Risks associated with ultrasound procedure - Essay Example

Risks Associated with Ultrasound Procedure Introduction Ultrasound is the mechanical transmission of energy all the way through a medium at a frequency above the upper limit of human hearing (i.e., above roughly 20 kHz). (White, 2006) At frequencies well above this limit, medical ultrasound applies high frequency (i.e., typically 0.5 to 20 MHz) acoustic energy to a region of interest for the purpose of diagnostic imaging or therapeutic treatment. (Vaezy, 2007) Ultrasonic imaging involves mechanical energy, in which pressure waves pass across tissue. Image is formed by the reflection and dispersing back to the transducer. Risks Associated with Ultrasound Procedures There are several physical effects of ultrasound, which are classified as follows: 1. Thermal effects It involves heating of tissues due to the absorption of ultrasonic waves by the tissues. Some heat is also generated at the surface of transducer; 2. Cavitation Cavitation involves the production of gas bubbles at extreme negative pressures; 3. Gas- Body Effect Exposure to Ultrasonic waves stimulates bioeffects within mammalian tissue through gas body activation which is non-thermal mechanism. (Vella, 2003) 4. Other mechanical effects Radiation forces may get access to body fluids and create pressure at tissue interfaces. These physical effects of ultrasound are discussed below in detail. Gas- Body Effect of Ultrasound (Nahirnyak, 2007) While diagnostic imaging is likely the most widely recognized (e.g., obstetric sonography) application of medical ultrasound, therapeutic ultrasound predates diagnostic medical ultrasound by at least two decades with the earliest investigation of focused therapeutic ultrasound reported over 60 years ago; study of lesion formation in ex vivo beef liver and in situ brain of cats and dogs). (Shaw A and Haar, 2006) But despite its early start, medical applications of therapeutic ultrasound waned due to the lack of adequate imaging for targeting and monitoring of treatments. (Smith, 2001) However, with recent advances in imaging, therapeutic ultrasound is making resurgence in the medical community. High-intensity focused ultrasound (HIFU) is such a therapeutic application, with intensities that are generally 1,000 to 10,000 times greater than traditional diagnostic ultrasound devices ( Table 1) for the purpose of selective destruction of tissues deep within the body. In its clinical application, acoustic energy is focused to a well-defined region of tissue (focal region or zone), and the energy that has not been scattered is absorbed and converted into heat. (Soneson, 2009) Due to the high intensities in the focal region, temperatures quickly rise to levels that will result in rapid cellular necrosis, creating a well-defined region of necrosed tissue while leaving the surrounding tissue undamaged. Penetration of Ultrasonic Waves in the Skin (Shaw, 2006) This lesion may be on the order of one to a few centimeters in length and one to a few millimeters in diameter. Electron microscopy has revealed that a narrow boundary of about six to ten cells in width exists between live cells and dead cells at the edges of the lesion. Such localization of the damaged region is clinically advantageous by permitting targeting in close proximity to sensitive structures (e.g., nerves). (Haar, 2001) Studies have demonstrated instantaneous thermal necrosis when temperatures in excess of 56oC are maintained for 1 second or longer. As such, high-intensity therapies seek to rapidly (i.e., within just a few seconds) achieve focal temperature rises in excess of 56oC. While these high-intensity ultrasound technologies offer many clinical advantages, they also present several limitations and challenges. (Haar, 2007) Limitations include reduced effectiveness through air-filled regions (e.g., lungs, bowels) and bone (e.g., brain, ribcage), costly and limited real-time monitoring of treatment, and the potential for incomplete ablation of targeted tissues (a concern for treatment of cancerous lesions). One of the primary challenges of high-intensity ultrasound is the prevention of unintended thermal ablation/damage to surrounding pre-focal and post-focal tissues. Of particular concern is the potential for energy absorption in bone, leading to thermal conduction to nearby tissues. For example, bone is a highly attenuating medium that has the potential to generate unintended temperature rises when exposed to high-intensity ultrasound and to then conduct its thermal energy to nearby tissues, presenting potentially significant safety concerns for diagnostic and therapeutic uses of high-intensity ultrasound systems. Sufficiently high temperatures can damage surrounding tissues such as nerves, resulting in potential acute and/or chronic discomfort, pain, and/or impaired function. (Haar, 2008) Table 1. Typical parameters of HIFU versus diagnostic ultrasound Little work has been done to quantify the effect of the presence of bone on the extent and magnitude of heating and of cellular necrosis within and around a region of tissue exposed to high-intensity ultrasound. (Protopappas, 2007) And, while there are FDA guidance documents and/or international consensus standards for lower-intensity applications of diagnostic ultrasound and physiotherapy (i.e., FDA guidance for diagnostic US, 1997; IEC 60601-2-5, 2000; IEC 1689, 1996), currently there are no available references for high-intensity systems (there are efforts in progress). Furthermore, the only consensus standards that address heating due to the presence of bone (i.e., AIUM/NEMA UD 3-2004 and IEC 60601-2-37, referred to as the “Output Display Standards”) are limited in the following ways: they predict steady-state temperature rise, only (not transient); they do not account for nonlinear propagation in tissue (as would occur in high-intensity applications); and, they do not directly account for thermal heating from shear waves in bone. Kourtis (2007) conducted an analytical study of longitudinal and shear wave propagation in tissue. The study found negligible heating from shear waves in tissue, compared with longitudinal waves, despite higher attenuation coefficients (67x103 dB/cm and 1.33 dB/cm at 5 MHz, respectively). Consequently, analytical and computational efforts described within this dissertation modeled the tissue as a viscous fluid (also as homogeneous). (Protopappas, 2007) Generation of an Ultrasound (Vaezy, 2007) Ren and Zhou (2009) conducted an experimental study of the heating in an in vivo dog thigh (muscle and femur) from 20 minutes of focused hyperthermia. (Ren X-L, Zhou X-D, Yan R-L, Liu D, Zhang, 2009) The study used transducers that ranged from 6 to 8 Watts of power, operated from 1 to 3.58 MHz. The temperatures were measured with 0.7 mm manganin-constantin thermocouples (this type is presently undefined) placed in an array in the dog thigh muscle; one thermocouple was placed in a hole drilled in the thigh bone. The study found that the maximum temperature occurred at the surface of the bone – this finding is not in agreement with Nahimyak (2007) which reported the maximum temperature to be ~ 7 to 8 mm within the bone in the cancellous region. (Nahirnyak, 2007) Myers (2005) conducted a computational study of the propagation and heating in a muscle-bone system from focused (Gaussian) hyperthermia. The study was limited to normal incidence, only; it did not model shear waves in the bone; it presumed the thermal conductivity and specific heat of the bone were equal to that of the tissue; and, it evaluated steady-state temperature rises, not transient temperatures. The study evaluated the heating from a 0.057 Watt, Gaussian-shaded transducer (I0 = 0.1 W/cm2) operating at 2 MHz. The article concluded that the temperature rise due to reflected wave (back into tissue) is significant and should be considered. It furthermore concluded that the location of the focus (be it at various points in the tissue or inside the bone) has essentially no effect on the location at which maximal heating occurs, and the maximum temperature increase was found to be ~ 13 degrees Celsius, just inside the bone (perfusion was not modeled), when the focus was at the interface. This conclusion is consistent with Fujii et al. (1999), except that Fujii found the maximum to be at interface (for normal incidence) – the Wu & Du finding that the maximum heating was just inside the bone surface is consistent with Myers (2005) and also with the findings of this research which found the peak temperature rise to occur at roughly ½ mm distal to the surface of the bone. (Myers, 2004) The lack of standardized methods to characterize the acoustic output and spatial intensity distributions of high-intensity transducers and to test and evaluate the thermal effects of bone has hampered preclinical research studies and subsequently has slowed the FDA regulatory review process for such devices. Most pregnant women in the UK are now offered at least one ultrasound examination as a routine part of their antenatal care. There is, however, considerable debate, in both the lay and medical press, over whether ultrasound should be used to examine all pregnant women rather than just those with clinical indications. Clement's (2004) review of routine ultrasound reported that `no clear benefit in terms of a substantive outcome measure like perinatal mortality can yet be discerned to result from the routine use of ultrasound'. (P. 1357) Exposure to Ultrasonic waves stimulates bioeffects within mammalian tissue through gas body activation which is non-thermal mechanism. Already present bodies of gas can be stimulated due to pressure amplitudes. For greater-pressure amplitudes, aggressive cavitation action with inertial disintegration of microbubbles can be initiated from hidden nucleation areas or from the deterioration of gas bodies. This reflex perturbation at the site of stimulation results in the development of biological effects on neighbouring cells and organs. Cavitational bioeffects were first observed during Shockwave lithotripsy within mammalian tissues, like injury in the kidney and hemorrhage etc. The thermal index is defined as the amount of power needed to generate a highest increase in temperature of 1°C. If the thermal index is 1, it means power generating a rise of temperature up to 1°C. If the thermal index is 2, it shows the double amount of power but would not essentially show a maximum increase of temperature up to 2°C. Since increase of temperature depends upon the type of tissue and is predominantly reliant on the existence of bone, hence the thermal index is further categorised into the following indices: 1. TIS i.e. thermal index used for soft tissue; 2. TIB i.e. thermal index with bone closed to the focus; 3. TIC i.e. thermal index for bone at the exterior. When scanning fetus, the maximum temperature rise would probably take place at the bone and thermal index with bone closed to the focus may cause the ‘worst case’ to happen. Both the thermal and mechanical index should be exhibited if the ultrasound system is able to go beyond an index of 1. Conclusion Due to the ability of ultrasound to transmit through tissue and take effect at a location within the body, remote from the transducer, medical ultrasound offers minimally invasive (i.e., can be used extra corporeally) diagnostic and therapeutic treatments that are faster than open surgery, reduce the need for general anesthesia, reduce complications and pain, reduce recovery time, permit access to remote/inaccessible locations, and permit localized treatment with minimal damage to surrounding tissues. Certain physical hazards have found to be associated with high intensity ultrasound procedure i.e. thermal effect, caviatation, gas body effect and mechanical effects like radiation pressure etc. References Clement, G. T, White PJ, and Kynynen K. 2004. “Enhanced ultrasound transmission through the human skull using shear mode conversion.” J. Acoust. Soc. Am., 115 (3), 1356-1364. Haar, G. 2001. “Acoustic surgery.” Physics Today, 29-34. 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