EDUCATION & DEBATE
Regular Review Magnetic resonance imaging -2: clinical uses Peter Armstrong, Stephen F Keevil
Academic Department of
Radiology, St Bartholomew's Hospital, London ECIA 7BE Peter Armstrong, FRCR, Mercer professor of diagnostic radiology Stephen F Keevil, MSC, honorary research fellow in medical physics BM7 1991;303:105-9
BMJ
VOLUME
303
13
In the first part of this review,' we introduced the basic principles of magnetic resonance imaging. We will now summarise the main established clinical uses of the technique. Magnetic resonance imaging is the most sensitive diagnostic technique for detecting multiple sclerosis (fig 1), being far superior to computed tomography.", 2 A normal magnetic resonance scan of the brain all but excludes the diagnosis of cerebral multiple sclerosis.3 But multiple areas of abnormal signal are non-specific, being seen not only in patients with multiple sclerosis but also in asymptomatic elderly patients4 and in a variety of conditions, notably ischaemic disease. The combination of the clinical features of multiple sclerosis and multifocal, well circumscribed plaques of altered signal with a predilection for the subependymal, periventricular spaces may, however, be very helpful in managing patients suspected of having the disease, though magnetic resonance imaging without the use of gadolinium diethylenetriaminepenta-acetic acid (DTPA) cannot distinguish between active lesions and those that have been present for many years.56 The use of magnetic resonanqe imaging enhanced with gadolinium DTPA to assess the activity of multiple sclerosis lesions appears promising. 'Another important use for magnetic resonance imaging is in showing parenchymal brain infection. Various types of abscess and infectious encephalitis, particularly herpes, are readily diagnosed in T2 FIG 1-Magnetic resonance imaging of head showing high signal weighted images, such images being more sensitive intensity lesions of multiple sclerosis in the cerebral substance, mainly than computed tomography at showing oedema in the periventricular areas. Two ofthe larger lesions are indicated by accompanying inflammation within the brain.7 The arrows. The scan is an axial T2 weighted image (courtesy ofDr Paul ability to show the site and extent of various infections Goddard, Bristol) in the central nervous system with magnetic resonance imaging is likely to be of particular use in patients with ofchoice for diagnosing or excluding the presence of an AIDS.8 arteriovenous malformation (fig 2). Similarly, cerebral infarction can be diagnosed at With the notable exception of acute subarachnoid an earlier stage than with computed tomography, haemorrhage, intracranial haemorrhages are readily largely because magnetic resonance imaging can show shown with magnetic resonance imaging (fig 3). The the extent and shape of focal cerebral oedema. But appearance of collections of blood in magnetic resomagnetic resonance imaging, although more sensitive nance images has attracted considerable attention,9 '0 than computed tomography in detecting early infarcts, and a simplified explanation is given here because the frequently cannot distinguish bland from haemorrhagic appearances of blood clots over time provide an insight infarction in early infarcts, a distinction that can into the complexity of information available from be achieved with computed tomography. Therefore, magnetic resonance imaging. Provided that such computed tomography is preferred to magnetic reso- complexities are understood, they are potentially very nance imaging in those patients who need imaging for useful in reaching diagnostic conclusions. stroke in the first 48 hours. The signal intensity of haemorrhage varies subAneurysms, including those responsible for cerebral stantially with time depending on the local chemical haemorrhage, are sometimes demonstrable by virtue of environment and 'the physicochemical state of the altered signal of fast flowing blood, and magnetic haemoglobin and its breakdown products in the area resonance imaging is particularly good at showing of haemorrhage. Within the first 24 hours a blood arteriovenous malformations, again based on alteration collection has the characteristics of a protein rich water of signal due to blood flow through the malformation. solution (high proton density, long T1 and long T2 Magnetic resonance imaging is so good in this particular relaxation times) and appears similar to normal brain respect that it can replace arteriography as the method or oedematous tissue. Thereafter, oxyhaemoglobin JULY
1991
105
Itf ; t;5it
rounding brain (fig 4). Whether or not this information is clinically useful varies substantially from patient to patient. But magnetic resonance imaging rarely provides less information than computed tomography, whereas computed tomography rarely provides more information than magnetic resonance imaging. Magnetic resonance imaging is therefore an appropriate initial test, obviating the need for computed tomography in most instances. Magnetic resonance imaging, like computed tomography, only occasionally predicts a specific histological process and may not even permit the distinction between such processes as neoplasm and infection. Also, differentiating the tumour itself from surrounding oedema may be difficult, although using gadolinium DTPA may help in this respect. Diagnosing metastases in asymptomatic patients presents a somewhat different problem. Here the ability of magnetic resonance imaging to detect small lesions-because of greater inherent contrast between
FIG 2-Axial Ti weighted image
through mid-cerebellum showing cerebellar artenovenous malformation with the serpiginous flow void characterstc offastflowing
FIG 3-HaeFnirrhage within substance ofright occipital lobea (solid white arrow) tracking over cerebiral hemisphere in_
occipitoparietal region (open
whitearrow). Note the high signal intesiy in the blood clot
due to methaemoglobin shortening TI and therefore causing increased signal intensity
i.fi5 _
'changes to deoxyhaemoglobmn at a rate that depends on
Regular Review Magnetic resonance imaging -2: clinical uses Peter Armstrong, Stephen F Keevil
Academic Department of
Radiology, St Bartholomew's Hospital, London ECIA 7BE Peter Armstrong, FRCR, Mercer professor of diagnostic radiology Stephen F Keevil, MSC, honorary research fellow in medical physics BM7 1991;303:105-9
BMJ
VOLUME
303
13
In the first part of this review,' we introduced the basic principles of magnetic resonance imaging. We will now summarise the main established clinical uses of the technique. Magnetic resonance imaging is the most sensitive diagnostic technique for detecting multiple sclerosis (fig 1), being far superior to computed tomography.", 2 A normal magnetic resonance scan of the brain all but excludes the diagnosis of cerebral multiple sclerosis.3 But multiple areas of abnormal signal are non-specific, being seen not only in patients with multiple sclerosis but also in asymptomatic elderly patients4 and in a variety of conditions, notably ischaemic disease. The combination of the clinical features of multiple sclerosis and multifocal, well circumscribed plaques of altered signal with a predilection for the subependymal, periventricular spaces may, however, be very helpful in managing patients suspected of having the disease, though magnetic resonance imaging without the use of gadolinium diethylenetriaminepenta-acetic acid (DTPA) cannot distinguish between active lesions and those that have been present for many years.56 The use of magnetic resonanqe imaging enhanced with gadolinium DTPA to assess the activity of multiple sclerosis lesions appears promising. 'Another important use for magnetic resonance imaging is in showing parenchymal brain infection. Various types of abscess and infectious encephalitis, particularly herpes, are readily diagnosed in T2 FIG 1-Magnetic resonance imaging of head showing high signal weighted images, such images being more sensitive intensity lesions of multiple sclerosis in the cerebral substance, mainly than computed tomography at showing oedema in the periventricular areas. Two ofthe larger lesions are indicated by accompanying inflammation within the brain.7 The arrows. The scan is an axial T2 weighted image (courtesy ofDr Paul ability to show the site and extent of various infections Goddard, Bristol) in the central nervous system with magnetic resonance imaging is likely to be of particular use in patients with ofchoice for diagnosing or excluding the presence of an AIDS.8 arteriovenous malformation (fig 2). Similarly, cerebral infarction can be diagnosed at With the notable exception of acute subarachnoid an earlier stage than with computed tomography, haemorrhage, intracranial haemorrhages are readily largely because magnetic resonance imaging can show shown with magnetic resonance imaging (fig 3). The the extent and shape of focal cerebral oedema. But appearance of collections of blood in magnetic resomagnetic resonance imaging, although more sensitive nance images has attracted considerable attention,9 '0 than computed tomography in detecting early infarcts, and a simplified explanation is given here because the frequently cannot distinguish bland from haemorrhagic appearances of blood clots over time provide an insight infarction in early infarcts, a distinction that can into the complexity of information available from be achieved with computed tomography. Therefore, magnetic resonance imaging. Provided that such computed tomography is preferred to magnetic reso- complexities are understood, they are potentially very nance imaging in those patients who need imaging for useful in reaching diagnostic conclusions. stroke in the first 48 hours. The signal intensity of haemorrhage varies subAneurysms, including those responsible for cerebral stantially with time depending on the local chemical haemorrhage, are sometimes demonstrable by virtue of environment and 'the physicochemical state of the altered signal of fast flowing blood, and magnetic haemoglobin and its breakdown products in the area resonance imaging is particularly good at showing of haemorrhage. Within the first 24 hours a blood arteriovenous malformations, again based on alteration collection has the characteristics of a protein rich water of signal due to blood flow through the malformation. solution (high proton density, long T1 and long T2 Magnetic resonance imaging is so good in this particular relaxation times) and appears similar to normal brain respect that it can replace arteriography as the method or oedematous tissue. Thereafter, oxyhaemoglobin JULY
1991
105
Itf ; t;5it
rounding brain (fig 4). Whether or not this information is clinically useful varies substantially from patient to patient. But magnetic resonance imaging rarely provides less information than computed tomography, whereas computed tomography rarely provides more information than magnetic resonance imaging. Magnetic resonance imaging is therefore an appropriate initial test, obviating the need for computed tomography in most instances. Magnetic resonance imaging, like computed tomography, only occasionally predicts a specific histological process and may not even permit the distinction between such processes as neoplasm and infection. Also, differentiating the tumour itself from surrounding oedema may be difficult, although using gadolinium DTPA may help in this respect. Diagnosing metastases in asymptomatic patients presents a somewhat different problem. Here the ability of magnetic resonance imaging to detect small lesions-because of greater inherent contrast between
FIG 2-Axial Ti weighted image
through mid-cerebellum showing cerebellar artenovenous malformation with the serpiginous flow void characterstc offastflowing
FIG 3-HaeFnirrhage within substance ofright occipital lobea (solid white arrow) tracking over cerebiral hemisphere in_
occipitoparietal region (open
whitearrow). Note the high signal intesiy in the blood clot
due to methaemoglobin shortening TI and therefore causing increased signal intensity
i.fi5 _
'changes to deoxyhaemoglobmn at a rate that depends on
