Computed tomography (CT)

CT is now widely available throughout Europe.  Furthermore, there have been recent important advances due to the development of spiral and multi-slice CT, which allow the acquisition of large amounts of data from a single breath hold.  Such advances have opened up new diagnostic opportunities, such as the use of multi-slice CT in the diagnosis of coronary artery disease.  Nevertheless, different hospitals will have their own policies about accepting CT requests.  It is worth remembering that CT imparts a relatively high x-irradiation dose.  Thus it is always worth considering alternatives, especially in view of the increasing role of MRI.  The UK NRPB have published several general recommendations with regard to CT in Protection of the patient in x-ray computed tomography [22], and they are currently reviewing the advice.

Like all radiological requests, any CT referral which falls outside established guidelines should be discussed with a radiologist.  Because of the need to minimise the extent of the examination (and thereby the cost and radiation dose), it is helpful if the clinical notes and previous imaging investigations are available for review by the imaging department at the time of the proposed CT.  

 A few further points:

• CT remains the optimal investigation for many clinical problems within the chest and abdomen, despite the radiation risks.

• CT is still widely used for intracranial problems, especially cerebrovascular accident and trauma.

• CT remains a simple method of staging many malignant diseases (e.g. lymphoma) and of monitoring the response to therapy.

• CT provides valuable pre-operative information about complex masses and is widely used to investigate postoperative complications.

• CT allows accurate guidance for drainage procedures, biopsies, and anaesthetic nerve blocks.

• CT has an important role in the management of trauma.

• CT images may be degraded by prostheses, fixation devices, etc.

• CT provides better anatomical detail than US in obese patients.  In thinner patients and children, US should be used whenever possible.

• CT of the abdomen imparts a radiation dose equivalent to about 500 chest x-rays.

lnterventional radiology (including angiography and minimal access therapy)

This area of radiology is now fully established.  Most abscesses in the abdomen are now treated by percutaneous drainage procedures using radiological guidance.  Likewise, the majority of liver biopsies are now performed by radiologists (using US guidance).  Lymph node biopsies are routine in most US and CT units.  While all departments of clinical radiology have been undertaking angiography and associated procedures (e.g. angioplasty) for many years, new techniques are constantly developing.

New technology is rapidly widening the range of interventional radiology yet further.  Innovations include:

• Percutaneous vertebroplasty for collapsed vertebral bodies

• Percutaneous insertion of grafts for abdominal aortic aneurysms

• Various techniques to treat inoperable hepatic lesions (e.g. radiofrequency ablation under imaging control)

• Interventional MRI with 'real-time' imaging to allow monitoring of therapeutic manoeuvres

These examples of recent innovations require close collaboration with clinical colleagues.  The precise arrangements vary considerably according to local expertise and availability of equipment.  There is continuing discussion at national level about the best arrangement for these interventional procedures.  Inevitably, requests for all such procedures call for detailed discussion involving various specialists.

Magnetic resonance imaging (MRI)  

There has been a substantial recent increase in the number of MRI systems across Europe.  Accordingly, there are numerous recommendations for the use of MRI.  Indeed, with the recent technical advances and increasing experience, the role of MRI continues to expand, and the limiting factor for further expansion is now often financial. 

Since MRI does not use ionising radiation, MRI should be preferred in cases where it would provide information of similar value to that provided by CT (and when both are available).  However, MRI is in danger of being subjected to inappropriate demands, which may lead to long waiting times.  Thus, all requests for MRI should be agreed with a radiologist.  A few further points:

• MRI usually provides more information than CT about intracranial, head and neck, spinal, and musculoskeletal disorders because of its high contrast sensitivity and multiplanar imaging capability.  This helps clinicians to establish the diagnosis and institute appropriate management with greater confidence.  It is increasingly being used in oncology.

• Major recent advances include: breast and cardiac MRI; angiographic and interventional techniques; magnetic resonance cholangiopancreatography (MRCP) and other fluid-sensitive MRI techniques; functional MRI imaging of the brain.  However, many of these techniques await full evaluation.

• MRI is not approved during the first trimester of pregnancy.  However, it may well prove to be safer than some of the alternative options.  All imaging of pregnant women should be discussed with the radiology department.

• There are some definite contraindications to the use of MRI: metallic foreign bodies (FBs) in the orbits, aneurysm clips, pacemakers, cochlear implants, etc.  Furthermore, MRI will give reduced image quality close to prostheses.  The full list of contraindications is provided in several textbooks and monographs.  Any uncertainty about contraindications should be discussed with the imaging department well in advance of the proposed investigation.

Nuclear medicine (NM)  

In some EU countries NM is an independent specialty and the use of unsealed sources of radionuclides for diagnosis and therapy is restricted to NM specialists.  In some countries other specialists, usually radiologists, provide NM services.  Whatever the local arrangements, an experienced specialist will be available to discuss the appropriate NM techniques for a given clinical situation.  The specialist will also be able to advise on which particular NM investigation should be used.

Accordingly, referring clinicians should indicate the precise clinical problem requiring investigation, because this will determine which radionuclide (or alternative) investigation is used.

Despite some misconceptions, the radiation doses imparted by most NM techniques compare favourably with those of many other imaging investigations regarded as 'safe'.  As shown in Table 1 the effective dose associated with most routine NM studies is considerably less than that for abdominal CT.

There is particular value in the functional data that can be provided by NM techniques.  At a basic level, NM can determine whether a distended renal pelvis shown by US is merely due to a capacious collecting system or is caused by an obstructing lesion.  The same investigation can provide data on the percentage of overall renal function provided by each kidney.  More complex studies can indicate the ejection fraction of the left ventricle or the distribution of blood flow to the cerebral cortex.

Positron emission tomography (PET) has recently made large strides, and its availability is gradually increasing.  Because of the short-lived nature of the key radionuclides (the glucose analogue F-18-fluorodeoxyglucose, FDG, is widely used), PET can only be offered within a reasonable distance of a cyclotron and radionuclide pharmacy.  PET can identify small foci of viable tumours, so it offers exceptional opportunities in the staging of various cancers (e.g. bronchus) and in cancer follow-up (e.g. lymphoma), where other imaging techniques may be unable to distinguish between residual fibrotic masses and active disease.  PET can also provide unique data about brain metabolism and myocardial viability, and there are several research units studying these aspects.  Over the next few years there will be an increasing uptake of PET into clinical practice, and its potential use is flagged for certain clinical problems in the ensuing recommendations.  

Nuclear medicine therapy  

Ultrasound (US)

Since the previous edition of these Guidelines, most departments of clinical radiology have experienced a large increase in referrals for US examinations.  During this period US equipment and expertise have advanced and the scope of referrals (colour Doppler, power Doppler, transvaginal gynaecological work, etc.) has widened.  These trends are to be welcomed because US does not employ ionising radiation.  However, there is scant evidence that the increase in US referrals has been accompanied by much reduction in referrals for other radiological investigations and a consequent reduction in total radiation dose to the public.  The one notable exception is the IVU, which is required much less often since the advent of US.  However, because US is non-invasive, the total number of patients investigated with urological problems has increased.  Departments of clinical radiology have developed different local policies for dealing with the increasing US workload.

The actual acquisition of US images has to be undertaken by an experienced operator, although such an operator may not be able to gain perfect images in every patient.  For example, US can be difficult and unsatisfactory in obese patients.

Furthermore, the distribution of bowel gas may mask some features.  Nevertheless, the cheap, quick, reliable, and noninvasive nature of US makes it an excellent initial investigation for a wide range of clinical referrals.  Accordingly, US has been recommended as the investigation of choice whenever appropriate.

Since US avoids ionising radiation and is relatively inexpensive, it is often recommended where more expensive studies (e.g. CT) cannot be justified or resources are limited.  Conversely, it is difficult to refuse a request for US on grounds of invasiveness or expense.  There is thus a danger of US departments being overloaded with requests that may be on the margins of appropriateness.  Referring clinicians therefore still have a duty to consider carefully whether each request for US is justified and whether the result (e.g. the presence of gallstones) will affect management (see Introduction: Why are guidelines needed?).

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