocols to cover every possible disease entity sometimes with duplication.

This “do everything” approach allows less reliance on the accuracy of the referrer and the clinical context, but is increasingly time consuming and expensive.

In many cases there are specific clinical questions that allow for a more targeted, optimised approach. This is particularly true for MRI because prior investigations such as CT, US and clinical acumen often allow the clinical question to be narrowed down.

Optimising exams for specific clinical questions will become more important in the next decade as we are increasingly challenged to provide value for money in our financially limited healthcare systems.

On the other hand, if our optimisation is too targeted then the cost of omitting a key sequence and having to recall the patient has to be weighed in the balance. This means that understanding the clinical context and our relationship with the referring clinicians will become an increasingly important aspect of MR exam optimisation.

Regrettably MRI examinations are not patient independent.

Patients tend to breathe, their GI tract peristalses, their blood vessels pulsate, they may move voluntarily or involuntarily.

May or may not be able to cooperate with instructions during an examination.

May not tolerate long in the MRI system

May be ventilated or sedated

May be elderly, deaf and relatively immobile

May be very young, physically small and very mobile.

May have severe renal failure or be pregnant - precluding administration of intravenous contrast medium.

May have intra-corporeal metal clips or implants that affect the image quality.

May have variant anatomy that confuses inexperienced operators.

Their age and gender may influence the likelihood of a particular diagnosis – eg hepatic adenoma.

We often gloss over this – but how accurate is your referring clinician?

Are they diagnostically experienced or not? How often are they way off the diagnostic target?

Less experienced referrers usually mean a more comprehensive exam is required to cover a wider range of pathologies.

It is obviously good clinical practice to understand why you are performing an MRI examination and what the clinical context and diagnostic questions are.

It is bad practice to perform an exam just because you are asked to do it and will get paid for it! The advent of modern electronic health records reduces the need to directly contact the referring clinician but this is still needed in some cases.

Need more information - “Abdo pain please scan”, “Weight loss and pain”

More reasonable – “Elevated Alk P and bilirubin + RUQ pain - obstruction? stones?”

Should you really be doing a more appropriate diagnostic examination such as US or CT?

The MRI system can be thought of as a complex toolbox and the acquisition sequences as “tools” to address specific questions.

Largely unappreciated - MRI is in fact highly operator dependent and requires that technicians / radiographers / radiologists understand and correctly use the MR “tools” available to them.

MR systems vary widely in capabilities relating to manufacturer, field strength, magnet design, system age and availability of both coils and software packages.

This requires all of the above factors to be taken into account and is linked to the issues discussed below -

Adequate SNR is a requirement to allow you to detect many lesions. Constable & Henkelman demonstrated elegantly in the 80s that as you increase the SNR you decrease the CNR and make lesions more difficult to detect.

SNR is influenced in all acquisitions by coil selection, and the image voxel size – and therefore matrix size, FOV, & slice thickness.

SNR is also influenced by many other factors such as flip angle, TE and TR but these are less likely to be modified in day to day optimisation.

Optimising SNR is more likely to be challenging in upper abdominal imaging where fast breath-hold acquisitions are routinely used.

Coil selection is typically dictated by availability, desired volume coverage and the size of the patient.

Coil selection is important when deploying receive acceleration methods such as SENSE, SMASH, ARC, GRAPPA that rely on suitably positioned multiple coil elements to provide spatial sensitivity variation.

Historically much MRI is performed in the axial plane, almost certainly because of CT but also because this is relatively efficient and the human body is tubular.

Matching different types of contrast sequence in terms of imaging plane, FOV, section and gap thickness is also routinely performed as it allows direct spatially matched comparison of the varying contrast features of a particular tissue or lesion.

Matching is becoming less critical with the use of 3D acquisitions which are now more practical for some abdominal and pelvic examinations.

Is possible with a variety of techniques

Is widely used with T1w sequences to emphasise the effect of iv gadolinium administration

Also has a role in suppressing artefacts from abdominal wall motion during respiratory triggered or navigated acquisitions

Can be used to discriminate between haemorrhage and fat e.g. in ovarian lesions

Normal blood flow can often be confusing and vary widely

Understanding the appearances of fast flow, slow flow and likely thrombus and how this varies with sequence types is important to avoid misdiagnoses.

The lecture will discuss examples of sequence optimisation covering as many of the following topics as possible -

Imaging Solid Organs

T1 sequences

T2 sequences

I/O Phase gradient echo

Balanced SSFP

DWi

Imaging Tubular Organs

Hydrographic sequences

Balanced SSFP

Imaging Blood Vessels

2D gradient echo

Balanced SSFP

3DT1w gradient echo

Phase Contrast

Quantification

Elastography & Liver Stiffness

T2/T2* and Hepatic Iron Concentration

Dixon Sequences and Hepatic Fat Fraction