Respiratory organ motion in interventional MRI : tracking, guiding and modeling

Respiratory organ motion is one of the major challenges in interventional MRI, particularly in interventions with therapeutic ultrasound in the abdominal region. High-intensity focused ultrasound found an application in interventional MRI for noninvasive treatments of different abnormalities. In order to guide surgical and treatment interventions, organ motion imaging and modeling is commonly required before a treatment start. Accurate tracking of organ motion during various interventional MRI procedures is prerequisite for a successful outcome and safe therapy. In this thesis, an attempt has been made to develop approaches using focused ultrasound which could be used in future clinically for the treatment of abdominal organs, such as the liver and the kidney. Two distinct methods have been presented with its ex vivo and in vivo treatment results. In the first method, an MR-based pencil-beam navigator has been used to track organ motion and provide the motion information for acoustic focal point steering, while in the second approach a hybrid imaging using both ultrasound and magnetic resonance imaging was combined for advanced guiding capabilities. Organ motion modeling and four-dimensional imaging of organ motion is increasingly required before the surgical interventions. However, due to the current safety limitations and hardware restrictions, the MR acquisition of a time-resolved sequence of volumetric images is not possible with high temporal and spatial resolution. A novel multislice acquisition scheme that is based on a two-dimensional navigator, instead of a commonly used pencil-beam navigator, was devised to acquire the data slices and the corresponding navigator simultaneously using a CAIPIRINHA parallel imaging method. The acquisition duration for four-dimensional dataset sampling is reduced compared to the existing approaches, while the image contrast and quality are improved as well. Tracking respiratory organ motion is required in interventional procedures and during MR imaging of moving organs. An MR-based navigator is commonly used, however, it is usually associated with image artifacts, such as signal voids. Spectrally selective navigators can come in handy in cases where the imaging organ is surrounding with an adipose tissue, because it can provide an indirect measure of organ motion. A novel spectrally selective navigator based on a crossed-pair navigator has been developed. Experiments show the advantages of the application of this novel navigator for the volumetric imaging of the liver in vivo, where this navigator was used to gate the gradient-recalled echo sequence

Simultaneous multislice triple-echo steady-state (SMS-TESS) T1, T2, PD, and off-resonance mapping in the human brain

Purpose To investigate the ability of simultaneous multislice triple‐echo steady‐state (SMS‐TESS) imaging to provide quantitative maps of multiple tissue parameters, i.e., longitudinal and transverse relaxation times (T1 and T2), proton density (PD), and off‐resonance (ΔB0), in the human brain at 3T from a single scan. Methods TESS acquisitions were performed in 2D mode to reduce motion sensitivity and accelerated by an SMS excitation scheme (CAIPIRINHA) with SENSE reconstruction. SMS‐acceleration factors (R) of 2 and 4 were evaluated. The in vitro and in vivo validation process included standard reference scans to analyze the accuracy of T1, T2, and ΔB0 estimates, as well as single‐slice TESS measurements. Results For R = 2, the quantification of T1, T2, PD, and ΔB0 was overall reliable with marginal noise enhancement. T1 and T2 values were in good agreement with the reference measurements and single‐slice TESS. For R = 4, the agreement of ΔB0 with the standard reference was excellent and the determination of T1, T2, and PD was reproducible; however, increased variations in T1 and T2 values with respect to single‐slice TESS were observed. Conclusion SMS‐TESS has shown potential to offer rapid simultaneous T1, T2, PD, and ΔB0 mapping of human brain tissues

Simultaneous acquisition of image and navigator slices using CAIPIRINHA for 4D MRI

Respiratory organ motion is still the major challenge of various image-guided treatments in the abdomen. Dynamic organ motion tracking, necessary for the treatment control, can be performed with volumetric time-resolved MRI that sequentially acquires one image and one navigator slice. Here, a novel imaging method is proposed for truly simultaneous high temporal resolution acquisition.; A standard balanced steady state free precession sequence was modified to simultaneously acquire two superimposed slices with different phase cycles, namely an image and a navigator slice. Instead of multiband RF pulses, two separate RF pulses were used for the excitation. Images were reconstructed using offline CAIPIRINHA reconstruction. Phantom and in vivo measurements of healthy volunteers were performed and evaluated.; Phantom and in vivo measurements showed good image quality with high signal-to-noise ratio (SNR) and no reconstruction issues.; We present a novel imaging method for truly simultaneous acquisition of image and navigator slices for four-dimensional (4D) MRI of organ motion. In this method, the time lag between the sequential acquisitions is eliminated, leading to an improved accuracy of organ motion models, while CAIPIRINHA reconstruction results in an improved SNR compared with an existing 4D MRI approach

Model-guided respiratory organ motion prediction of the liver from 2D ultrasound

With the availability of new and more accurate tumour treatment modalities such as high-intensity focused ultrasound or proton therapy, accurate target location prediction has become a key issue. Various approaches for diverse application scenarios have been proposed over the last decade. Whereas external surrogate markers such as a breathing belt work to some extent, knowledge about the internal motion of the organs inherently provides more accurate results. In this paper, we combine a population-based statistical motion model and information from 2d ultrasound sequences in order to predict the respiratory motion of the right liver lobe. For this, the motion model is fitted to a 3d exhalation breath-hold scan of the liver acquired before prediction. Anatomical landmarks tracked in the ultrasound images together with the model are then used to reconstruct the complete organ position over time. The prediction is both spatial and temporal, can be computed in real-time and is evaluated on ground truth over long time scales (5.5 min). The method is quantitatively validated on eight volunteers where the ultrasound images are synchronously acquired with 4D-MRI, which provides ground-truth motion. With an average spatial prediction accuracy of 2.4 mm, we can predict tumour locations within clinically acceptable margins

Ultrasound-driven 4D MRI

We present an ultrasound-driven 4D magnetic resonance imaging (US-4DMRI) method for respiratory motion imaging in the thorax and abdomen. The proposed US-4DMRI comes along with a high temporal resolution, and allows for organ motion imaging beyond a single respiratory cycle. With the availability of the US surrogate both inside and outside the MR bore, 4D MR images can be reconstructed for 4D treatment planning and online respiratory motion prediction during radiotherapy. US-4DMRI relies on simultaneously acquired 2D liver US images and abdominal 2D MR multi-slice scans under free respiration. MR volumes are retrospectively composed by grouping the MR slices corresponding to the most similar US images. We present two different US similarity metrics: an intensity-based approach, and a similarity measure relying on predefined fiducials which are being tracked over time. The proposed method is demonstrated on MR liver scans of eight volunteers acquired over a duration of 5.5 min each at a temporal resolution of 2.6 Hz with synchronous US imaging at 14 Hz-17 Hz. Visual inspection of the reconstructed MR volumes revealed satisfactory results in terms of continuity in organ boundaries and blood vessels. In quantitative leave-one-out experiments, both US similarity metrics reach the performance level of state-of-the-art navigator-based approaches

Impact of internal target volume definition for pencil beam scanned proton treatment planning in the presence of respiratory motion variability for lung cancer: A proof of concept.

PURPOSE Motion management is crucial in scanned proton therapy for mobile tumours. Current motion mitigation approaches rely on single 4DCTs before treatment, ignoring respiratory variability. We investigate the consequences of respiratory variations on internal target volumes (ITV) definition and motion mitigation efficacy, and propose a probabilistic ITV based on 4DMRI. MATERIALS AND METHODS Four 4DCT(MRI) datasets, each containing 40 variable cycles of synthetic 4DCTs, were generated by warping single-phase CTs of two lung patients with motion fields extracted from two 4DMRI datasets. Two-field proton treatment plans were optimised on ITVs based on different parts of the 4DCT(MRI)s. 4D dose distributions were calculated by considering variable respiratory patterns. Different probabilistic ITVs were created by incorporating the voxels covered by the CTV in at least 25%, 50%, or 75% (ITV25, ITV50, ITV75) of the cycles, and compared with the conservative ITV encompassing all possible CTV positions. RESULTS Depending on the selected planning 4DCT, ITV volumes vary up to 20%, resulting in significant variation in CTV coverage for 4D treatments. Target coverage and homogeneity improved with the conservative ITV, but was associated with significantly increased lung dose (~1%). ITV25 and ITV50 led to acceptable plan quality in most cases without lung dose increments. ITV75 best minimised lung dose, but was insufficient to ensure coverage under all motion scenarios. CONCLUSION Irregular respiration significantly affects CTV coverage when ITVs are only defined by single 4DCTs. A probabilistic ITV50 provides an adequate compromise between target coverage and lung dose for most motion and patient scenarios investigated

Molecular oxygen loading in candidate theranostic droplets stabilized with biocompatible fluorinated surfactants: Particle size effect and application to in situ19F MRI mapping of oxygen partial pressure

Perfluorocarbon nano- and micron-sized emulsions are a new field of investigation in cancer treatment due to their ability to be used as imaging contrast agents, or as delivery vectors for pharmaceuticals. They also demonstrated capability to enhance the efficiency of high intensity focused ultrasound thermo-therapy. In the context of new biomedical applications we investigated perfluorooctyl bromide (PFOB) theranostic droplets using 19F NMR. Each droplet contains biocompatible fluorinated surfactants composed of a polar Tris(hydroxymethyl)aminomethane head unit and hydrophobic perfluorinated tail (abbreviated as F-TAC). The influence of the droplet size on the oxygen loading capacity was determined from longitudinal relaxation (T1) data of 19F NMR signal