ABSTRACT
The propagation of ultrasound through complex biological media, such as the human calvarium, poses a great challenge for modern medicine. Several ultrasonic techniques commonly used for treatment and diagnosis in most of the human body are still difficult to apply to the human brain, in part, because of the properties of the skull.
Moreover, an understanding of the biomechanics of transcranial ultrasound may provide needed insight into the problem of blast wave induced traumatic brain injury (TBI). In the present study, the spatial variability of ultrasonic properties was evaluated for relevant frequencies of 0.5, 1, and 2.25 MHz.
A total of eighteen specimens from four donors were tested using a through‐transmission configuration. With the aid of a two interface model, the ultrason ic attenuation coefficient was determined from the total energy loss at various locations on the specimens. With the same setup, speed of sound through the bone layer at the same locations was also determined. Mean volumetric densities at various locations on the samples were determined from computed tomography images.
The results show good correlation between attenuation and volumetric density, particularly for the higher frequencies. In addition, the spatial variability of the attenuation, within a single person and with respect to different people, was found to be much larger than expected. These results are anticipated to have a major impact on transcranial biomedical research.
BACKGROUND
Acoustic waves, also known as pressure waves, travel through matter carrying mechanical energy. When these pressure waves vibrate at a frequency above the human hearing range (20 kHz) they are known as ultrasound. The presence of a medium is essential to the transmission of ultrasonic waves, i.e., sound waves cannot propagate in vacuum
Longitudinal Waves:
From all the types of waves, the longitudinal, L wave, or also called compression wave, has been the most widely used since it will travel in liquids, solids, or gases and is easily generated and detected.
Longitudinal waves have been used for non-destructive testing (NDT) and materials characterization (Aggelis et al., 2007; Ghoshal, 2008; Lionetto et al., 2005) and longitudinal ultrasound transmission through the human skull is used for imaging, doppler imaging, and therapeutic urposes (Fry, 1977; Kinoshita et al., 2006; Pichar do et al., 2011). They exist when the motion of the particles in a medium is parallel
to the direction of wave propagation.
Ultrasonic Transducers:
The ultrasonic transducer is the element used to generate and detect ultrasound. By analogy, one can think of a transducer in transmission mode as a stereo speaker and the transducer in reception mode as the human ear. There are different types of transducers depending on the application for which they are designed.
The Basic Elements of an Ultrasound Transducer Are: Housing for Protection of Internal Components, Circuitry to Input and Output Electrical Signals, Piezoelectric Element, an Impedance Matching Material, and Insulator for Noise Control.
A and C Data Acquisition Modes:
The most common mode for ultrasound data collection is the A-mode (A stands for amplitude)(Christopher F. Njeh et al., 1999). It consists of a time-base display like the one shown at the top.
The display shows the amplitude of the signal received by the input transducer as a function of time. In UTWin®, the software employed with the immersion system used in the present work, various parameters such as gain, time-window, sampling rate, and number of averages per second can be adjusted.
Typical UTWin® Display Showing both A-mod e and C-mode.
CALVARIUM SPECIMENS
The human skeleton, as any other endoskeleton of other vertebrates, is composed of rigid structures called bones. Bones are a type of connective tissue and their main purpose is to provide support to other organs in the body. The hierarchical structural composition of bone plays a major role in its mechanical and ultrasonic properties. For instance, the known compressive strength of bone can be attributed more to the microstructure than to the total bone mass. Furthermore, it has been discussed in various publications that its high ultrasound attenuation is the result of the complexity of its internal microstructure.
Human Calvarium:
Since bone is a biological material, it is expected to have a large local variation in its properties. At both the micro and macro-scales the human calvarium and its composing bones exhibit this local variability in properties as well (Smith, 2001). Major changes in curvature, thickness, and density can be encountered in even small skull areas. This makes the human skull one of the most difficult materials for ultrasound propagation (Aubry et al., 2003).
The Human Skull Is Composed by Several Irregularly Shaped Bones Including the Lower Mandible. The Frontal, Two Parietal, Two Temporal, the Occipital, and the Sphenoid Bones Conform the Calvarium.
DENSITY FROM COMPUTED TOMOGRAPHY
The principle behind computed tomography (CT) was proposed prior to the development of the first automated processors. Perhaps the first fully functional CT scanner used for medical applications was designed by Godfrey N. Hounsfield in 1967. Currently CT is one of the most popular techniques for biomedical non-invasive evaluation.
The image reconstruction that takes place in a CT scanner, and that was first enunciated in the 1940’s, follows the idea that by evaluating 2D projections of an object from multiple angles, the internal composition of such 3D object can be regenerated.
A 3d Ct Image Is Reconstructed From Sever Al 2d X-rays. These 2d X-rays Are Taken at Slightly Different Angles Around the 360 Degrees. Then, a Code Following a Simple Algorithm First Proposed in the 1940’s, Makes Sense of All the Different Projections and Composes the 3d Image.
PROPAGATION OF ULTRASOUND THROUGH HUMAN CALVARIUM
Equipment:
Transducers
Four ultrasound transducers with central frequencies of 0.5, 1, 2.25, and 5 MHz, respectively, were used in the present investigation. These frequencies are found in the frequency profile of blast waves and are also relevant for medical purposes in both diagnoses and treatment of neural diseases.
Sound Absorbing Foam:
When the sound beam propagates through the water in the test tank, it hits the target (calvarium fragment) and scatters in different directions. These scattered waves can then reflect on other objects in the tank and the walls of the tank itself. Part of these reflections may be received by the transducer creating a noisy signal. To prevent this artifact it is often recommended to pad the tank internally with sound absorbers. In this work it was decided to place a dividing wall between the sample and the source transducer.
Multi-transducer Fixture Made With T-slotted Framing Allowed the Placement of Transducer Holders and Sound Absorbing Foam Wall. The Distance Between Transducers Can Be Adjusted.
RESULTS AND DISCUSSION
Wave speed:
In the present study, the average group velocity for each calvarium fragment has been calculated at 0.5, 1 and 2.25 MHz. It is important to note that neither the setup used in this work, nor the ultrasonic transducers selected were optimized for wave speed measurements. The broadband transducers employed produce very short pulses (approximately 3 cycles).
CONCLUSIONS AND FUTURE WORK
In this thesis, experimental equipment and protocols have been established for measurement of speed of sound and attenuation coefficient on human calvarium fragments under ex-vivo conditions.
A pulse-echo propagation configuration has been effectively employed to obtain thickness and curvature of excised human calvarium fragments. These thickness and curvature data have been later used to calculate ultrasonic properties of skull bone. From thickness data, great variability of the thickness in even small regions in the calvarium fragments can be seen.
From through-transmission pulses propagated through human calvaria, along with thickness data from pulse-echoes, the speed of sound has been determined at multiple locations on the samples. Some limitations of thickness and angle of incidence have been encountered such that calculations of sound speed from through-transmission pulses in human calvaria were difficult. However, average speeds of sound for the eighteen fragments were obtained and the values were in agreement with those reported by previous authors.
Attenuation of ultrasound in human calvaria has been assessed at frequencies of 0.5, 1 and 2.25 MHz at various locations on the samples. The attenuation coefficients have been obtained from the measurements of pressure drop in multiple locations on the total of eighteen samples from four head donors. Moreover, the spatial variation of the attenuation coefficient has been explored from the numerous data extracted from the experiments. The attenuation coefficient data distributions for the eighteen fragments at the same frequencies were obtained.
These histograms along with the results of a t-test performed on the data suggest that the attenuation coefficient varies spatially much more than expected. Additionally, a high variation of attenuation coefficients can be expected in calvaria from different donors. Furthermore, such spatial variation seems to increase with frequency for the three frequencies studied, indicating an increased sensitivity of attenuation with changes in the microstructure of bone.
Quantitative densitometry has also been performed on the human calvarium samples. Polymer phantoms have been used as density references during the computed tomography sessions. Bone density information has been obtained from computed tomography images through an adapted Matlab code.
Average volumetric densities have been correlated with the ultrasonic properties. No strong correlation between mean volumetric density and speed of propagation of sound through calvarium bone has been found. On the contrary, attenuation coefficient, especially for higher frequencies, shows some correlation with mean volumetric density. This result again suggests a stronger sensitivity of attenuation in calvarium with changes in the microstructure that are related to density.
From the results obtained in the present work, it can be concluded that the statistics of the variation of the ultrasonic properties of calvarium for different frequencies, locations and donors, have to be considered in computational and analytical models of this phenomenon. A realistic model of the propagation of ultrasound through calvarium should replicate the behavior described here.
Moreover, future research should explore practical experiments that can be performed in-vivo and provide information to predict the speed of sound, attenuation coefficient and its variability at different locations of the calvarium. Thus, diffuse ultrasonic backscatter may be used to obtain scattering information that can be related to speed of sound and attenuation coefficient.
Source: University of Nebraska
Author: Armando Garcia Noguera
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