Table Of ContentIgnasi Carrio
Pablo Ros
Editors
PET/MRI
Methodology and
Clinical Applications
123
PET/MRI
Ignasi Carrio (cid:129) Pablo Ros
Editors
PET/MRI
Methodology and Clinical
Applications
Editors
Ignasi Carrio Pablo Ros
Department Medicina Nuclear Department of Radiology
Autonomous University of Barcelona University Hospitals Case
Hospital Sant Pau Medical Center
Barcelona Cleveland , OH
Spain USA
ISBN 978-3-642-40691-1 ISBN 978-3-642-40692-8 (eBook)
DOI 10.1007/978-3-642-40692-8
Springer Heidelberg New York Dordrecht London
Library of Congress Control Number: 2013956349
© Springer-Verlag Berlin Heidelberg 2014
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Contents
1 PET/MR System Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Gaspar Delso and Sibylle Ziegler
2 Image Distortions in Clinical PET/MR Imaging. . . . . . . . . . . . 21
S.H. Keller, A.E. Hansen, S. Holm, and T. Beyer
3 Workfl ow and Practical Logistics. . . . . . . . . . . . . . . . . . . . . . . . 43
N.F. Schwenzer, H. Schmidt, and C.D. Claussen
4 MR-PET in Breast Cancer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Christian Buchbender, Thomas C. Lauenstein, Andreas Bockisch,
Gerald Antoch, and Till A. Heusner
5 PET/MRI in Evaluating Lymphomas: Preliminary
Experience and Potential Future Applications . . . . . . . . . . . . . 71
Maria Chiara Gaeta and Karin Anna Herrmann
6 PET/MRI of the Liver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Raj Mohan Paspulati and Andres A. Kohan
7 PET/MR in Colorectal Cancer . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Sasan Partovi, Andres Kohan, Raj Mohan Paspulati,
Pablo R. Ros, and Karin A. Herrmann
8 PET/MR in Brain Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
Wolf-Dieter Heiss and Alexander Drzezga
9 MR-PET in Cardiology: An Overview
and Selected Cases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
Stephan G. Nekolla and Christoph Rischpler
10 Risks and Safety Aspects of MR-PET . . . . . . . . . . . . . . . . . . . . 139
Gunnar Brix, Elke Nekolla, and Dietmar Nosske
11 Health-Care Costs and Impacts . . . . . . . . . . . . . . . . . . . . . . . . . 153
Mathias Goyen
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
v
1
PET/MR System Design
Gaspar Delso and Sibylle Ziegler
Contents Abstract
The combination of clinical MR and PET
1.1 Introduction .................................................... 1
scanners has received increasing attention
1.2 Compatibility Issues ....................................... 2
in recent years. In contrast to currently used
1.2.1 Static Magnetic Field ........................................ 2
1.2.2 Magnetic Susceptibility .................................... 3 PET/CT machines, PET/MR offers not only
1.2.3 Eddy Currents ................................................... 3 improved soft-tissue contrast and reduced
1.2.4 Coil Loading ..................................................... 3 ionizing radiation but also a wealth of avail-
1.2.5 Temperature ...................................................... 4
able MR variations such as functional, spec-
1.2.6 Mechanical Vibration ....................................... 4
1.2.7 Interference ....................................................... 5 troscopic, and diffusion tensor imaging.
1.2.8 Gamma Attenuation ......................................... 5 This combination, however, has proven to
be very challenging, due to the detrimental
1.3 Design Options ................................................ 5
1.3.1 Photodetectors .................................................. 5 effect of the scanners on each other’s per-
1.3.2 Radiofrequency Shielding ................................ 6 formance. Signifi cant progress has been
1.3.3 Data Transmission ............................................ 7
made in the last 10 years to solve the vari-
1.3.4 Gamma Shielding ............................................. 8
ous technical issues, culminating with the
1.3.5 Cooling ............................................................. 8
1.3.6 Hardware Attenuation ...................................... 9 recent release of clinical whole-body hybrid
scanners.
1.4 PET/MR System Designs ............................... 10
1.4.1 Sequential Architecture .................................... 10 In this chapter, we review the technological
1.4.2 Insert Architecture ............................................ 11 challenges of PET/MR design, briefl y describ-
1.4.3 Integrated Architecture ..................................... 12 ing the different available architectures for
1.5 Discussion ........................................................ 16 hybrid clinical scanners, their capabilities, and
limitations.
References ................................................................... 16
1.1 Introduction
Despite the extensive research effort dedicated to
software coregistration in the 1990s, the introduc-
tion of combined scanners capable of acquiring
positron emission tomography (PET) and com-
G. Delso (*) (cid:129) S. Ziegler puted tomography (CT) data sequentially [2 ] was
Nuklearmedizinische Klinik und Poliklinik , Klinikum
met with enthusiasm by the medical community.
rechts der Isar der Technischen Universität München ,
Nine out of every ten PET scanners purchased
Ismaninger Str. 22 , 81675 Munich , Germany
e-mail: [email protected]; [email protected] nowadays are combined machines. The key to this
I. Carrio, P. Ros (eds.), PET/MRI, 1
DOI 10.1007/978-3-642-40692-8_1, © Springer-Verlag Berlin Heidelberg 2014
2 G. Delso and S. Ziegler
success was not just the straightforward solution design. We discuss the advantages and limitations
to the coregistration problem in most applications of the available architectures, giving the reader
but also the signifi cant improvement in workfl ow an overview of the different setups investigated
derived from both scanning the patient just once by the various groups having reported on work-
and not needing to perform a transmission scan ing PET/MR prototypes.
for generating attenuation correction data.
PET/CT is currently fully integrated in clinical
routine, but, though its advantages are many, CT 1.2 Compatibility Issues
still provides limited soft-tissue contrast and, when
used for whole-body diagnosis, exposes the 1.2.1 Static Magnetic Field
patients to high radiation doses (over 10 mSv) [1 5 ].
An alternative source of anatomical information At the heart of magnetic resonance imaging is the
would be magnetic resonance imaging (MR) [7 4 ]. static fi eld , a powerful magnetic fi eld used to
The combination of MR and positron emission align the magnetic moments of atomic nuclei. In
tomography scanners, however, has proven to be clinical practice, fi eld intensities of 1.5 and 3 T
very challenging due to the potentially detrimental are currently used. More advanced medical appli-
impact of the scanners on each other’s perfor- cations are being investigated with fi eld intensi-
mance. The intense static magnetic fi eld, quickly ties of up to 9.4 T.
changing gradient fi elds, and radiofrequency (RF) The magnetic moments of nuclei in a mag-
signals from the MR scanner affect the light yield netic fi eld can be oriented parallel or antiparallel
of scintillator materials [ 9 ], prevent the normal to the fi eld. Due to the interaction between the
operation of photomultiplier tubes, and induce nuclear magnetic moment and the external mag-
interference in the front-end electronics of PET netic fi eld, these two states have slightly different
detectors. Conversely, the presence of the PET energy levels. The moments will therefore be in
detector causes inhomogeneities in the magnetic an equilibrium state governed by Maxwell-
fi eld, degrades the performance of the coils, and Boltzmann statistics. When an electromagnetic
can induce interference in the MR acquisition path. excitation is applied with a frequency that
However, in the last years, progress has been matches the energy difference between the two
made in identifying scintillators with adequate states, a resonant absorption is generated.
magnetic properties [7 1 ], developing suitable As the resonance frequency of the magnetic
PET detectors which either use optical fi bers to moments is proportional to the strength of the
guide the scintillation light away from the MR static fi eld, the homogeneity of this fi eld is of
magnetic fi elds [6 , 32 , 3 6 , 50 , 57 , 5 8 ] or replace critical importance to ensure the desired distribu-
the photomultipliers by magnetic-fi eld- tion of excitation over the sample. This is
insensitive solid-state detectors [5 , 21 , 41 , 44 , 45 , achieved by a careful consideration of the geom-
60 , 68 ], and designing shielded PET electronics etry of the system and the susceptibility of the
to avoid electromagnetic interference [2 8 ]. materials (Sect. 1 .2.2 ), as well as by the use of
To this day, several research groups have suc- passive and active s himming .
cessfully developed small PET/MR prototypes The excited nuclei will gradually return to an
for animal studies [5 , 35 , 45 , 49 , 57 , 58 , 68 ], and equilibrium state by releasing energy at the reso-
each of the three main manufacturers (General nant frequency. To be able to discern the origin of
Electric, Philips, and Siemens) has recently these emissions, a set of spatially variant g radient
released their own proposal of whole-body hybrid. fi elds are superimposed with the static fi eld, lead-
However, these systems present radically differ- ing to local variations in the frequency and phase
ent architectures, and it has to be shown which is of the emitted signal. The resulting information
the best confi guration for clinical practice. can be processed by a r econstruction algorithm
In this chapter, we briefl y describe the main to create two- or three-dimensional images of the
issues and choices involved in PET/MR system resonance signal.
1 PET/MR System Design 3
Consequently, inhomogeneities of the static spatial encoding (e.g., 200 T/m/s), will induce
magnetic fi eld can alter the spatial encoding of currents in any conductive structures present in
the sample, leading to artifacts in the recon- the scanner.
structed images, such as geometrical distortions These local current loops, known as eddy cur-
in the readout direction. rents or Foucault currents , will in turn create
Another important effect of the static mag- magnetic fi elds, degrading the homogeneity of the
netic fi eld is the generation of Lorentz forces on static fi eld. If the eddy currents decay before the
charged particles moving through it. This can signal readout, the result will be a spatially variant
have a positive effect, like an improvement in phase change of the resonance signal. However, if
PET resolution due to the reduction of the aver- the eddy current still persists during readout, the
age distance traveled by positrons prior to annihi- associated magnetic fi eld will alter the spatial
lation [2 3 ]. However, it has a severely detrimental encoding of the signal, leading to a distortion of
effect in the performance of the photomultiplier the reconstructed images [ 3 , 51 ]. Eddy currents
tubes generally used in PET detectors [2 2 ]. during slice selection may cause imperfect refo-
cussing of the spins, leading to signal loss.
For conventional imaging with a rectilinear
1.2.2 Magnetic Susceptibility raster of the k-space, the effect of eddy currents is
mainly visible in the phase image. However, for
Closely related to the issue of static fi eld homo- sequences with more complex raster patterns
geneity is that of magnetic susceptibility. V olume (such as echo planar imaging), the k-space shift-
magnetic susceptibility is a proportionality con- ing and phase accumulation can lead to ghosting,
stant indicating the degree of magnetization (the compression, shearing, or image displacement.
magnetic dipole moment per unit volume) of a Particularly strong artifacts can arise in sequences
material in response to an applied magnetic fi eld. that require accurate phase images (such as
In other words, materials placed in the static fi eld phase-contrast imaging) or that combine multiple
will become magnetized, altering the overall images (such as diffusion-weighted imaging). In
magnetic fi eld in their vicinity. the case of spectroscopy studies, eddy currents
As a reference, an object with a susceptibility lead to time-varying shifts in the resonance fre-
of 10− 6 (1 ppm) produces a maximum perturba- quency, degrading the resolution of the acquired
tion of about ±1 ppm in the surrounding fi eld and, spectrum [3 0 ].
consequently, in the resonant frequency. If the Other undesired side effects of eddy currents
susceptibility is 10 ppm, the effect is ten times as are coil loading, heating, mechanical vibration,
large, etc... [3 3 , 53 ]. and interference, which are discussed in the fol-
Since MR image quality is strongly dependent lowing sections.
on the uniformity of the static fi eld, materials
with high susceptibility (in absolute value)
should be either avoided or placed as far from the 1.2.4 Coil Loading
fi eld of view as possible.
The magnetic fi eld generated by an eddy current
always opposes the change of the fi eld that cre-
1.2.3 Eddy Currents ated the eddy current in the fi rst place (Lenz’s
law). The energy lost due to the generation of
Faraday’s law of induction states that a time- eddy currents has the effect of altering the load
varying magnetic fl ux through a closed circuit on the gradient and radiofrequency coils. As the
will generate an electromotive force proportional load deviates from the ideal value for which the
to its rate of change. The consequence of this is coils have been designed, their performance
that the radiofrequency excitation fi eld, as well as degrades: The resonance frequency of the coil
the switching of the gradient fi elds used for shifts and its quality factor is reduced.
4 G. Delso and S. Ziegler
In the case of radiofrequency coils, this leads 1.2.6 Mechanical Vibration
to a reduction of the signal-to-noise ratio of the
images. In the case of gradient coils, it limits the Due to the interaction of the static magnetic fi eld
rate of change of the gradient fi elds (or s lew rate ) with the time-varying currents used to generate
and alters spatial encoding. the magnetic fi eld gradients, the gradient coils
are subject to Lorenz forces causing vibrations in
the acoustic range. The vibration amplitude will
1.2.5 Temperature increase with the magnetic fl ux density, the inten-
sity of the current, and the coil radius. Lighter
As with any electric current circulating through a coils will generally present larger amplitudes.
nonideal conductor, eddy currents will cause The length of the coil and the elastic properties of
Joule heating. This effect can cause discomfort the coil materials will determine the frequency of
or, in extreme cases, burns when patients are in the vibration modes, whereas the viscosity of the
contact with metallic objects [1 4 , 39 ]. materials will affect how the vibrations decay
More importantly for the purpose of system [6 6 ]. It is worth mentioning that in scanners using
design, the performance of some electronic actively shielded gradient coils, these vibrations
components can be sensitive to temperature are reduced by mechanically coupling them with
[1 0 , 1 6 , 6 1 ]. This is of particular relevance the shielding coils. This is due to the different
when using solid-state photodetector technol- polarity of the currents that make the coils vibrate
ogy (Sect. 1 .3.1 ), where an increase of tempera- in opposed phase.
ture will reduce the gain of the electron The airborne transport of these vibrations is
avalanche due to increased collisions with the the cause of the well-known acoustic noise in
semiconductor lattice [8 ]. Indeed, temperature MR scanners, which can be of the order of
variations have been shown to cause signifi cant 115 dB(A) in 3 T scanners. However, the mechan-
drifts (3.5 % per Kelvin) in the gain of ava- ical vibration can also be directly transmitted
lanche photodiodes. In the case of silicon pho- through the different support structures in the
tomultipliers, temperature changes alter the system [5 2 ]. Despite the use of passive and active
breakdown voltage, leading to variations in dampeners, some of these vibrations will reach
photodetection effi ciency, dark counts, and other subsystems in the scanner.
gain. These effects degrade the energy resolu- It is a known fact that MR scanners are sensi-
tion of PET detectors, cause sensitivity varia- tive to mechanical vibration, especially at low fre-
tions, and, ultimately, result in reconstruction quencies (<100 Hz). External vibrations are more
artifacts and quantifi cation errors [1 0 , 1 6 , 6 1 ]. detrimental than those introduced by the scanner
The temperature issue is aggravated by the itself, which are often in phase with the experi-
preamplifi cation required by solid-state photode- ment. But self-induced resonance effects can still
tectors. Power consumption in the preamplifi er is cause artifacts in the reconstructed images [1 9 ].
a constant source of intense heat in close proxim- We are not aware of any published studies on the
ity to the sensitive photodetector. Although pre- effect of mechanical vibrations on PET detec-
amplifi er heating can have a severely detrimental tors. However, two issues should be considered:
effect on the performance of the PET system, it Firstly, measures should be taken to prevent a
can be effectively managed by an adequate cool- degradation of the PET detector assembly due
ing system (Sect. 1 .3.5 ) and accounted for in the to the exposure to frequent, irregular mechanical
system calibration. This is not always true for vibrations; secondly, the geometry and mechani-
the localized, time-varying warming caused by cal properties of the materials used in the con-
the intense eddy currents induced by the more struction of the PET detector should be carefully
demanding MR sequences, another reason to selected, preventing resonance effects leading to
design the system in such a way that minimizes vibrational modes potentially deleterious for the
eddy current load. MR acquisition.
