Janne Heikkilä

Investigation of Optimal Method for Inducing Harmonic Motion in Tissue Using a Linear Ultrasound Phased Array — A Simulation Study:

Many noninvasive ultrasound techniques have been developed to explore mechanical properties of soft tissues. One of these methods, Localized Harmonic Motion Imaging (LHMI), has been proposed to be used for ultrasound surgery monitoring. In LHMI, dynamic ultrasound radiation-force stimulation induces displacements in a target that can be measured using pulse-echo imaging and used to estimate the elastic properties of the target. In this initial, simulation study, the use of a one-dimensional phased array is explored for the induction of the tissue motion. The study compares three different dual-frequency and amplitude-modulated single-frequency methods for the inducing tissue motion. Simulations were computed in a homogeneous soft-tissue volume. The Rayleigh integral was used in the simulations of the ultrasound fields and the tissue displacements were computed using a finite-element method (FEM). The simulations showed that amplitude-modulated sonication using a single frequency produced the largest vibration amplitude of the target tissue. These simulations demonstrate that the properties of the tissue motion are highly dependent on the sonication method and that it is important to consider the full three-dimensional distribution of the ultrasound field for controlling the induction of tissue motion.

Local Harmonic Motion Monitoring of Focused Ultrasound Surgery—A Simulation Model

In this paper, a computational model for localized harmonic motion (LHM) imaging-based monitoring of high-intensity focused ultrasound surgery (FUS) is presented. The LHM technique is based on a focused, time-varying ultrasound radiation force excitation, which induces local oscillatory motions at the focal region. These vibrations are tracked, using pulse-echo imaging, and then, used to estimate the mechanical properties of the sonication region. LHM is feasible for FUS monitoring because changes in the material properties during the coagulation process affect the measured displacements. The presented model includes separate models to simulate acoustic sonication fields, sonication-induced temperature elevation and mechanical motion, and pulse-echo imaging of the induced motions. These 3-D simulation models are based on Rayleigh-Sommerfield integral, finite element, and spatial impulse response methods. Simulated-tissue temperature elevation and mechanical motion were compared with previously published in vivo measurements. Finally, the simulation model was used to simulate coagulation and LHM monitoring, as would occur with multiple, neighbouring sonication locations covering a large tumor.

Hypofractionated stereotactic body radiotherapy for localized prostate cancer – first Nordic clinical experience

Abstract Background: The use of hypofractionated stereotactic body radiotherapy (SBRT) as primary treatment modality in clinically localized prostate cancer (PCa) is emerging, because the low α/β-ratio favors the use of high dose per fraction in PCa. There is a need for more data about SBRT, especially in high-risk PCa patients. The purpose of this retrospective study was to evaluate the safety and the short-term efficacy of robotic SBRT in a clinical patient cohort with localized PCa including also high-risk patients (D’Amico risk stratification). Materials and methods: A total of 240 consecutive patients with clinically localized PCa were treated primarily with SBRT to total doses of 35 Gy or 36.25 Gy in 5 fractions using a robotic SBRT device (CyberKnife®). All risk groups (D’Amico risk stratification) were represented as follows: 48 (22%), 59 (27%) and 111 (51%) of the patients representing low-, intermediate- and high-risk group, respectively. Data on acute and intermediate-term toxicities and early PSA responses were analyzed. Results: Neither acute grade 3 or higher GU nor rectal toxicity was observed. Regardless of the fact that 29 (13.3%) patients experienced intermediate-term toxicity requiring diagnostic interventions, the rates of intermediate-term grade 3 GU, rectal and infectious toxicity were low, 1.8%, 0.9% and 1.4%, respectively. A biochemical relapse was observed in ten (4.6%) patients. With the median follow-up time of 23 months the biochemical relapse-free survival (bRFS) rate was 100%, 96.6% and 92.8% in low-, intermediate- and high-risk group, respectively. Conclusions: The toxicity of robotic SBRT in a large clinical cohort of PCa patients was tolerable and the early PSA response was good in all risk groups. The hypofractionated SBRT offers a possibility to high dose per fraction and to provide the whole radiotherapy treatment within two to three weeks.

Simulations of lesion detection using a combined phased array LHMI-technique

Ultrasound based elasticity imaging techniques have been developed during the past decades. Some of these techniques are based on an internal radiation force stimulation in which a transient or dynamic radiation force is produced by using a single or dual-frequency sonication. In addition, sonication and data acquisition can be implemented using combined or separate transducers. In this simulation study of lesion detection using localized harmonic motion imaging (LHMI), we used a combined phased array designed for simultaneous thermal ablation and lesion detection. In the sonication mode, a focused single-frequency amplitude-modulated sonication is used to induce harmonic motion and in the tracking mode, some of the array elements are used for pulse-echo tracking of the induced displacements. The results showed that the size of the lesion affected the induced displacement around the sonication point. The displacement tracking simulations demonstrated that these changes in the displacement distributions can be detected using only a few of the array elements in the tracking mode but the exact size of the lesion can not be detected accurately. The simulations also showed that two lesions having the radius of 2.5mm can be distinguished if distance between these lesions is at least 2.5mm.

3D simulations of difference frequency effects on a blood vessel in ultrasound-stimulated vibro-acoustography

Ultrasound-stimulated vibro-acoustography (USVA) is one of the recent noninvasive techniques that have been developed to explore mechanical properties of soft tissues. USVA is based on the interference of two focused ultrasound beams that have a slightly different frequency. This interference produces time variation to the acoustic radiation force that vibrates the joint focal region at the difference frequency. In this paper, we have simulated displacement amplitudes in the blood vessel using different difference frequencies and elasticity parameters. The difference frequency was varied between 300 Hz and 2.5 kHz, and two different stiffness parameters (60 kPa and 120 kPa) for the blood vessel wall were used. The stimulation transducer used in the simulations consists of two concentric and focused ring elements whose base frequency is 0.75 MHz. The stimulation fields have been computed in a homogeneous domain using the Rayleigh integral. The simulations of the vibrations have been computed using the finite element method. The displacement simulations are computed in an inhomogeneous domain that consists of soft tissue and a blood vessel. From the simulations, it can be seen that USVA is sensitive to the mechanical properties of vessel wall and that the displacements are highly dependent on the difference frequency and the material parameters.

Simulations of localized harmonic motions on a blood vessel wall induced by an acoustic radiation force used in ultrasound elastography

Many noninvasive techniques have been developed recently to explore the mechanical properties of soft tissue. In this paper, dynamic acoustic radiation force induced vibrations on a blood vessel wall were simulated using different stimulation frequencies and stiffness parameters for the vessel wall. The stimulation frequency was varied between 20 Hz and 20 kHz and the stiffness parameter (Youngs modulus) was varied between 60 kPa and 360 kPa. The vibration simulations were computed using a finite-element method in a 3D geometry that contained a vessel wall surrounded by soft tissue. The results indicate that vibrations caused by acoustic stimulation are sensitive to the changes in mechanical properties of the vessel wall and that the vibrations are highly dependent on the stimulation frequency and target structure. Therefore, measurements of absolute stiffness parameters may not be accurately achieved because this method is so dependent on the whole target structure, whereas the monitoring of changes during some process may be feasible.

Dosimetric evaluation of modern radiation therapy techniques for left breast in deep-inspiration breath-hold

PURPOSE The dosimetric differences between four radiation therapy techniques for left sided whole breast irradiation were evaluated side by side in the same patient population. METHODS Radiotherapy treatment plans were retrospectively created with Accuray TomoDirect (TD), Elekta Volumetric Modulated Arc Therapy (E-VMAT), Varian RapidArc (RA) and Field-in-field (FinF) technique for 20 patients, who had received left breast irradiation during deep-inspiration breath-hold. Dose characteristics of planning target volume and organs at risk were compared. RESULTS The E-VMAT, TD and RA treatment plans had higher target coverage (V95%) than FinF plans (97.7-98.3% vs. 96.6%). The low-dose spillage to contralateral breast and lung was smaller with FinF and TD (mean 0.1 and 0.3 Gy) compared to E-VMAT and RA (mean 0.6 and 0.9 Gy). E-VMAT, RA and TD techniques were more effective than FinF in sparing left anterior descending artery (mean 4.0, 4.2 and 4.7 Gy vs. 6.1 Gy, respectively). CONCLUSIONS In whole breast irradiation TD, E-VMAT and RA plans generated in this study achieved higher dose coverage and sparing of organs from the high dose in the vicinity of the PTV. The advantage of calculated FinF plans is the lowest dose on contralateral organs. The choice of the technique used should be weighted by each institution taking into account the dose characteristics of each technique and its fit with patient anatomy bearing in mind the increased workload of using modulated techniques and the increased beam on time.

A Simulation Model for Local Harmonic Motion Monitoring of Focused Ultrasound Surgery

A computational model for local harmonic motion (LHM) imaging‐based monitoring of high‐intensity focused ultrasound surgery (FUS) is presented. LMH technique is based on a focused ultrasound radiation force excitation, which induces local mechanical vibrations at the focal region. These pulse‐echo imaged vibrations are then used to estimate the mechanical properties of the sonication region. LHM has been proven to be feasible for FUS monitoring because changes in the material properties during the coagulation affect the measured displacements. The presented model includes separate models to simulate acoustic fields, sonication induced temperature elevation and mechanical vibrations, and pulse‐echo imaging of the induced motions. These simulation models are based on Rayleigh integral, finite element, and spatial impulse response methods. Simulated temperature rise and vibration amplitudes have been compared with in vivo rabbit experiments with noninvasive MRI thermometry.

Simulations of Localized Harmonic Motion Imaging for Ultrasound Surgery Monitoring

During the past few years, many noninvasive ultrasound techniques have been developed to explore mechanical properties of soft tissues. Some of these methods have been proposed to be useable for ultrasound surgery monitoring. In Localized Harmonic Motion Imaging (LHMI), a dynamic ultrasound radiation force stimulation is used to cause displacements in a target which are measured and used in the estimation of elastic properties of the target. In this study, we simulated displacements in soft tissue using different stimulation configurations while elastic parameters and absorption coefficients were varied as a function of temperature. The displacements were computed using a finite element method in an inhomogeneous domain that consisted of a spherical target surrounded by soft tissue. From the vibration simulations, it can be seen that displacements in LHMI are dependent on the stimulation technique and on the properties of the target when temperature changes. The results indicate that the method was able t...

A photographic technique for quick assessment of mechanical isocenter of a linear accelerator

Highlights • A photography-based method for precise determination of the mechanical isocenter is introduced.• The method is based on a modified front pointer, camera and automatic image-processing algorithm.• The method can be used to determine and visualize the size and shape of the isocenter in 3D.• Importance of the 3D analysis of the isocenter is justified in the results of the feasibility tests.• The developed tool produces also trend curves to detect slight temporal changes of the isocenter.