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Imaging in Particle Therapy
limited by the inconsistencies of the forward-projection model. These inconsistencies
between the iRad and the weDRR of the ground truth iCT have been quantified to
be in the same order of magnitude as the intrinsic inaccuracies of the treatment
planning x-ray CT. In Monte Carlo simulations of clinical data, an accuracy of 2%–
5% for integration-mode data and 1%–3% for list-mode data has been found, thus
resulting insufficient to minimize the intrinsic inaccuracies in the calibration curve
(Gianoli et al 2020).
6.4 Artificial intelligence in ion imaging
In the last few years, the use of artificial intelligence has been proposed in ion
imaging. In particular, machine learning has been adopted to improve the forwardprojection model and speed up its calculation for list-mode data (Lazos et al 2021),
which is typically based on the MLP. Further, machine learning has been proposed
to overcome the forward-projection model itself by means of a data-driven
approach, as the inconsistencies of the model are comparable to the intrinsic
inaccuracies of the treatment planning x-ray CT (Gianoli et al 2022). Machine
learning has been used for the correction of the iCT (proton CT) along with the
prediction of the uncertainties of the image in terms of noise and scattering (CollinsFekete et al 2020, Nomura et al 2021) and for the estimation of treatment planning
x-ray CT based on dual-energy x-ray CT (Wang et al 2020). Machine learning has
been proposed also to identify possible sources of range mismatches on proton
radiographies (Seller Oria et al 2020) and to estimate the scattering on proton
radiographies when multi-energy proton imaging system is available (van der
Heyden et al 2021).
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Imaging in Particle Therapy
Current practice and future trends
Chiara Paganelli, Chiara Gianoli and Antje Knopf
Chapter 7
Magnetic resonance imaging in particle therapy
C Paganelli, B Oborn, A Hoffmann and M Riboldi
7.1 Introduction
Up til now, computed tomography (CT) has been the clinical imaging standard for
treatment planning, whereas image guidance during treatment commonly relies on
2D x-ray projections. More recently, volumetric imaging has become available
(chapter 5), including gantry, nozzle, or couch-mounted cone beam computed
tomography (CBCT), robotic C-arm CBCT, and CT-on-rails (Landry and Hua
2018). Although x-ray imaging is the current state-of-the-art modality, it presents
several limitations such as the poor soft-tissue contrast, the inability to directly track
moving tumours in real time and the need for extra non-therapeutic imaging dose.
Due to the greater sensitivity of particles to anatomical and density changes along
the beam path, advanced image guidance is therefore essential to make the most of
particle therapy (PT) benefits.
In recent years magnetic resonance imaging (MRI) has gained importance in
radiation therapy for different anatomical sites thanks to its intrinsic advantages: it
is a radiation-free modality, with improved soft-tissue contrast compared to x-ray
imaging, fast dynamic pulse sequences, and functional imaging (Hunt et al 2018,
Paganelli et al 2018b, Corradini et al 2019, Kurz et al 2020). MRI in PT is expected
to combine the ability of MRI to visualize anatomy and biological heterogeneity
with the unique dose-deposition and biological properties of PT. This brings
potential novel opportunities to improve cures by biological dose escalation in
specific cancer types, including pancreatic, central lung, liver, oesophagus, brain and
oligometastatic cancers (Keall et al 2022, Pham et al 2022), as well as in pediatric
patients, where radiation levels must be carefully controlled.
Up-to-date MRI is adopted offline in the clinical routine of PT for precise tumour
contouring and to support treatment planning and verification, with near-room
systems installed in some of the facilities. However, the integration of MRI with
linear accelerators (i.e. MRI-linac) is the most recent advancement in image-guided
conventional radiotherapy, with different configurations being proposed, ranging
doi:10.1088/978-0-7503-5117-1ch7 7-1 ª IOP Publishing Ltd 2024
Imaging in Particle Therapy
Figure 7.1. Potential of MRI in particle therapy with respect to x-ray imaging. Adapted from Pham et al
(
2022). Copyright (2022), with permission from Elsevier.
from in-room (Jaffray et al 2014) to in-beam modalities (Fallone 2014, Keall et al
2014, Lagendijk et al 2014, Mutic and Dempsey 2014). Among the latter, two
commercial systems are currently available (ViewRay MRIdian and Elekta Unity;
Olsen et al 2015, Raaymakers et al 2017, Klüter 2019), which have demonstrated
significant improvement in clinical treatment outcomes, e.g., by enabling dose
escalation for pancreatic cancer (Rudra et al 2019) or benefit from online plan
adaptation (Intven et al 2021, Nierer et al 2022, Kishan et al 2023). Given the higher
dosimetric sensitivity and biological effectiveness, PT would greatly benefit from
MRI guidance, through online delivery monitoring and treatment adaptation (Pham
et al 2022).
A true MRI-integrated proton therapy system could offer superior targeting
accuracy compared with offline imaging options, especially for moving or hard to
visualize tumours ( figure 7.1). A recent study has shown that margin reduction
enabled by a hypothetical MRI-integrated proton therapy could significantly
decrease the normal tissue complication probability with respect to MRI-linac,
offline MRI-guided proton therapy and online CBCT-guided proton therapy for
liver tumours (Moteabbed et al 2021). Several challenges need to be accounted for
in the integration of an MRI scanner with proton beam delivery, but a first preclinical prototype system is currently exploring the mutual interactions between
the t wo systems (Gantz et al 2020) and the prospect of the first clinical MRI-
integrated proton therapy prototype system is on the horizon (Oborn et al 2017,
Hoffmann et al 2020). If MRI-integrated proton therapy seems t o be the next
engineering challenge, the integration of an MRI system with a carbon-ion beam
seems more far from reality. In the meant ime, however, a dedicated near-room
MRI scanner in the PT suite to facilitate offline treatment adaptations and
biological targeting could play an important role to improve treatment efficacy
(Moteabbed et al
2021).
In this chapter, we will revise MR imaging modalities for static and moving
organs (section 7.2), the perspectives for in-beam MRI proton therapy (section 7.3)
and the features of an MRI-guided PT workflow (section 7.4). The book by Liney
et al (Liney and Van der Heide 2019) can also complement this chapter.
7-2
Imaging in Particle Therapy
7.2 MR imaging
MRI forms images via the detection of unpaired nuclear spins. Although a number of
nuclei possess an unpaired nuclear spin and can in principle be detected using MRI, the
hydrogen nucleus is by far the most commonly utilised due to its prevalence in the body
and strong MRI signal. To enable detection of nuclear spins, the imaging object is
placed in a static magnetic field. A combination of radio frequency pulses and magnetic
field gradients is then applied to detect and localise the signal within the so-called kspace data, which is then transformed into the image domain via the application of an
inverse Fourier transform. The flexibility of MRI in providing different contrasts and
information makes this modality particularly suited to image the anatomo-pathological
condition of the patient. In this section, we report an overview of the imaging modalities
for static and moving organs that are currently available in off-line scanners or MRIlinac units and thus can be adopted in off-line workflows or potentially integrated in
future in-room/in-beam MRI-PT systems. A deeper description of MRI basic physics is
beyond the scope of this chapter, and can be found in McRobbie et al (2017).
7.2.1 Imaging of the static anatomy
MR imaging of the static anatomy is usually performed using 3D imaging protocols,
relying on either stacked 2D slices or native 3D MR acquisition sequences. The
former approach has been reported for low-field MRI-linacs using a bSSFP (balanced
steady state free precession) sequence (Gao et al 2018), whereas the latter for 1.5 T
MRI-linacs is relying on 3D SPGR (spoiled gradient recalled acquisition in steady
state; Raaymakers et al 2017), 3D T1 weighted FFE (fast field echo; Werensteijn-
Honingh et al 2019), or 3D T2-weighted scans (Intven et al 2021). These sequences can
be also acquired off-line in conventional near-room diagnostic scanners and with
contrast media to increase tumour visibility and subsequent delineation. Image
resolution depends on the investigated anatomical site, ranging from 3 mm in the
brain (Dirix et al 2014) to high resolution (e.g. 0.5 mm) for ocular treatments (Via et al
2021). Native 3D acquisitions are favorable as they can potentially minimize slice
distortions that occur in 2D imaging due to B
verification of the organ contours in the three anatomical planes (Kurz et al 2020).
This is meant to facilitate inter-fractional treatment adaptation, by capturing daily
images at minimal spatial distortion. Anatomical acquisitions can be also coupled
with quantitative MRI data, such as MR perfusion and diffusion sequences (see
chapter 11; Gurney-Champion et al 2020,Thorwarthet al 2020) which can support
treatment planning for the definition of a biological target volume (BTV; Ling et al
2000) and thus, implement biological adaptation. An interesting application in this
direction is the use of diffusion MRI for glioblastoma and sarcoma (Gao et al 2017),
and central nervous system tumours (Lawrence et al 2021).
inhomogeneity and facilitate the
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7.2.2 Imaging of the moving anatomy
At present, MRI approaches capable of resolving organ motion can be broadly
classified as either time-resolved or respiratory-correlated (4D); the former delivers
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Imaging in Particle Therapy
Figure 7.2. Image acquisition approaches for moving anatomy (adapted from Paganelli et al 2018b). © 2018
Institute of Physics and Engineering in Medicine. All rights reserved.
organ motion data in real-time at comparatively low spatial resolution, whilst the
latter provides better spatial sampling, but relies on retrospective reconstruction (i.e.,
it is not real-time). Ideally, MRI for motion quantification would involve real-time
3D MRI (i.e., sub-second 3D imaging), but due to the intrinsic trade-off between
spatial and temporal resolution, this is still a challenge (figure 7.2).
The limited frequency at which full 3D volumes can be acquired requires the
patient to breathe slowly or limits image quality (Dinkel et al 2009); as such, images
are created by retrospective sorting or prospective acquisition acquired over several
breathing cycles to reconstruct one average representation, in a similar fashion to
conventional 4DCT. Different techniques have been proposed in the literature for
retrospective sorting, including navigator slices, image-based approaches or k-space
data sorting (Stemkens et al 2018, Paganelli et al 2018b). Navigator slices are
typically acquired interleaved with imaging data to be sorted and provide a robust
internal surrogate (von Siebenthal et al 2007, Peteani et al 2023). Image-based
approaches instead rely on extracting internal surrogates directly from MRI data
and use such information for data labelling (Paganelli et al 2018a, Meschini et al
2019). Conversely, k-space techniques sort the raw data into respiratory bins prior to
reconstructing into image space (Mickevicius and Paulson 2017a), by continuously
sampling the center of the k-space in the form of the so-called self-gated or selfnavigated acquisition. While image-based approaches can be easily implemented in
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