Integrating a MRI scanner with a radiotherapy
accelerator: a new concept of precise on line radiotherapy
guidance and treatment monitoring
B.W. Raaymakers, J.J.W. Lagendijk, U.A. van der Heide, J. Overweg1, K. Brown2, R. Topolnjak, H. Dehnad, I.M.
Jürgenliemk-Schulz, J. Welleweerd, C.J.G. Bakker3
Department of Radiotherapy and Radiology3, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands.
Philips Research Hamburg1, Germany, Elekta2, Crawley, United Kingdom.
Abstract
Within a collaboration between the UMC Utrecht, Elekta and Philips Research, Hamburg, we are investigating the
integration of a MRI scanner with a radiotherapy accelerator. This system must provide on-line, soft tissue based position
verification, but also on line monitoring of treatment results. The technical feasibility study led to a design that will be
presented in this paper. Basically the design is a single energy accelerator mounted in a ring around a conventional 1.5 T
MRI scanner.
Keywords
Position verification, MRI, accelerator, treatment monitoring, IMRT, fiducial markers, treatment planning
Introduction
Precise image guided radiotherapy has to deal with the
uncertainties in the exact daily positioning of the radiation
fields with respect to the changing anatomy due to internal
organ motion and tumour response to treatment. Huge effort
is being put in the development of tumour visualisation and
position verification technology. The most commonly
applied technique is position verification using the
megavoltage treatment beam for imaging. The image
contrast however is poor and allows merely the visualisation
of the bony anatomy or implanted fiducial gold markers
[1,2]. Bony structures and markers may also be visualised
using ceiling mounted kilovolt X-ray imaging [3]. More
sophisticated are the new developments that integrate conebeam CT functionality [4] or integrate megavoltage CT with
the accelerator [5]. Also the integration of ultrasound
imaging with an accelerator is being investigated [6].
The excellent soft tissue contrast of MRI can be used for
two, interconnected purposes, i.e. treatment guidance and
treatment monitoring. If it is possible to integrate MRI
functionality with a radiotherapy accelerator this may
provide unequalled, on-line, soft-tissue based position
verification. Besides, MRI has the potential to characterise
the tumour in 3D [7]. Parameters related to blood flow,
hypoxia, pH, vascularity, etc. can be measured with MRI or
MR spectroscopy. New developments in this are the use of
antibody or receptor specific nanosensors, contrast agents
which are visible with MRI [8]. All this biological imaging
can be used for achieving biological conformality [9].
Besides the expected gain on position verification of both
tumour and surrounding tissues, a system that integrates
MRI and an accelerator can incorporate, and contribute to,
these new imaging developments. In collaboration with
Elekta and Philips Research, Hamburg, we are investigating
the feasibility of such an integrated MRI accelerator system.
Technical feasibility and design
The design of the MRI accelerator combination is basically
a small accelerator mounted on a ring around a modified,
Figure 1: Impression of an integrated MRI accelerator. The inner
grey drum represents the MRI system, the blue ring around it
contains the accelerator.
closed-bore MRI system, see figure 1.
The magnetic stray field potentially disturbs the operation
of the accelerator. On the other hand, magnetisation of the
moving accelerator components would lead to modulation
of the 1.5 T magnetic field with image distortion and ghost
artifacts as possible consequences. Active magnetic
shielding has to be applied to magnetically uncouple the
MRI system and the accelerator. In order to limit cost and
complexity, we decided to base the concept on a cylindrical
closed-bore main field magnet, with the accelerator
irradiating the patient from outside the MRI system. This
choice implies beam transmission through the MRI field
generating system. The MRI magnet and the gradient coil
need to be modified in order to minimise both scatter and
absorption heterogeneity.
Furthermore, the photon beam travels unperturbed in the
magnetic field. However the actual dose deposition, an
avalanche of secondary electrons, is affected by the
presence of the permanent 1.5 T magnetic field.
The technical feasibility study has focussed mainly on
3 topics:
 Magnetic interference of the MRI system and the
accelerator
 Beam transmission through the closed bore MRI
 Dose deposition kernel in a 1.5 T field
Main magnet. The magnetic interference is hampering the
integrated system in two ways: the operation of the
accelerator is distorted by the presence of the magnetic stray
field. Especially the gun section with its low energy
electrons before full acceleration is sensitive. On the other
hand, ferromagnetic accelerator components might become
magnetised and thus induce a distortion on the main
magnetic field. In order to limit image distortion, the main
field homogeneity has to be of the order of few ppm over
the entire field of view,.
In collaboration with Philips Research, modifications of the
coil configuration of a standard Intera 1.5 T magnet have
been investigated. The magnetic interference is minimized
by active magnetic shielding. By slightly modifying the
number of turns of the shield coil of a standard actively
shielded magnet design, it is possible to achieve a very low
field in a toroidal volume in the midplane of the MRI
magnet in which the accelerator can be placed. This
modification leads to a slight increase of the far field of the
magnet, but the field at larger distances is still sufficiently
low to allow installation in the vicinity of conventional
accelerators (typically at 10 m distance).
The coil configuration of the main magnet can also be
modified in such a way that there are no superconducting
coil windings in the transversal mid-plane of the MRI
system. This means that the equivalent of approximately
6 cm (or less) of homogeneous aluminium remains in the
beams eye view, significantly reducing both scatter
induction and absorption heterogeneity. This modified
magnet is not much different in construction, and induces
no significant compromises on the central field quality, i.e.
the new design would yield the same imaging performance
as a standard Philips system.
Gradient coil system. The magnet system, i.e. the cryostat
with the superconducting coils can be adjusted to create a
beam portal without compromising its performance. The
remaining MRI components through which the beam travels
are the gradient coils and the RF coils. The RF coils can
easily be made very transparent to radiation. A conventional
gradient coil however poses a severe scatter and absorption
problem. Several patterns of thick copper conductors with
gaps of a few mm in between are located on top of each
other. When irradiating through this, intensity contrasts upto
80% at a spatial resolution of mm can be expected.
Therefore also radiation portals in the gradient coils are
required. Again in collaboration with Philips Research,
Hamburg, the impact of a 20 cm gap in the gradient coils
was investigated. For the gradient in the axial direction (Zdirection), the windings can easily be moved out of the
central area. The coils for the transversal gradients (i.e. Xand Y-direction) needed significant modification, but
suitable transverse coil geometries without conductors
covering the central 20 cm were indeed found. Figure 2
shows magnetic field plots for a possible design.
Figure 2: Gradient of the magnetic field with a 20 cm gap in
the gradient coils. The left hand panel shows the transversal
mid plane, gradient quality similar to a conventional coil, the
right hand panel shows the sagital mid plane, in this direction
the field of view is limited to 25 cm.
These preliminary investigations have provided confidence
that the gradient coils can be placed outside the beam’s eye
view. For this preliminary design, the image quality in the
transversal mid-plane will be unaffected and the field of
view in the axial direction will be limited to approximately
25 cm, which is enough for this application.
Beam characteristics As discussed, the beam has to travel
through various structures. After modifying the magnet and
gradient coils, the total amount of (homogeneous) mass in
the beams’ eye view is the equivalent of approximately 10
cm Aluminium (~27 gr/cm2). This means the scatter
induction at 10 cm from the border of a 10x10 cm2 field is
rel. dose %
approximately doubled compared to the open beam
situation, as shown in figure 3. Here the scatter induction as
function of mass in the beam was measured for a 10x10 cm2
field. In these measurements the mass was compressed into
a single slab. The next step will be measuring the scatter
induction for a range of fields in the actual MRI accelerator
configuration, as well as performing Monte Carlo
simulations in order to precisely quantify the scatter
induction.
2,5
2
1,5
1
0,5
0
0
10
20
30
40
weight gr/cm2
Figure 3: Scatter induction as function of the amount of
mass in the beam. The scatter induction is measured at 5 cm
depth at 10 cm from the border of a 10x10 cm2.
The scatter induction can be further reduced by limiting the
system to a discrete number of gantry positions. This allows
the creation of beam portals in both the carrier of the
superconducting coils in the cryostat, the thermal shields
and the carrier of the gradient coils. Whether this is
necessary with respect to scatter induction will be further
investigated.
The only absorption heterogeneity is found in the RF coils,
these structures represent an equivalent of approximately
2.3 cm thick aluminium, yielding maximum intensity
contrasts of 25% at a spatial resolution of mm. Furthermore
there is the helium level in the cryostat for specific gantry
angles. A maximum of 15 cm helium can be in the beam’s
eye view, leading to approximately 10% intensity contrast.
Another source of heterogeneity is the omission of a
flatness filter in order to minimise scatter induction and
increase the accelerator output. The remaining
heterogeneity can be flattened using MLC based [10] or
compensator based IMRT.
Dose deposition kernel The impact of the 1.5 T magnetic
field on the dose deposition of a 6 MV beam was modelled
using Monte Carlo simulations, see accompanying paper
[11]. The conclusion is that the impact is small and can be
taken into account in a conventional treatment planning
procedure. The feasibility and calibration of dosimetry
equipment in the presence of 1.5 T has to be studied.
Technical and clinical challenges The technical feasibility
of various issues has been addressed and briefly discussed.
The conclusion is that integrating a MRI with an accelerator
is technically feasible. Still the actual technical
implementation has to be done. First, the preliminary results
need further and more detailed investigation. Monte Carlo
simulations will be applied for quantifying for instance the
scatter induction and the dose deposition kernel in the actual
MRI accelerator configuration. The way of compensating
the remaining absorption heterogeneities has to be decided;
MLC based IMRT versus compensators. The first option
provides lower spatial resolution, the second option induces
additional scatter, which can be partially dealt with by
introducing additional shielding. A comparative assessment
between these choices has to be made. The present design
aims to keep the two systems as physically separated as
possible. A dedicated ring around the MRI has to be
constructed in which the accelerator can be mounted. When
choosing for compensators to solve the absorption
heterogeneity, their placement is a topic of study. They can
be placed on the outside of the MRI system, but preferably
they will be placed in between the gradient coil and the
magnet, there they are positioned closely to the system’s
heterogeneity (i.e. the RF coil). Conventionally here the
passive shielding system is located, but this system is placed
outside the central area to minimise the amount of mass in
the beam.
Various separate topics need to be investigated to fulfil the
aim of the MRI accelerator, i.e. accurate radiation delivery
at mm resolution, for instance the problem of absolute
positioning. When using MRI images for treatment
guidance it is crucial that the images are geometrically
correct and that the exact relation between the MRI
coordinate system and the accelerator coordinate system is
known. At our department a full quality assurance system
for correction MR images has been developed [12,13].
System dependent image distortions could be reduced from
13 mm to 2 mm and patient induced distortions could be
corrected to the order of the image pixel size. These
corrections and dedicated MR scanning procedures were
developed explicitly for radiotherapy treatment planning
purposes. This work will be continued with the aim to
achieve sub-mm accuracy.
The main clinical question is: what is the expected benefit
for treatment outcome when using a MRI system integrated
with an accelerator for daily, on-line position verification of
both tumour and surrounding structures? This is being
studied parallel with the development of the MRI
accelerator. For this study a 1.5 T MRI radiotherapy
simulation system serves for repeated, daily and continuous
imaging in order to investigate daily anatomy variations
[14,15]. The next step is to investigate how to handle this
information. Registration with treatment planning images,
on-line judgement of position accuracy and position
correction and ultimately on-line re-planning are topics that
need to be investigated. It is the explicit goal of these efforts
to come to highly conformal, adaptive plans with the
minimum feasible margins.
Conclusion
The feasibility of an integrated MRI accelerator system is
being investigated. Preliminary results indicate that the
design is feasible. Such a system will provide superb on-line
position verification of both tumour and normal tissue
structures and will facilitate on line treatment optimisation
and treatment response monitoring.
Acknowledgement
[7] Stubbs M. 1999, Application of magnetic resonance
techniques for imaging tumour physiology. Acta Oncol.
38, 845-853
[8] Weissleder R, Moore A, Mahmood U, Bhorade R,
Benveniste H, Chiocca EA, Basilion JP. 2000, In vivo
magnetic resonance imaging of transgene expression.
Nat. Med. 6, 351-354
[9] Ling CC, Humm J, Larson S, Amols H, Fuks Z, Leibel
S, Koutcher JA. 2000. Towards multidimensional
radiotherapy (MD-CRT): biological imaging and
biological conformality. Int. J. Rad. Biol. Phys. 47, 551560
[10] Topolnjak R, Van der Heide UA, Lagendijk JJW.
2004, IMRT sequencing for a six-bank multi-leaf
collimator system. Submitted to 14th ICCR, Seoul, South
Korea.
This work is supported by a grant from the Dutch
Technology Foundation STW.
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