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| Susanne C. Ladd is Senior
Radiologist at the Department
of Diagnostic and Interventional
Radiology and Neuroradiology
(Director: Professor Michael
Forsting) at the University
Hospital Essen, Essen, Germany.
After studying physics and
medicine at the University of
Münster, Germany, she received
her postgraduate training
in general radiology at the
University Hospital Zurich and
the University Hospital Hannover
(Director: Professor Michael
Galanski). Her main interests are
whole-body screening MRI and,
recently, high field MRI. |
Whole-body MRI
Susanne C. Ladd
Department of Diagnostic and Interventional Radiology
and Neuroradiology, University Hospital Essen, Essen,
Germany
Address for correspondence:
Susanne C. Ladd MD
Department of Diagnostic and Interventional Radiology
and Neuroradiology, University Hospital Essen
Hufelandstrasse 55, D-45122 Essen, Germany
Tel. +49-201-723-84534 Fax +49-201-723-1548
Email: susanne.ladd@uni-duisburg-essen.de
Abstract
This article discusses recent developments in whole body
magnetic resonance imaging (wb-MRI) and its
clinical indications. Improvements in hardware, such
as extended table translation, and a high number of
simultaneous receiver channels are prerequisites for
wb-MRI. Generally, two different strategies can be
followed: multi-station wb-MRI and data acquisition
during continuous table movement, both permitting
a variety of different sequences and contrasts.
Wb-MR angiography facilitates the visualisation of
the entire arterial system from head to toe with
the exception of the coronary arteries. Wb-MRI can
replace skeletal scintigraphy in the detection of bone
marrow metastases by using fluid-sensitive sequences,
while fast contrast-enhanced sequences can be used
as an alternative approach in the search for tumours
and metastases. Such a wb-MRI approach can even
be superior to a combination of positron emission
tomography and computer tomography in the
detection of distant metastases.
One of the challenges for which the radiologist must be
prepared in wb-MR protocols is a dramatic increase in
image data, which might result in an increase in false
negative findings.
Further developments in wb-MRI aim at multi-contrast
imaging and further improvements in coil technology
for faster data acquisition (parallel imaging), with the
goal of reducing total examination costs.
Recent developments
Whole-body (wb) magnetic resonance imaging (MRI)
has evolved as a valuable alternative diagnostic tool to
other imaging modalities. It is important to recognize
that the term 'wb-MRI' was introduced as far back as
1980 and denotes the examination of body regions
beyond the neuroradiological domain - i.e. thorax,
abdomen or extremities.
Until a few years ago, only a single very restricted
part of the body could be assessed within reasonable
examination time. Today, however, wb-MRI actually
means imaging the whole body in one MR examination
'from head to toe'.
The first wb-MRI technique, wb-MR angiography,
depicted the arterial system from head to lower legs in
one examination following the administration of only
one contrast agent bolus (Figure 1). Soon afterwards,
wb-MRI began to be used in the search for tumours and metastases in bones. Today the assessment of the
musculoskeletal system, the parenchymal organs, the
vessels and the heart, and even the colon, has become
possible within a single setting. This article aims to
discuss the technical requirements as well as the
potential of wb-MRI and also some of its diagnostic
implications.
Overview of techniques
Various pulse sequence types can be applied to create
many different tissue contrasts. Other than computer
tomography (CT), MRI has always needed a clear
diagnostic hypothesis, as only single, restricted body
regions with a cranio-caudal field-of-view of 40-50 cm
could be examined within a single session. For more
extended body coverage, multiple body regions each
with a field of view of 40-50 cm are examined one after
the other. Due to restricted maximum table translation
range, the patient has to be repositioned from 'head
first' to 'feet first' to bring cranial and caudal body
parts into the isocentre of the magnet in succession.
Due to the restricted number of signal reception
channels, repositioning of the various radiofrequency
coils onto the body region currently being examined
in the isocentre is also required. If intravenous contrast
agent injection is required, this can only be applied to
one or two body regions. All these steps lead to long
examination times and, consequently, to high stress
for patients.
Recent technical achievements in hardware have
enabled radiologists to overcome these problems, thus
making wb-MRI possible.
Multi-station wb-MRI
 |
Figure 1. Multi-station MR angiography; patient without arterial stenoses.
Contrast-enhanced 5 consecutive 3D datasets (spoiled gradient-echo),
maximum intensity projections.
Figure 2. Whole-body MRI, acquired during continuously moving table. |
Among the first MR techniques for imaging a larger
field-of-view in a short time were a rolling platform with
extended field of view, which allowed wb examinations
without repositioning,(Ref: 1) and 'moving-bed infusion-tracking
MR angiography',(Ref: 2) both of which are dependent on
a rolling table platform. Today, commercially available
scanners offer a table range of 200 cm and up to several
dozen simultaneous receiver channels. In addition, the
patient can be covered with coils from 'head to toe',
so the repositioning of patient or coils is not required.
The high number of simultaneous receiver channels
additionally allows for 'parallel imaging' which can, for
example, be used for increased spatial resolution while
keeping acquisition times constant.(Ref: 3)
It should be noted that the actual cranio-caudal field
of view still is limited to 40-50 cm; it depends on the
homogeneity of the main magnetic field B0, and this has not changed (because of the length of the magnet
and bore diameter required). Hence, the acquisition
times and the field-of-view for the continuing blockwise
acquisitions remain constant. Additionally, there is loss
in efficacy due to pauses in data acquisition during table movement from one body region to the next.
This has implications, especially for MR angiography, in
which fast 'bolus chasing' is crucial. Due to magnetic
field 'inhomogeneities', the borders of the different data
blocks will not match perfectly - border artefacts will
occur, even if 5 cm of overlap is allowed (Figure 1).
Continuously moving table
The field-of-view can, however, be virtually increased to
200 cm by acquiring data during continuous movement
of the table (Figure 2).(Ref: 4-8) Time efficiency is increased
compared with multi-station MRI as no pauses occur
during the examination. In addition, a single image with
large field-of-view that does not suffer from border
artefacts is obtained.
Adequate image reconstruction requires knowledge
about the table position at every time point, which
has only recently become possible. With this technique,
high spatial-resolution two-5 or three-dimensional9 data
sets of various contrasts(Ref: 10) can be acquired, in analogy to
CT. MR angiographies can be obtained without border
artefacts (Figure 3).(Ref: 4,11) An attractive application is the
acquisition of images in short bore magnets; despite the
small field-of-view, the scanner could be used for wb-MRI.
Further developments in continuous table movement
aim at the reduction of breathing artefacts - i.e. by
creating algorithms that compensate for free breathing(Ref: 12)
- as well as the acquisition of multi-contrast sequences
during a single table translation(Ref: 10) and the adaptation
of table velocity with respect to the varying travel
velocities of the contrast agent bolus in different body
territories.(Ref: 13)
Indications and clinical experience
Atherosclerosis
MRI offers a unique opportunity to assess what damage,
if any, has already been inflicted by atherosclerosis on
the peripheral arterial system in the single individual.
MR angiography has been shown to be almost
equivalent to invasive techniques in virtually all single
vascular territories including the carotid,(Ref: 14) the renal(Ref: 15)
and the peripheral arteries.(Ref: 16,17 )
The implementation of faster gradient systems laid the
foundation for wb-MRA from the carotid arteries to
the trifurcation vessels of the calves.(Ref: 2,16,18,19) In wb-MR
angiography, acquisition times as well as the required
amount of contrast agent do not increase linearly
with the field-of-view; rather, the performance of
the gradient systems allows for sufficiently fast data
acquisition to 'chase' the contrast agent bolus through each of the four to six body regions or field-of-views.
This is reflected in the relatively low costs for wb-MR
angiography. Further improvement in image quality
is expected from the use of the recently introduced
continuous table movement techniques (variable table
movement velocity, seamless image quality)(Ref: 4,11) and
automated table velocity control.(Ref: 20)
Cardiovascular survey
Based on a wb-MR angiography approach,(Ref: 16) a fairly
comprehensive combined protocol can be developed that
achieves the depiction of the brain, the heart and the
peripheral arteries from the carotids to the ankles.(Ref: 21,22)
This protocol optimises the logistics of intravenous
contrast agent administration. It offers high image
quality and can be performed in less than 60 minutes.
Not only are macroscopic changes of the arterial system
(except the coronaries) visualised but potential organ
damage such as myocardial infarction, stroke or cerebral
microangiopathy can also be observed.
|
|
| Figure 3. MR angiography acquired during continuous table movement.
No border artefacts occur (maximum intensity projection of 3D data set). |
Figure 4. Six out of approximately 500 axial slices; contrast-enhanced fatsaturated
3D spoiled gradient echo data sets. A thrombus is seen as a clear
filling defect in the right popliteal vein (arrow). |
MR venography
Venous MR angiography traditionally was performed by
use of flow-sensitive pulse sequences (i.e. time-of-flight).
Since the introduction of paramagnetic contrast agents
in 1988, contrast-enhanced techniques are increasingly
used. Many departments perform MR angiographies of
the pulmonary arteries in a one-field-of-view technique;
these can be complemented by MR venography of the
lower extremities and abdominal-pelvic region (Figure
4) to detect potential sources for pulmonary emboli.(Ref: 23,24)
The high accuracy of MR venography in comparison with
conventional phlebography can have a particular impact
in the care of pregnant women (given due consideration
of recommendations for contrast agent administration in
pregnancy).(Ref: 25) Another potential application for wb-MRI of
vessels is the search for arterio-venous malformations.(Ref: 26)
Wb tumour and metastasis search
There is broad clinical consensus about the need for
screening for metastases in cancer patients, especially as the presence of metastases might alter therapy from
surgical to non-surgical alternatives.(Ref: 27) Usually, screening
for metastases is performed by a set of multiple
diagnostic methods such as bone marrow scintigraphy,
computer tomography, ultrasound, laboratory findings or
more dedicated nuclear medicine techniques, depending
on the tumour type. MRI in this context has traditionally
been used for specific diagnostic hypotheses in restricted
parts of the body (i.e. metastasis search in regional
lymph nodes, liver, brain or spine) and until recently
could not provide a complete survey, especially for
M (metastasis) staging.
The first wb-MR studies used a rather slow fluidsensitive
sequence (short tau inversion recoverysequence:
'STIR'), which was applied station-wise
throughout the whole body. It could depict bone
marrow infiltration by lymphomas and metastases but
also by infection with high sensitivity.(Ref: 28,29) The technique
additionally allows the (rather superficial) assessment
of parenchymal organs, so that MR could perhaps
substitute for bone scintigraphy and CT.(Ref: 30) Metastases
in the vertebral bodies are better depicted by STIR MR
compared with skeletal scintigraphy.(Ref: 31)
Thin-slice axial three-dimensional T1 images after
contrast agent application offer higher spatial resolution
in all three dimensions, compared with STIR images.
They potentially better characterise lesions in
parenchymal organs and bone and soft tissues and can
more easily be acquired in breath-hold techniques. This
approach(Ref: 32,33) allows for the evaluation not only of the
bones but also the lung, the abdominal parenchymal
organs and the lymphatic structures. MR detects almost
all lung and all brain metastases seen by CT. With respect
to skeletal scintigraphy, vertebral and pelvic bones in
particular are better evaluated. As in-room time is under
15 minutes, this MR approach could be an alternative to
skeletal scintigraphy screening for metastases.(Ref: 32)
Recent studies have compared positron emission
tomography combined with CT (PET-CT) with wb-MRI
with respect to staging accuracy in malignant disease.(Ref: 32,34)
In detecting distant metastases, both modalities
performed similarly well. MR performed better in the
detection of bone and liver metastases,(Ref: 35,36) but had the
tendency to underdiagnose T (tumour) and N (node)
stage. In case PET-CT is not available, or in the case of
repeated examinations of children (where radiation
exposure can be a risk), wb-MRI can be a valuable
alternative. This might also hold true for patients
with a known allergy to iodinated contrast agents.
The 'total' protocol
Not only can cardiovascular disease be assessed
in one setting but also by adding MR of the lungs,
contrast-enhanced cerebral MR and MR colonography,
a comprehensive examination of virtually 'all'
morphologically visible pathologies results (limitations
will be described below).(Ref: 21) MR colonography has been
shown to be adequately sensitive for detection of polyps
over 5 mm diameter.(Ref: 37)
Post mortem MRI
Wb-MRI has been performed in human corpses,
rendering valuable additional information on the cause
of death. Problems with this indication include the low
potential of coronary visualisation, and of differentiating
between thrombi and emboli.38 However, wb-MRI
could play a role in examining cases without consent
to autopsy or in case of distinct risk of infection.(Ref: 39,40)
In many countries, however, the legal basis for post
mortem imaging remains undetermined.
Estimation of fat and muscle mass
Axial wb-MRI allows for assessment of total and
compartmental adipose tissue as well as quantification
of muscle mass of humans.(Ref: 41,42) Fat-water separation has
also been shown to be possible with continuous table
movement.(Ref: 43,44) In this regard, success control of physical
training (i.e. body building) becomes possible.(Ref: 45) In the
future, wb-MRI could also play a role in the assessment
of systemic diseases such as polymyositis(Ref: 46) or muscular
dystrophy.(Ref: 47)
Wb screening
MRI is a natural candidate for screening, a term that
refers to the search for occult disease - disease that has
not yet become symptomatic (secondary prevention).
The aim of screening is detection of disease in an early
stage, which allows for more efficient therapy and may
result in reduction of morbidity and mortality.
MRI fulfils many requirements for a screening technique:
no ionizing radiation, no known effect on individuals
with normal renal function, high diagnostic accuracy
and high patient acceptance. The only drawback is the
high cost involved. However, the combination of several
single examinations in one combined screening protocol
can reduce the relative costs.
The application of such a combined MR screening
protocol in a large group of presumably healthy
volunteers resulted in a very low detection rate of
arterial and organ pathologies.(Ref: 48) This highlights the need
for adequate pre-selection for reaching an acceptable
cost:benefit ratio.
Advantages and disadvantages of wb-MRI
Wb-MRI is capable of meeting a variety of needs in a
single examination. However, there are some limitations
to the technique. Many anatomic structures cannot be
assessed: among them are the small bowel and stomach,
breast, prostate, joints, and coronary arteries. Some of
these structures can be assessed by dedicated MRI (for
example, MR mammography screening especially in
younger patients with high risk of breast cancer(Ref: 49)) but
they all remain unassessable in a non-specific wb-MRI,
as presented above.
Another challenge is the enormous increase in image
data in wb-MRI, which not only implies an increase in
evaluation time for the radiologist but also increases
the potential for false negative findings. Additionally, the
radiologist cannot restrict him or herself to assessing only the target structures (i.e. myocardial morphology
and function, arterial pathologies, tumours or colonic
polyps). In particular, the three-dimensional nature
and the large field-of-view of wb-MRI necessitate the
work-up of all visible structures: bones and soft tissues,
parenchymal organs, lymph nodes and the venous
system. The radiologist must assume responsibility for
chance findings, and the patient must consider the
possibility that indistinct findings may be made and that
these might have to be assessed with further, potentially
invasive tests.
Future directions
MR manufacturers have acknowledged current demands
and now offer scanners allowing high-speed data
acquisition and data reconstruction (parallel imaging
strategies), extended table movement range and a set
of dedicated surface coils for the whole body. Future
developments will be targeted toward high-frequency
technology and pulse sequence design for ultrafast,
high-quality imaging and concomitant reduction of total
examination costs.
Additionally, there is a tendency towards higher field
strengths. These render higher signal:noise ratios, which
can be translated into higher spatial resolution but
also into drastically lower acquisition times. Three Tesla
systems have entered clinical practice, and the growing
number of 7 Tesla systems or higher in research centres
will certainly open new options, not only for brain
imaging but also for wb applications.
Computer-aided diagnosis could be another keyword for
future developments in wb-MRI, serving as competent
second reader, as is possible in CT.(Ref: 50)
The coronary arteries remain a focus of interest; they
will certainly profit from future technical improvements.
Depiction of the coronaries would enhance the
assessment of cardiovascular disease as the major cause of death in Western countries.
Finally, the inherently higher sensitivity of MR compared
with CT for many indications makes a combination
of positron emission tomography with its functional
information and MRI with its high soft tissue contrast
(PET-MRI) a very promising dual-modality alternative to
PET-CT for the work-up of tumour patients.(Ref: 51)
Discussion
Wb-MR is a novel technique for examining a large
portion of the body. Within a single MR setting, a variety
of clinical questions can be answered which, until
recently, required multiple, distinct examinations, usually
on different days. Wb-MRI has already been established
in some radiological departments and the potential
indications for the technique are quite well refined.
Wb-MRI is sufficiently accurate to be used as a side step
if other wb modalities cannot be applied.
In the clinical setting, oncological wb-MR is already a
valuable alternative if PET-CT is unavailable, for certain
tumour types (tracer-negative tumours) or for follow-up
of malignant conditions in children. Up to now, the lack
of cost assumptions by health insurance companies has
been a hindrance to the use of wb-MR in the clinical
setting in many countries.
Today's limitations and pitfalls of wb-MRI include the
lack of information on small bowel, breast, prostate,
joints and coronary arteries (although organ-specific MRI
may be valuable for these sites). Another drawback is the
enormous increase in image data, which not only leads
to an increase in reading time for the radiologist but also
might increase the rate of false negative findings.
However, the continued development of MR scanner
hardware will further facilitate wb-MRI. Thus, the
concept of wb-MRI is likely to remain a focus of active
research in the coming years.
Key Learning
• Wb-MRI became possible because of improvements in hardware, such as extended table translation, and the
availability of a high number of simultaneous receiver channels
• Wb-MRI approaches will shift the spectrum of indications from dedicated characterisation of single pathologies
towards assessment of systemic disease (atherosclerosis, metastatic disease, malformations)
• Wb-MRI appears to be a valuable alternative to PET-CT in children or in FDG-negative tumours
• Combined wb-MR protocols lead to a dramatic increase in image data, and might result in an increase in false
negative findings
• Further developments in wb-MRI aim at multi-contrast imaging and further improvements in coil technology for
faster data acquisition (parallel imaging), with the goal of reducing total examination costs
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