Summary of Ordering Guidelines
The following summaries are based on the Canadian Association of Radiologists Diagnostic Imaging Referral Guidelines (http://www.car.ca/en/standards-guidelines/guidelines.aspx). Physicians are encouraged to consult the full guidelines for more detailed information on recommended imaging for the wide range of patient conditions.
Central Nervous System (Brain), Head and Neck Conditions
CT is ALWAYS preferred over MRI for the following indications:
- Assessment of trauma to jaw, face and skull bones
- Assessment of paranasal sinus inflammation
- Initial assessment of scalp and skull lesions
- Rule out intracranial hemorrhage
CT is more efficient and provides satisfactory diagnostic information for the following urgent indications:
- Rule out space occupying lesion
- Rule out hydrocephalus
- Rule out ischemic stroke
- Rule out intracranial hemorrhage (intracerebral, subarachnoid, subdural, epidural)
MRI is preferred over CT for the following indications, but please note that wait time will vary on a case-by-case basis with the strength of the indication (relevant details from the history, physical examination and laboratory studies will greatly assist the triaging radiologist):
- New focal neurological deficit referable to brainstem/cranial nerve(s)
- Rule out encephalitis
- Rule out or follow-up demyelinating disease
- Rule out or follow-up sellar lesion (e.g. pituitary adenoma)
- Rule out or follow-up cerebellopontine angle lesion (e.g. vestibular schwannoma)
Depending on the condition in question, it may still be most appropriate to refer a patient for a consultant’s opinion prior to requesting an imaging procedure. Please consider the following common neuroimaging requests from this perspective:
Headache – Imaging seldom alters the management of patients with chronic or recurrent headache, lacking “red flags” (e.g. rapidly changing pattern, waking from sleep, new neurological deficit, history of cancer or immunodeficiency). Nevertheless, imaging may be important to provide reassurance to patient and doctor regarding potential serious pathology. In this scenario, CT can rule out a mass, hydrocephalus and hemorrhage quickly and accurately.
Intracranial aneurysm – The risk of harbouring an asymptomatic intracranial aneurysm increases steeply for patients who have TWO or more affected FIRST DEGREE relatives. Patients with autosomal dominant polycystic kidney disease are also at markedly increased risk. The choice to screen or not to screen must take into consideration the dilemma that may arise if a tiny aneurysm is found. MRA and CTA are essentially equivalent for screening purposes. MRA is preferable, as it avoids radiation and contrast enhancement. Wait times are long, commensurate with the very low annual risk of rupture of most asymptomatic aneurysms.
Dizziness – The yield of imaging, with CT or MR, in patients with uncomplicated dizziness (i.e. no specific brainstem or cranial nerve findings) is low. Patients with true vertigo are best served by ENT evaluation. Only a subset of these patients will benefit from further assessment with MRI to rule out retrocochlear pathology.
Dementia – Imaging most commonly yields non-specific findings. From a practical point of view, CT can rule out a space occupying lesion. CT can provide evidence in support of the clinical diagnoses of vascular dementia and normal pressure hydrocephalus. MRI is more sensitive than CT for small vessel ischemic changes, if vascular dementia remains a clinical concern after a non-contributory CT examination. Neurological and/or neuropsychological assessment may be the next best step if less common etiologies of dementia that have specific MRI findings are being considered.
Orbital pathology – MRI is best suited to study the soft tissues of the orbit. However, MRI examination is best used in a targeted fashion, either to confirm an abnormality suspected on fundoscopy or to identify an abnormality that is not accessible to fundoscopic examination. In some cases, MRI must address inflammatory or neoplastic differential diagnostic considerations, while in other cases, vascular imaging may be more appropriate. Therefore, ophthalmological referral is the best first step to determine the need for MRI and also to guide the MRI examination, potentially maximizing its yield.
Head and neck masses – CT is routinely used to confirm and characterize cystic and solid masses in the head and neck, and to stage cancers of the head and neck. MRI can accomplish the same without ionizing radiation. However, MRI is more susceptible to motion artefact (swallowing, breathing, vascular pulsation). Both modalities are susceptible to artefact from dental amalgam and appliances. MRI is typically used as a problem-solving tool in patients with a documented abnormality that is incompletely characterized on CT or ultrasound.
Temporomandibular joint pathology – MRI is superior to plain radiography and CT, because it can demonstrate meniscal pathology as well as bony pathology. However, the precise definition of meniscal pathology is most important for the planning of interventions in patients with established TMJ disease. MRI should be reserved for this purpose.