AK&M 20 November 2024 14:21
Scientists from the Faculty of Physics of Moscow State University, together with radiologists from the University Clinic of the Medical Scientific and Educational Institute of Lomonosov Moscow State University (MNOI MSU), investigated the possibilities of improving the method of noninvasive ultrasound therapy of the brain. This method, only recently introduced into clinical practice, is based on controlled irradiation of small areas of the human brain with focused ultrasonic waves. To date, operations using focused ultrasound are quite lengthy and require lengthy preparation. On the basis of a pre-performed volumetric computed tomography (CT) of the human head, a detailed three-dimensional acoustic model of it is built and radiation planning is carried out, calculating the passage of ultrasound through the heterogeneous bones of the skull and determining the phases on each of the elements of a multi-element ultrasonic array to correct distortions (aberrations) of the wave field introduced by the skull.
Scientists from Moscow State University conducted a study on the possibility (in the near future) of simplifying, reducing the cost and improving the accuracy of neurosurgical operations using focused ultrasound. Usually, in such operations, focused ultrasound treatment is performed under the control of magnetic resonance imaging (MRI) thermometry. CT and MRI images of the patient’s head are combined and irradiation is performed using pre-calculated phases. The data obtained allow us to hope that it will be possible to plan and perform such treatment procedures using only MRI data. The study showed that, although CT data currently provide more accurate information at the planning stage, MRI data already allow for the initial selection of patients for whom treatment is possible and who are promising for more accurate phasing of ultrasound elements during irradiation. In the future, it is planned to use neural networks to synthesize a CT analogue from an MRI image. The work on the project was carried out within the framework of the Scientific and Educational School of Moscow State University “Photonic and Quantum Technologies. Digital medicine”.
Such studies aim to create the basis for the development and implementation of a new direction in medicine – ultrasound neurosurgery. The idea of the method of noninvasive ultrasound neurosurgery is to focus an intense ultrasound beam with frequencies of the order of hundreds of kilohertz through the skull into specified areas of the brain. This leads to local heating of brain tissues and their subsequent destruction. This procedure is an attractive alternative to surgery, as it does not require opening the skull and allows you to affect hard-to-reach central areas of the brain (the thalamus region). In practice, this method of remote and local effects on brain structures has already been used to treat, for example, diseases such as essential tremor, tremor caused by Parkinson’s disease, and muscular dystonia, and is being implemented in many countries of the world, including Russia.
The main obstacle for ultrasound on the way to the site of exposure is the bones of the skull, which differ greatly in their acoustic properties from the surrounding soft tissues. In particular, the density of bone tissue and the speed of sound in it are significantly higher than in water and brain tissues. When passing through the skull, the ultrasonic beam is partially reflected, and also experiences noticeable absorption and strong distortions (aberrations). Due to the corresponding energy losses, the intensity of the wave in the target irradiation area may not be sufficient to destroy the biological tissue. If you compensate for the loss by increasing the power of the emitter, the patient’s skull may be overheated. Due to aberrations caused by inhomogeneous thickness and internal structure of the bones of the skull, the focus is blurred, hot spots appear in undesirable places, i.e. areas that were not intended for irradiation may be damaged. The introduction of ultrasound neurosurgery into medical practice turned out to be possible only relatively recently, after it was possible to compensate for the aberrations of the wavefront of the beam.
Currently, the aberration compensation procedure is being implemented based on computed tomography (CT) data obtained before the operation.
“Each voxel of the computed tomography image has its own value in Hounsfield units,” explains Elena Mershina, a leading researcher at the Department of Radiation Diagnostics of the Medical Scientific and Educational Institute of Moscow State University, Associate professor of the Department of Radiation Diagnostics and Therapy at the Faculty of Fundamental Medicine of Moscow State University. – The Hounsfield scale shows how a particular tissue weakens X-ray radiation when passing through it. For water, this value in Hounsfield units is zero. There is a linear correlation between the values on the Hounsfield scale and the density of biological tissue, therefore, CT data, unlike, for example, magnetic resonance imaging (MRI) data, allow us to obtain tissue density values in each voxel of the image.”
Knowing the density, it is possible to calculate the speed of sound and, thus, restore the acoustic parameters of tissues. Information about the speed of sound helps to calculate the phase delays on the radiator elements in order to form the necessary wavefront in front of the patient’s head surface. This stage of the operation can be called preparatory, the calculation of phases is carried out before irradiation for each target point in the brain and uses CT data. The irradiation itself is carried out under the control of MRI thermometry and includes several stages. The first stage is combining CT and MRI images and aiming. The irradiation is carried out at low power, when the heating is not enough to destroy the tissues, but it is visible on MRI. If at the first stage the target point and the heating area coincide, then the power is slightly increased and causes reversible disorders of the brain cells. After that, the patient is examined by a neurologist, with successful exposure (for example, the patient has lost tremor, muscle stiffness has decreased), this point is irradiated at high power. Irreversible tissue destruction occurs, i.e. necrosis, and the patient has a decrease in symptoms of the disease. As you can see, the procedure requires careful preparation, high positioning accuracy, as well as a large amount of data, both CT and MRI. In this case, a separate task is to combine the previously obtained CT image and the real-time measured MRI image.
The disadvantage of using CT for surgery planning is its potential harm (X-ray irradiation is used), as well as the impossibility of additional phase correction already during irradiation. In this regard, one of the tasks of ultrasound brain surgery is to simplify and facilitate the operation, and to investigate the possibility of using only MRI data. A team of scientists from Moscow State University conducted a series of numerical experiments to answer this question. Acoustic models of the patient’s head were constructed based on an anonymized set of CT and MRI data from the same patient. The kit was taken from the archival data of the Department of Radiation Diagnostics of the Medical Scientific and Educational Institute of Moscow State University. The patient underwent both studies as prescribed by the attending physician. The spatial boundaries of the tissues were reconstructed from the scan data. The restoration of boundaries is equally well performed according to both CT and MRI data. Modeling the propagation of an ultrasonic beam in such acoustic models has shown that information about the internal structure of the skull bones is important for the most accurate compensation of aberrations (phase selection on the radiator elements to form a converging front in the brain).
“The possibilities of these two approaches, CT and MRI, are different in terms of specifying the structural details of the environment,” notes Daria Chupova, a participant in the project, a graduate student at the Faculty of Physics at Moscow State University. – In each voxel of the model based on CT data, the density and speed of sound can be calculated. The acoustic parameters in the model based on MRI, on the contrary, are spatially homogeneous for each type of tissue and a priori unknown, for their assignment it is necessary to use the averaged values of the data obtained from CT.”
It would seem that these considerations do not allow us to abandon CT. However, the results of the work showed that MRI data can be used to select patients, pre-plan surgery and predict the outcome. “The CT—based model is more complete,” says Oleg Solontsov, a graduate student at the Laboratory of Medical and Industrial Ultrasound at Moscow State University. – However, artificial intelligence technologies are actively developing now and work is already underway to train a neural network to synthesize a CT image based on MRI data. We also plan to move in this direction to create more accurate, safe and effective protocols for ultrasound brain surgery.”
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