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INSIDE
Basic concepts
Safety and
contraindications
Requesting MRIs
Head and neck
The limbs
The trunk
Conditions better
assessed by CT
the authors
Dr Garvin Williamsz
radiologist, staff specialist, Hunter
New England Imaging, New
Lambton Heights, NSW.
MRI in general
practice
Associate Professor
Peter Stanwell
MRI physicist, associate
professor in medical radiation
sciences, school of health
sciences, University of Newcastle,
Callaghan, NSW.
Introduction
MAGNETIC resonance imaging
is a non-invasive technique that
provides structural and functional
information. Its origins date back
to the 1930s when magnetic characteristics of atomic nuclei were first
described.
However, it was not until 1971
that nuclear magnetic resonance
(NMR) was applied in biomedical
applications, when Dr Raymond
Damadian measured T1 and T2
relaxation times (see ‘Basic concepts’ section) in rat neoplasms and
observed that neoplastic tissue possessed longer T2 relaxation times
than those of normal tissue. At that
same time Dr Damadian correctly
predicted that the NMR technique
might prove useful in the detection
of malignant neoplasms.
In 1977 the first magnetic resonance images of humans were
produced (the ‘N’ was dropped to
avoid negative connotations associated with ‘nuclear’).
Fundamental work in diffusion imaging in 1984 laid the
groundwork for functional MRI
techniques, and in 1986, the first
reports of the calculation of diffusion coefficients occurred. In the
following years, diffusion tensor
imaging and functional MRI using
blood oxygenation level-dependent
techniques were also developed.
Today, an ever-growing array of
novel techniques for MRI is currently under investigation to increase
the number of structural and functional (eg, tissue angiogenesis, tissue ultrastructure) correlations that
can be assessed using magnetic resonance-based techniques. MRI does
not use ionising radiation, thus in
situations where MRI provides
similar, or improved, information
compared with ionising radiation
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techniques (eg, CT, X-ray, nuclear
medicine) MRI may be a safer
choice. This is particularly relevant
to children and young adults who
are at greater risks of harm from
ionising radiation than older adults.
In this article we aim to provide
an overview of the physics behind
MRI scanning, an understanding
of the techniques used to enhance
the effectiveness of MRI, how and
when MRI may be used to assist
in a patient’s management and the
potential adverse events and safety
issues that should be addressed.
cont’d next page
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Australian Doctor
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27 June 2014 | Australian Doctor |
21
How To Treat – MRI in general practice
Basic concepts
Physical basis
MRI produces cross-sectional
images that bear some similarity to
those of CT scans, but the physical
foundation underlying both techniques is entirely different. MRI
is based on signals derived from
the protons associated with tissue
water and, to a lesser extent, fat.
The images are essentially maps
of the spatial distribution of water
and fat within the body.
Energising and aligning protons
The physical principle underlying MRI is the weak magnetic
property of the hydrogen proton.
When protons experience a strong
magnetic field, there is a tendency
for them to spin (‘precession’) and
align with an external magnetic
field. When a brief radiofrequency
pulse is applied with a frequency
matching that of the hydrogen
protons (‘nuclear resonance’), the
net magnetisation direction of the
protons is altered, and the protons
are transiently in phase (‘phase
coherent’). A weak electronic signal is associated with these perturbed, or ‘energised’, protons.
During an MRI scan the patient
is placed in a large static magnetic
field causing some of the patient’s
own protons to align with the
static magnetic field (about one in
1012 protons). These protons are
then energised with the application of a radiofrequency pulse of
short duration. Once this pulse is
switched off, the energised protons
emit a characteristic signal specific to their chemical composition
that is detected as the MRI signal.
Smaller magnetic fields are used to
localise the origin of the detected
signal (figure 1).
Table 1: MRI techniques and explanations
Imaging technique
Explanation
T1 weighting
A T1-weighted image is obtained at short repetition time (TR) and short echo time (TE), resulting
in structures having short T1 relaxation times appearing with the highest signal intensity.
Therefore tissues with a high lipid content (eg, normal adult bone marrow, normal subcutaneous
fat) will appear brighter on a T1-weighted image
T2 weighting
A T2-weighted image is obtained at long TR and long TE, resulting in structures that have a
long T2 relaxation time, ie, having higher signal intensity than structures with short T2 relaxation
times. Therefore tissues with a high water content (eg, CSF, pathological tissue oedema) will
appear brighter on a T2-weighted image
STIR (short tau
inversion recovery)
STIR is an imaging sequence that results in suppression of magnetic resonance signal from fat.
It is used extensively in body imaging (thorax, abdomen, pelvis) and musculoskeletal imaging
showing pathological oedema change in marrow and soft tissue and provides a high degree of
contrast
Gadolinium chelate
Gadolinium chelates are the magnetic resonance contrast agents that are most commonly
used in a clinical setting. Gd3+ is a paramagnetic ion that markedly shortens T1 and T2 times of
hydrogen protons, even in low concentrations
Magnetic resonance
angiography (MRA)
MRA is a technique that provides a non-invasive method for imaging the vascular and cardiac
systems. The most common form, time-of-flight (TOF) MRA selectively images blood flow while
suppressing signal from stationary tissue to provide three-dimensional images of blood vessels
Diffusion-weighted
imaging (DWI)
DWI is an MRI technique that maps molecular movement of water molecules. Using DWI in
stroke, ischaemic regions can be detected as early as 15 minutes after arterial occlusion. More
recently, DWI has been shown to improve tumour detection and can serve as a non-invasive
biomarker of tumour aggressiveness
Perfusion-weighted
imaging (PWI)
PWI is an MRI technique that allows for non-invasive, high-resolution measurements of tissue
perfusion at the microvascular or capillary level
Diffusion tensor
imaging (DTI)
DTI is an MRI technique that measures and maps the orientation of myelin fibres on the basis of
their anisotropy. DTI has been employed to study the integrity of white matter in the brain during
disease processes (eg, brain tumour, stroke, traumatic brain injury)
Magnetic resonance
spectroscopy (MRS)
Proton MRS is the most commonly used magnetic resonance-based method to non-invasively
evaluate metabolic changes in the human body. Proton MRS aims to help characterise tissues
by assessing biochemistry in vivo
Blood oxygen level
dependent (BOLD)
functional MRI (fMRI)
BOLD–fMRI is a technique used to obtain maps of human brain activity on the basis of signal
changes during resting and stimulated sensory states. This is accomplished by exploiting
the magnetic differences between oxygenated and de-oxygenated blood and neural tissue
utilisation of oxygen during task activation
Excitation
Emission
Figure 1: Physical principles in
creating an MRI image. An external
magnetic field is applied to the
patient with a brief radiofrequency
pulse that transiently aligns the
hydrogen protons in the body. When
the external field is switched off, the
perturbed protons send out a weak
electronic signal that is picked up and
translated into the image, by the MRI
scanner.
T1 and T2 parameters
As soon as the matched external electronic pulse (the applied
radiofrequency pulse) stops, the
inherent signal from the perturbed
protons begins to decay. The signal loss is the result of two independent factors:
• T1 relaxation time (or spin–lattice relaxation), where protons
begin to realign with the static,
external magnetic field.
• T2 relaxation time (or spin–spin
relaxation), where interactions
between nearby molecules disrupt phase coherence.
Both processes take place independently of each other and
undergo exponential kinetics.
Table 1 lists the common imaging
techniques used in MRI and a brief
explanation of their respective uses.
Applying to human body images
To create magnetic resonance
images from the human body it
is necessary to encode these weak
signals spatially to obtain spatial information about the origin
(position in three-dimensional
space) of detected signals. This is
accomplished by superimposing
smaller magnetic fields (gradient
magnetic fields) in addition to the
external magnetic field to allow
accurate localisation of the decaying signals in three-dimensional
space. These detected signals are
then mathematically transformed
(Fourier transformation) into the
recognisable magnetic resonance
images based on a grey scale,
where traditionally black codes
for zero (‘absence of’) signal, and
white codes for maximal signal.
Imaging techniques
These fundamental processes allow
for the generation of images, using
different types of ‘sequences’ (T1,
T2, diffusion-weighted imaging,
magnetic resonance angiography).
These sequences generate differing
tissue contrast to show anatomy
and where present, associated
pathologic change. Imaging is
undertaken in any plane — transverse (or ‘axial’), coronal, sagittal,
oblique — using two-dimensional
and three-dimensional (or volume)
techniques. The lack of ionising
radiation as well as the combination of demonstration of the interrogated body part using numerous
contrast mechanisms with multiplanar imaging, represent many
of the strengths of magnetic resonance imaging.
Practical considerations
Keeping patient still
In comparison with CT, patient cooperation or immobility is much
more important. Most MRI tests
need a series of separate ‘acquisitions’ or ‘sequences’ (at least three,
commonly 4-6, sometimes as
many as 8-10). Each sequence typically lasts 3-4 minutes, sometimes
longer. Additional dynamic contrast-enhanced acquisition demonstrating contrast flow or perfusion
can significantly add to the examination time. It is essential that the
patient remains still during these
periods of image acquisition, as
the slightest motion degrades the
image acquisition process, and can
necessitate the rescanning of all, or
part, of the study. Sedation, and
general anaesthesia with young
children, may be required when
clinically indicated.
Patient monitoring
Patient monitoring is also more
challenging in magnetic resonance scanning compared with
other imaging modalities, but is
not impossible. The monitoring
devices and instrumentation taken
into an MRI scanner room must
be MRI-compatible, where no ferrous metal-containing component
is present.
Safety and contraindications
Safety
LIMITATIONS of MRI include
time constraints and safety concerns
related to the strong external magnetic fields, applied radiofrequency
pulses and rapidly switching gradient magnetic fields. A safety check
will always be performed on-site
at the radiology practice before
a patient proceeds to MRI. However, it is recommended that GPs
complete a simple pre-MRI safety
checklist in the GP rooms to limit
patients encountering problems on
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the day of scanning (see ‘Pre-MRI
safety checklist’ box).
Potential adverse events
Strong external magnetic fields
may cause a ‘missile effect’, where
loose metallic objects are rapidly
attracted by the strong external
field and pulled toward the scanner
bore, even from within the patient’s
body. Human body tissue may heat
up from the applied radiofrequency
pulses. Strict guidelines govern the
amount of energy deposition, but
care should be taken when nonferrous metallic implants (eg, joint
replacements) or patients with
impaired thermoregulation are
scanned. Rapidly switching gradient magnetic fields may cause
unexpected and uncomfortable
peripheral nerve stimulation, while
induced voltages may have harmful effects on implanted wires (eg,
cardiac pacing leads, deep brain
stimulators). Care must be taken
to carefully assess each patient and
attendant entering the vicinity of
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the MRI system for metal within,
or on the body.
Safe objects
Many objects, including most surgical clips and orthopaedic hardware are composed of non-ferrous
metals (eg, stainless steel, titanium)
that are not significantly attracted
to a magnet, and thus do not pose
a significant risk to the patient.
Most cardiac pacemakers and defibrillators, among other devices (eg,
cochlear implants, certain aneurysm
clips), are contraindications to MRI
and fatal incidents have occurred
where proper procedures have not
been followed.
Lists are available regarding
the safety of implanted medical devices, and all MRI centres
have specific screening procedures before undertaking MRIs
(see Online resources). Likewise,
objects to be taken into the vicinity of the magnet must be carefully
screened as ferromagnetic objects
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How To Treat – MRI in general practice
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can become dangerous projectiles
in the MR scanner environment.
Noise and claustrophobia
The MRI scanning environment
is noisy owing to mechanical
vibrations inherent with the scan
technique. While being scanned,
earplugs are used to attenuate
noise levels. Some patients may
prefer headphones with music
to minimise the impact of the
mechanical sounds, and thus help
with feelings of claustrophobia.
About 4% of patients experience
feelings of claustrophobia and
cannot tolerate the enclosed space
of the MRI scanner.
Contrast media
IV contrast agents are sometimes
required for MRI and are commonly needed when detecting or
analysing characteristics of neoplastic or inflammatory lesions.
Contrast agents used in MRI (ie,
gadolinium chelates) are different from those used with CT and
X-ray studies (ie, iodinated contrast media) and have lower rates
of anaphylactic reaction. They
also are given at much smaller volumes than those used for CT.
Gadolinium-based
contrast
media are mainly based on low
molecular weight chelates of the
gadolinium ion (Gd3+). They are
used to increase the conspicuousness of certain tissues, abnormalities or disease processes during MRI
scans. In the intravascular space,
these agents are used to shorten
T1 relaxation time in the proton,
mainly in the water molecule and
Figure 2:
Gadopentetate
dimeglumine is
one of several
gadolinium
chelates that are
used as contrast in
MRI, as opposed to
iodine that is used
as contrast in CT.
Pre-MRI safety checklist
Does the patient have any metallic
implants? If so, are these surgical
or traumatic?
Are there any implanted
devices, such as pacemakers or
neurostimulators?
Have any wires been left in the
patient?
Has the patient had a previous eye
injury, which may result in a metallic
foreign body being present in the
orbit or globe?
Does the patient have significantly
impaired renal function or risk
factors for administration of MRI
contrast agents?
Is the patient claustrophobic?
thus demonstrate tissue enhancement on T1-weighted imaging
sequences, where water(oedema)
abnormally accumulates. Gadolinium-based agents are usually
injected intravenously, but may also
be injected intra-articularly for some
examinations, especially to examine
small structures in joints, such as the
shoulder, hip, elbow and wrist.
If contrast is clinically necessary
to gain significant further information, normal renal function
ascertained by measuring eGFR
is used to help guide the decision.
If the patient’s eGFR is below
30mL/minute/1.73m2 then IV
gadolinium chelates are generally
not given. Some centres may give
a ‘half dose’ if eGFR is between
30mL and 60mL/minute/1.73m2.
It is always helpful to have
recent renal function test results
available before the patient
tating condition nephrogenic systemic fibrosis (NSF) if they are
given IV gadolinium chelates as
MRI contrast agents. Deaths have
been reported.
NSF presents typically with
skin abnormalities from the day
of exposure, although it can
take months before changes are
seen. These abnormalities may
include pruritus and erythematous
plaques with oedema and induration. Manifestations in the skin
are usually symmetrical, affecting
the extremities and trunk and can
eventually lead to joint contractures. Multiple organ involvement
can result in severe morbidity, and
while not directly causing death, it
may be a contributing factor.
Pregnancy
attends for the MRI test — especially in the outpatient or private
practice setting: if the eGFR is low,
the MRI provider may need to discuss relevant pretreatment days
before the patient attends for the
scan, or they may need to advise
other options for investigation of
the pathology in question.
Renal risk
There is a low risk of aggravating renal impairment associated
with using IV contrast in MRI. It
is important that GPs inform the
radiology practice if the patient
has significant renal impairment
(ie, eGFR <30mL/minute/1.73m2).
As mentioned above, patients
whose eGFR is between 30-90mL/
minute/1.73m2 may need additional management before their
MRI with contrast. This prior
management may include additional fluid intake and renal
function monitoring, with repeat
serum creatinine testing 24-48
hours after the contrast injection.
Nephrogenic systemic fibrosis
Patients with severe renal disease
are at risk of the rare but debili-
There are no known adverse
effects of MRI during pregnancy.
However, if the MRI is not clinically necessary, it is sensible to
wait until after pregnancy, or at
least until after the first trimester.
Similarly, contrast-enhanced studies are generally avoided during
pregnancy unless absolutely necessary.
Paediatric patients
MRI is more suitable for the paediatric population who are more
sensitive to ionising radiationinduced side effects (including
cancer development). The only significant consideration is that sedation or general anaesthesia may be
required to minimise any motion
artefact and an anaesthetist or
relevant qualified medical practitioner is needed to administer this.
Requesting MRIs
MRI provides excellent contrast
resolution and is able to produce
detailed images of soft tissue (eg,
brain, nerves, cartilage, tendons,
ligaments) making it a superior
modality in some clinical situations. However, it should be cautioned that the more sensitive the
imaging modality, the more likely
it is to find an incidental ‘abnormality’ that is not clinically relevant and will not progress to
having a clinical manifestation.
Reports may be more
relevant and accurate
in some cases if the
GP discusses the case
with the radiologist
before MRI referral.
Getting the most out of MRIs
When requesting MRIs, GPs
should ideally specify the clinical
questions that the MRI is expected
to answer and offer a probable
diagnosis (or differential diagnoses) based on clinical findings. A
decision to request a test usually
follows on from analysis of presenting symptoms and the relevant
clinical examination. The purpose
of the MRI should be clear, such
as to gain more information to
confirm a clinical impression and
aid in decision-making, including the potential need for further
investigations and management.
Reports may be more relevant
and accurate in some cases if the
GP discusses the case with the
radiologist before MRI referral.
For instance, such a discussion
may determine whether IV or
intra-articular contrast agents
are required for a particular MRI
scan.
Equally important to consider
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| Australian Doctor | 27 June 2014
and address are the imaging expectations of patients before referral.
Patients need to be advised that
structural changes detected on
MRI do not necessarily correlate
well with symptoms.
ALARA principle
Choosing medical imaging tests
that do not use ionising radiation
as the basis of the technology is the
ideal, and applying the “as low as
reasonably achievable” (ALARA)
principle when using imaging as
an investigation tool should be a
primary goal for everyone. There
is always a preference to minimise
any ionising radiation (eg, with an
X-ray, CT or nuclear medicine) in
the younger age group, especially
children, as all forms of ionising
radiation are cumulatively additive
during a lifetime, with associated
increased risk of radiation-induced
side effects (see Online resources for
recommendations and protocols).
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Billing Medicare
Medicare rebates have recently
been extended to cover a limited
range of requests from GPs. The
RACGP has produced a guideline
for its members to use when deciding whether an MRI is appropriate
and eligible for a Medicare rebate
for the condition being investigated (see Online resources). Each
item number has a clause using age
as a condition, where eligibility
may be decided based on whether
the patient is younger or older
than 16 years.
Each item also has two separate fee schedules: ‘K’ or ‘NK’.
This classifies equipment according to its age and the location of
the practice (eg, remote). Older
equipment can be exempted from
the age requirements if it is located
in a remote area. Every piece of
diagnostic equipment has a ‘life
age’. With MRI, this is 10 years
(and maximally 15 years if regular
upgrades have been maintained).
The K schedule item (for newer
equipment) is close to twice the
NK (older equipment) rebate per
item.
Medicare rebates are only payable to an imaging-accredited
provider. This includes scanners
available in public hospitals as well
as in private practice. Not all scanners are fully eligible for all MRIrelevant Medicare rebates. Thus
patients should ask the radiology
practice whether their particular test is eligible for a Medicare
rebate and whether there may be
an out-of-pocket payment.
The
following
discussion
includes imagings that receive a
Medicare rebate when referred
by a GP (many are age-limited to
those under the age of 16) and
ones that do not (eg, stroke, base
of skull soft tissue, neurological
conditions such as MS, thoracic
and lumbosacral spine, and surveillance or staging MRI for the
chest and abdomen).
Head and neck
Unexplained headache or seizures
IN children and adolescents under
the age of 16 with an unexplained
headache, the diagnosis of exclusion is commonly a space-occupying lesion, typically a neoplasm.
Many neoplasms can be detected
with CT if they are large or causing
major mass effect. However, some
lesions may be difficult to discriminate from normal adjacent brain
tissue on non-contrast CT, which
may commonly be improved with
the use of an iodinated contrast
agent.
If imaging findings are still negative following contrast-enhanced
CT (as can be the case with poorly
perfused or very small lesions) the
subtle morphological tissue alterations associated with these types
of lesions are often better depicted
with MRI.
Detection is the first objective
in suspected cases of intracranial
neoplasm. The second objective
is the analysis and characterisation of the neoplasm if present.
However, not every neoplasm has
a specific signature. Most importantly, despite the high prevalence
of headache, neoplasm as a cause
is extremely uncommon.
Generalised seizures are also not
commonly associated with abnormal imaging findings. With both
these potential diagnoses, a CT
scan when indicated is often the
first test performed and a good
screening test. But in patients with
chronic intractable symptoms,
a negative MRI result may give
an extra level of assuredness that
there is no actively treatable lesion.
Sinuses
A Medicare rebate is also available for GPs requesting an MRI
of the sinuses where conservative
management has failed. It is typically necessary to first perform a
CT scan, as bony sinus margins
Figure 3: T2weighted axial
brain scan
demonstrating
excellent
differentiation
between grey and
white matter.
can be difficult to perform. It is
also difficult to check if the patient
has a biomedical implant if the
level of consciousness is decreased.
Other neurological conditions
Neurodegenerative disease; MS;
and metabolic, inflammatory,
infectious and toxic conditions
can all be seen in good detail on
tailored MRIs.
Some pathologies of the neck,
especially the thyroid, can be well
demonstrated with ultrasound.
MRI can also be used to facilitate
accurate needle biopsy of neck
masses.
Cervical spine
are poorly seen with MRI. The
diagnoses of exclusion are occult
neoplasms, and atypical (eg, fungal) infections.
Suspected pathology of the
brain, skull base, soft tissue of
the neck
MRI affords excellent soft tissue
detail; it is able to differentiate
between grey and white matter,
midbrain, cerebellum and brainstem, and the structures in the spinal cord in detail that cannot be
matched with CT (figure 3).
Lesions at the skull base, pharynx,
paranasal regions and oral cavity,
their presence and extent of infiltration, are best defined with MRI due
to the demonstration of detailed
anatomy and subtle changes with
much of the pathology.
Stroke
MRI can assess very early ischaemic changes in a stroke. At the
same time, arterial vasculature can
also be assessed. Compared with a
special (and expensive) high-end
‘perfusion CT’, MRI (including
perfusion MRI) can better evaluate the ischaemic core for irreversible damage and the ischaemic
penumbra for hypoperfused tissue
at risk of irreversible damage and
can help in aiding with prognosis.
In any event, perfusion CT with
analysis may not be available in
hospitals without an acute stroke
unit.
However, motion can significantly degrade image quality and
in stroke patients who may have
involuntary movement or are unable to follow commands an MRI
In patients younger than 16, a
Medicare-rebatable MRI may
be requested following an X-ray
where there is: significant and/or
clinically suspected trauma; unexplained neck or back pain with
neurological signs; or unexplained
back pain where significant pathology is suspected.
In patients over the age of 16, suspected trauma and cervical radiculopathy are the indications for an
MRI scan.
While an MRI may provide more
details on the soft tissue structures
in the neck, the protocol for trauma
in ED usually requires ‘clearance’
of significance fracture with a CT
first. Trauma may also be initially
screened with a single lateral cervical spine X-ray, so despite the
given indications, MRIs are not the
first-line investigation for suspected
trauma.
In MRI investigation of the
patient with radiculopathy, assuming ‘red flag conditions’ have also
been assessed (including tests with
other modes of imaging), MRI is
particularly relevant in excluding
a major surgically treatable cause,
such as a large disc herniation.
This can sometimes be difficult
to detect even with a good CT
scan. This is particularly the case
in the lower cervical region, below
C4-5, as a result of scan artefacts
from shoulders, especially in large
body habitus.
False-negative and false-positive
findings
A full MRI scan series can be long
and any patient motion can compromise image quality. This may
lead to a false-negative diagnosis
in cases of disc herniation or cord
oedema because motion can create
artefacts in critical areas (eg, in the
spinal cord) and subtle imaging
changes may not be identified as
a result.
Furthermore, ageing creates
its own morphological (eg, spur,
osteophyte and ‘disc bulge’) and
physiological changes (increased
facet joint fluid and peri-articular
‘degenerate cysts’), which can be
variably reported as observations
or as significant findings.
However, these findings do not
always correlate with the cause
of the presenting symptoms, especially if an explanation for pain or
radiculopathy is sought, giving rise
to false-positive radiological findings.
Effect on management
Finally, even if the patient were to
have typical signs of radiculopathy and a corresponding imagingdemonstrated disc herniation,
conservative management is still
commonly preferred because a
small percentage of patients with
non-sequestrated herniated discs
(especially in the lumbar region)
show spontaneous, sometimes
complete regression over time
(weeks to months).
At this time, there are no Medicare rebates for GP-referred MRI
scans of the thoracic or lumbosacral regions.
The limbs
Knee
Figure 4: Series
of four images
from a knee MRI
demonstrating the
lateral and medial
menisci (top left
and bottom right),
and anterior and
posterior cruciate
ligaments.
Patients under 16
FOR patients under 16 years of
age the clinical indication for an
MRI according to Medicare is suspected “internal derangement” of
the knee joint. There is no clear
and universally accepted definition of what “internal derangement” entails clinically. It is a
catch-all phrase with a similar
implication in meaning as the term
‘lumbago’ (and even ‘sciatica’)
has to the back; that is, it simply
requires that something abnormal
and potentially surgically treatable must be present within the
joint. The generally accepted paradigm when determining whether
a presentation satisfies this clinical requirement is a finding of
significant pain and/or instability
implying significant cruciate and/
or meniscal injury which may need
surgical intervention. This item
also mandates an X-ray be performed before MRI.
under Medicare is to assess the knee
joint following acute knee trauma,
specifically to exclude acute meniscal and ACL tears, where a clear
clinical diagnosis cannot be made
(see figure 4).
An X-ray is not mandatory before
the MRI, although radiologists do
commonly prefer to request one for
correlation with the MRI, to assess
the status of the knee, to define
additional changes of arthritis, the
presence of small bony avulsions or
calcified intra-articular structures, or
loose bodies, which may be difficult
to define with MRI.
Again the prevalence of morphological changes in the soft tissue of the knee and relevant bony
structures increases as one ages,
and can be demonstrated on MRI
images (eg, bony spurs, cartilage
degeneration, subchondral degenerate cysts). However demonstration of an ‘abnormality’ does not
imply it is symptomatic.
Hip
Patients over 16
For patients over the age of 16, the
main clinical indication for an MRI
Medicare rebates apply for MRI
in patients under the age of 16
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27 June 2014 | Australian Doctor |
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How To Treat – MRI in general practice
from previous page
after an X-ray has been performed
to specifically exclude suspected
septic arthritis, slipped capital
femoral epiphysis (SCFE) or suspected Perthes’ disease. The initial
X-ray may not show early SCFE
or early Perthes’ changes and typically does not show an effusion
or bone changes in a septic hip.
However, these conditions commonly show changes associated with
abnormal marrow oedema as the
earliest signs, which are well demonstrated with MRI.
Elbow
Under Medicare, a rebate is available for MRI in patients under the age
of 16 after an X-ray has been taken,
specifically to exclude a fracture or
avulsion that can significantly alter
management. MRI will best define
a cartilage fracture, as this structure
is not seen on X-rays nor is it well
defined with CT.
Wrist
Magnetic resonance examination
attracts a Medicare rebate specifically to exclude a scaphoid fracture
post X-ray in patients under the age
of 16. It can take 7-10 days to demonstrate early X-ray changes in an
initially occult scaphoid injury and
an MRI may be requested if there
is strong clinical suspicion of a fracture despite a normal X-ray. Other
imaging modalities (eg, radionuclide
bone scan) may be ordered for this
as early as 6-8 hours after the injury.
MRI can also clearly define the soft
tissue or ligamentous component of
a significant injury.
Other musculoskeletal
The many joints in the body are
eminently suitable to be scanned
using MRI, for a variety of degenerative, traumatic and inflammatory conditions, including early
changes of ischaemia and infection.
The trunk
Chest
Loco-regional
gynaecological
cancer staging can be accurately
assessed with MRI. These include
uterine, cervical and increasingly,
ovarian cancer.
MRI is also an important investigative tool in local rectal cancer
staging and anal fistula mapping.
With the ever-increasing trend to
more conservative management
and surveillance for prostate cancer,
prostate MRI is playing an increasingly significant role in watchful
waiting.
There may be a time in the near
future, where a short ‘prostate
screening MRI’ examination will
be cost-effective in detecting and
localising small lesions that would
be suited to targeted MRI-guided
biopsies, needing 1-2 directed needle passes. This may overcome the
need for multi-pass transrectal ultrasound-guided biopsy, now the most
common method for histological
assessment of an abnormally raised
PSA level.
MRI can be used to aid prostate
evaluation and staging if a needle
biopsy result shows malignancy. It
may help to eventually minimise the
overtreatment of the lower grade
prostate cancer.
Although not used to look for
metastatic bony disease, there
is current research into whole
body skeletal MRI (with limited
sequences), which can demonstrate
bony spread.
A radionuclide bone scan is still
typically the first investigation of
choice for this indication.
investigation and not usually the
first-line imaging test for a suspected fracture in typical clinical
presentations.
The densely compact cortical
bone creates a rigid meshwork of
bound proteins in the hydroxyapatite matrix. Though the proton
‘seen with MRI’ is still stimulated
in the magnetic field, its ‘signal
energy’ is rapidly dissipated within
the tightly bound structural matrix
and no significant measurable signal is emitted. The lack of measurable signal then renders this
structure black on most magnetic
resonance images.
In contrast, cancellous or medullary cavity bone has abundant
fatty and/or red marrow with
water that contains freely excitable protons. The fatty and/or red
marrow as well as any diseaseassociated alteration in this structure can be demonstrated well
with MRI. For example, where the
mass of a neoplasm involves marrow and breaches cortical bone,
MRI is able to demonstrate the
distinctions between intraosseous and extraosseous soft tissue
components well, with the distribution of the neoplastic mass and
adjacent intramedullary reactive
oedema being seen clearly. This
is more accurate in showing the
cont’ page 28
MRI may be used to evaluate congenital cardiac and great vessel disease. Additionally, cardiac masses
can be better defined with MRI if
echocardiography is insufficient.
Recently, there has been ongoing
research into MRI assessment of
pulmonary pathology, particularly
in children (eg, cystic fibrosis).
Breast
There is excellent evidence supporting the use of breast MRI in screening the small population of high-risk
women who are BRCA gene mutation carriers (a Medicare rebate is
available for patients up to the age
of 50).
Recent meta-analyses and reviews
have shown however, there is no
long-term benefit in performing an
MRI in patients who have diagnosed breast cancer or have had
surgery for breast cancer.
Abdomen
Liver and biliary tree
MRI provides particularly good
access to the liver. It allows for the
evaluation of focal or diffuse lesions,
and helps better characterise solitary
masses and detects subtle lesions in
organs that are already severely diseased (eg, in cirrhosis).
Contrast that is excreted by
hepatocytes may be given to provide
additional analysis in characterising
the intrahepatic lesion.
MRI can help demonstrate a
pathologically dilated biliary tract
and define a primary biliary cause
or a secondary cause such as an
extra-biliary mass or stricture, even
at the pancreatic level. IV contrast is
not always needed.
Bowel
Small bowel disease has always
been difficult to assess with imaging, apart from traditional barium
‘small bowel follow-through’. MRI
investigation in conditions such as
Crohn’s disease is valuable, particularly in the younger age group
who are more susceptible to the
cumulative harms of ionising radia-
tion. MRI allows more regular and
frequent scanning in patients whose
conditions are difficult to manage.
Obstetric
Fetal MRI is increasingly being used
in the second and third trimester to
better evaluate ultrasound-detected
morphological abnormalities. MRI
can also be used in the assessment
of the position of certain grades of
placenta praevia to aid with management of delivery.
Pelvis
Conditions better assessed by CT
Intracranial bleeding
SMALL volumes of recent extraaxial bleeding can be difficult to
detect with MRI, even with specialised sequences. Also, if the
patient cannot keep still because
of bleed-related central neural
irritation, the motion can easily
result in artefacts during the image
acquisition.
Older residual products of
bleeding within the brain are
better seen with MRI, as well as
chronic low-volume recurrent subarachnoid haemorrhage. The latter ‘coats’ the brain surfaces with
haemosiderin, a by-product of
red cell breakdown, which is well
defined on MRI.
Acute intracranial blood typically still relies on CT to demonstrate acute change, particularly if
the bleeding is subarachnoid and
of a small volume. Of course, CT
can be negative if there is insufficient active bleeding. Therefore,
even with a negative CT a diagnostic lumbar puncture may be neces-
26
| Australian Doctor | 27 June 2014
sary if it is clinically indicated. If
the haemorrhagic focus is large
enough, it can be detected with
MRI days to weeks after the initial
event.
Bone
MRI is not primarily used to diagnose a fracture. It is an expensive
www.australiandoctor.com.au
Online resources
Department of Health
Medicare eligibility criteria for MRIs
www.health.gov.au/internet/main/
publishing.nsf/content/di-factsheetmri
The Alliance for Radiation Safety in
Pediatric Imaging
Protocols and recommendations
imagegently.dnnstaging.com/
Procedures/InterventionalRadiology/
Protocols.aspx#1989769-protocolrecommendations
RACGP
Clinical Guidance for MRI Referral
www.racgp.org.au/download/
documents/guidelines/mri%20
referrals/mrireferrals_complete.pdf
RANZCR (Royal Australian and
New Zealand College of Radiology)
Information on various imaging
tests, including MRI
www.insideradiology.com.au
How To Treat – MRI in general practice
from previous page
pathology compared with a radionuclide bone scan or CT and commonly shows greater extension
than on a plain X-ray.
Undisplaced fractures may be
detected on an MRI by visualisation of the associated sub-periosteal oedema, or marrow oedema
or bleeding, as a result of trauma
and early healing. The fracture line
itself is better defined with X-ray
or CT, although if the separation
is sufficiently wide (ie, millimetres)
a gap in the dense cortical bone
may be demonstrated with MRI.
Some bony neoplasms are
detected after symptomatic presentation and X-ray examination. Analysis of bone lesion patterns is still best
assessed with radiographs and CT,
to evaluate fine detail of margins
and the patterns of destruction (eg,
pure lytic, with or without a sclerotic margin, ill-defined permeative
margin or periosteal bone reaction).
As discussed above, the medullary
extension and infiltration into the
soft tissue compartments is better
defined with MRI.
Air in the lungs, which has rapidly
moving particles, has little proton
content per unit volume relative
to adjacent soft tissue. This means
that air cannot be detected on
MRI and is coded black in images.
Normal interlobular septae are
barely visible on high-resolution
CT. Even moderate pathological
thickening and small masses may
not be clearly seen on an MRI
because it takes several minutes to
acquire each MRI scan (as opposed
to sub-second scanning for a CT
per slice) and the long acquisition
time results in respiratory motion
blurring artefacts. These artefacts
prevent clear definition of fine
detail and small structures.
Although large masses and
consolidation, multiple small- to
moderate-sized (1-3cm or larger)
masses and thickened pleural disease (including fluid) may also
be visualised and analysed using
MRI, CT is still the preferred
imaging modality to diagnose and
stage cancers involving the lung.
High-resolution CT in particular is
the modality of choice in assessing
interstitial disease patterns.
functional and anatomical imaging.
There continue to be developments in techniques of scanning,
usually in an attempt to reduce
acquisition time, improve resolution and assess other technological
parameters that may help improve
the accuracy of the diagnostic result.
Today, MRI serves as a powerful
adjunct to more traditional medical
imaging solutions and in certain clinical conditions, is now the primary
diagnostic modality.
Lung
Conclusion
MRI is most frequently used in the
investigation of disease in the musculoskeletal system, CNS, soft tissue of the neck/spine, certain pelvic,
breast and some abdominal areas.
Although MRI scanning is excellent
at demonstrating anatomy in exqui-
site detail, in most parts of the body,
its impact on clinical outcomes is still
evolving.
Since its initial clinical use in the
early 1980s, MRI has revolutionised the diagnosis and treatment
of a wide variety of medical con-
ditions. Over the past 30 years,
different MRI techniques, pulse
sequence acquisition strategies and
novel hardware components have
been developed to shorten image
acquisition times, improve image
quality, and facilitate advanced
Instructions
How to Treat Quiz
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MRI in general practice
— 27 June 2014
1. W
hich TWO statements are correct
regarding the basic physics behind MRI?
a) MRI uses ionising radiation
b) In an MRI, different signals are acquired
because the energised protons emit a
characteristic signal specific to their chemical
composition
c) MRI captures images by magnetising iron
atoms in the body and sensing their orientation
d) Black represents absence of signal, white
maximal
2. W
hich THREE statements are correct
regarding the imaging techniques used in
MRI?
a) Water is dark on T1-weighted images but
bright on T2-weight images
b) Diffusion-weighted images can detect arterial
occlusion of parts of the brain by mapping
molecular movement of water molecules
c) Gadolinium chelates work as contrast agents
in MRI by shortening both T1 and T2
d) Magnetic resonance angiography is a
technique that uses intra-arterial injection of
contrast to show areas of poor perfusion
3. W
hich TWO statements are correct
regarding the practical considerations
when performing an MRI?
a) Fixed implanted devices such as cardiac
pacemakers and cochlear implants are safe in
MRI machines
b) The potential for adverse effects with sedation
should be considered in children and those
with cerebral irritation
c) Patient monitoring devices with no ferrous
metal-containing component are safe to use in
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an MRI machine
d) Past eye injury does not need clinical
assessment prior to an MRI
4. W
hich TWO statements are correct
regarding safety and potential adverse
effects of MRI?
a) About 4% of patients experience feelings of
claustrophobia in MRI scanning
b) IV gadolinium can be safely given to any patient
with an eGFR below 30mL/minute/1.73m2
c) The skin manifestations of nephrogenic
systemic fibrosis may take months to be
observed
d) MRI is contraindicated in pregnancy because of
the potential for injury to the developing fetus
5. Which TWO statements are correct
regarding the use of MRI in brain, head and
neck areas?
a) Suspected lesions at the base of the skull and
soft tissues of the neck are best defined by MRI
b) The usefulness of MRIs to image the neck
below C4-5 is limited because of scan
artefacts from the shoulders
c) CTs, when indicated, are preferred over MRI
as the first screening test for generalised
seizures
d) MRIs are the best investigation for assessing
the sinuses
6. Which TWO statements are correct
regarding the use of MRI in the limb areas?
a) ‘Internal derangement of the knee’ as an
indication for MRI refers specifically to a
suspicion for a meniscal tear
b) MRI may be useful in excluding a scaphoid
fracture early when X-ray is negative despite
strong clinical suspicion
c) M
RI cannot see changes associated with
Perthes’ disease of the hip
d) MRI of the elbow is more sensitive than X-rays
and CTs in picking up a cartilage fracture
7. Which TWO statements are correct
regarding the use of MRI in the trunk
areas?
a) M
RI follow-up in patients who had surgery for
breast cancer have been shown to increase
five-year survival
b) MRI is poor in delineating focal liver lesions
particularly in cirrhosis because of its already
distorted anatomy
c) M
RI monitoring of Crohn’s disease in younger
patients is a good alternative to small bowel
follow-through radiographs
d) MRI can be useful in the assessment of
placenta praevia
8. Which TWO statements are correct
regarding the conditions or body areas that
are better assessed by a CT than an MRI?
a) A
cute subarachnoid bleeding is better seen on
CT than an MRI
b) CT is better than MRI in demonstrating a
malignancy that involves both marrow and
cortical bone
c) C
T is better than MRI at visualising old
intracranial bleeding
d) High-resolution CT is the investigation of
choice, not MRI, for interstitial lung disease
9. Charlie is a 21-year-old man who, while
driving, was rear-ended by a car travelling
at about 70km/h. He presents with back
pain and tingling in the right foot. You
consider imaging for his symptoms. Which
TWO statements are correct regarding his
assessment with an MRI?
a) Blood oxygen level dependent functional MRI
(BOLD-fMRI) is the MRI technique used to
ascertain whether Charlie has a disruption to
the vessels in his foot
b) Charlie should be advised that if an MRI
shows a corresponding non-sequestrated
disc herniation, he may be managed
conservatively in the first instance
c) A negative MRI may not rule out cord oedema
or disc herniation because of motion artefacts
d) If Charlie has neck symptoms from
being rear-ended, an MRI is the first line
investigation to exclude fracture-related
pathology
10. John is a 72-year-old man with a
significantly rising PSA and chronic
heart failure who presented with a
chronic cough and dyspnoea. Further
investigation is ordered. Which TWO
statements are correct regarding his
assessment with an MRI?
a) MRI is the investigation of choice to assess
for metastatic prostate disease in the bones
b) It is possible that even an MRI may miss
small metastases in John’s lungs
c) If IV contrast for MRI is needed, John may be
reassured that gadolinium is typically safer
than IV contrast used for CT examinations
d) MRI is contraindicated in John’s cardiac
condition because it may induce fatal
electrical cardiac activity
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the quiz by post or fax. However, we have included the quiz questions here for those who like to prepare the answers before completing the quiz online.
how to treat Editor: Dr Steve Liang
Email: [email protected]
Next week Vasectomy is the only method of male contraception that is both highly effective and well accepted by patients. However, due to changed circumstances, men sometimes require a
vasectomy reversal. The next How to Treat discusses the indications and clinical considerations for the procedures of vasectomy and its reversal. The author is Dr Robert Woolcott, reproductive
microsurgeon, Vasectomy Reversal Australia, Sydney; and director, Genea, Sydney, NSW.
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| Australian Doctor | 27 June 2014
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