Thermal Effects of Diagnostic Ultrasound in an Anthropomorphic Skull Model

Abstract

Purpose: Exposure to diagnostic ultrasound (US) can significantly heat biological tissue although conventional routine examinations are regarded as safe. The risk of unwanted thermal effects increases with a high absorption coefficient and extended insonation time. Certain applications of transcranial diagnostic US (TC-US) require prolonged exposure. An anthropomorphic skull model (ASM) was developed to evaluate thermal effects induced by TC-US of different modalities. The objective was to determine whether prolonged continuous TC-US application results in potentially harmful temperature increases.

Materials and Methods: The ASM consists of a human skull with tissue mimicking material and exhibits acoustic and anatomical characteristics of the human skull and brain. Experiments are performed with a diagnostic US device testing four different US modalities: Duplex PW (pulsed wave) Doppler, PW Doppler, color flow Doppler and B-mode. Temperature changes are recorded during 180 minutes of insonation.

Results: All measurements revealed significant temperature increases during insonation independent of the US modality. The maximum temperature elevation of + 5.25° C (p < 0.001) was observed on the surface of the skull exposed to duplex PW Doppler. At the bone-brain border a maximum temperature increae of + 2.01 °C (p < 0.001) was noted. Temperature increases within the brain were < 1.23 °C (p = 0.001). The highest values were registered using the duplex PW Doppler modality.

Conclusion: TC-US induces significant local heating effects in an ASM. An application duration that extends routine clinical periods causes potentially harmful heating especially in tissue close to bone. TC-US elevates the temperature in the brain mimicking tissue but is not capable of producing harmful temperature increases during routine examinations. However, the risk of thermal injury in brain tissue increases significantly after an exposure time of > 2 hours.

Zusammenfassung

Ziel: Thermische Effekte stellen ein potenzielles Risiko bei der Verwendung von prolongierter Ultraschall(US)-Anwendung dar. Ein entscheidender Faktor ist die Expositionszeit, welche die Gefahr von thermischen Schäden am Gehirn, aber auch an den dem Schallkopf näherliegenden Geweben erhöht. Anhand 1eines anthropomorphen Schädelmodells wurde die Temperaturentwicklung über eine Applikationsdauer über 180 min analysiert.

Material und Methoden: Es wurden 4 unterschiedliche diagnostische US-Modalitäten getestet: Duplex-PW-Doppler, PW-Doppler, Color-Flow-Doppler und B-Mode. Gemessen wird die Temperaturveränderung während einer Expositionszeit von 180 min an unterschiedlichen Messpunkten entlang des Schallverlaufs mittels Thermosensoren.

Ergebnisse: Unabhängig von der Modalität zeigten sich signifikante Temperaturanstiege in allen Messlokalisationen. Der höchste Anstieg von + 5,25 °C (< 0,001) wurde zwischen Muskel und Schädelknochen gemessen. Im Bereich zwischen Knochen und Gehirn betrug die maximale Temperaturanstieg + 2,01 °C (p < 0,001). In tiefer liegenden Hirngewebsschichten fand sich kein potenziell schädigender Temperaturanstieg (< 1,23°C; p < 0,001).

Schlussfolgerung: Transkranieller diagnostischer US erzeugt signifikante Temperaturanstiege entlang des Schallverlaufes. Eine prolongierte Anwendungsdauer verursacht potenziell schädliche thermische Effekte in knochennahen Geweben. Auch die Temperatur in gehirnäquivalentem Gewebe wird signifikant erhöht. Es entstehen jedoch keine thermischen Schäden innerhalb einer Applikationszeit von 2 h.

Key words

ultrasound - brain - safety - transcranial

• References

• 1 Jaeger KA. Ultrasound has Fundamentally Transformed Vascular Medicine. Ultraschall in Med 2008; 29: 600-603
• 2 Zipper SG. Transcranial Color-coded Sonography (TCCS): A Helpful Tool In Clinical Neurology. The Internet Journal of Neurology 2001; 1
• 3 Alexandrov AV, Molina CA, Grotta JC et al. Ultrasound-enhanced systemic thrombolysis for acute ischemic stroke. The New England journal of medicine 2004; 351: 2170-2178
• 4 Eggers J, Konig IR, Koch B et al. Sonothrombolysis with transcranial color-coded sonography and recombinant tissue-type plasminogen activator in acute middle cerebral artery main stem occlusion: results from a randomized study. Stroke 2008; 39: 1470-1475
• 5 Pfaffenberger S, Devcic-Kuhar B, Kastl SP et al. Ultrasound thrombolysis. Thromb Haemost 2005; 94: 26-36
• 6 Tsivgoulis G, Alexandrov A. Ultrasound-enhanced thrombolysis: from bedside to bench. Stroke; a journal of cerebral circulation 2008; 39: 1404-1405
• 7 Schlosser HG, Doepp F, Nolte CH et al. Does routine transcranial duplex ultrasound heat up the patient brain?. Ultraschall in Med 2009; 30: 37-41
• 8 Nakagawa K, Ishibashi T, Matsushima M et al. Does long-term continuous transcranial Doppler monitoring require a pause for safer use?. Cerebrovasc Dis 2007; 24: 27-34
• 9 Horder MM, Barnett SB, Vella GJ et al. Ultrasound-induced temperature increase in the guinea-pig fetal brain in vitro. Ultrasound Med Biol 1998; 24: 697-704
• 10 Barnett SB. Intracranial temperature increase from diagnostic ultrasound. Ultrasound Med Biol 2001; 27: 883-888
• 11 O'Brien Jr WD, Deng CX, Harris GR et al. The risk of exposure to diagnostic ultrasound in postnatal subjects: thermal effects. J Ultrasound Med 2008; 27: 517-535 ; quiz 537 – 540
• 12 American Institute of Ultrasound in Medicine (AIUM); National Electrical Manufacturers Association (NEMA). Standard for Real-Time Display of Thermal and Mechanical Indices on Diagnostic Ultrasound Equipment. 1992
• 13 Koch C. Thermal effects of ultrasound. Ultraschall in Med 2001; 22: 146-152
• 14 Food and Drug Administration. FDA Revised 510(k) Diagnostic Ultrasound Guidance for 1993. Rockville: 1993
• 15 Duggan PM, Murcott MF, McPhee AJ et al. The influence of variations in blood flow on pulsed doppler ultrasonic heating of the cerebral cortex of the neonatal pig. Ultrasound in medicine & biology 2000; 26: 647-654
• 16 Kollmann C, Bezemer RA, Fredfeldt KE et al. A test object for quality control of the instrument for doppler (duplex) ultrasonography, based on the Draft IEC 61685 Standard. Ultraschall in Med 1999; 20: 248-257
• 17 Teirlinck CJ, Bezemer RA, Kollmann C et al. Development of an example flow test object and comparison of five of these test objects, constructed in various laboratories. Ultrasonics 1998; 36: 653-660
• 18 Duck FA. Physical Properties of tissue a comprehensive reference book. Academic Press; 1990
• 19 American Institute of Ultrasound in Medicine. Conclusions Regarding Heat. Laurel, MD: American Institute of Ultrasound in Medicine; 1997
• 20 Daffertshofer M, Gass A, Ringleb P et al. Transcranial low-frequency ultrasound-mediated thrombolysis in brain ischemia: increased risk of hemorrhage with combined ultrasound and tissue plasminogen activator: results of a phase II clinical trial. Stroke 2005; 36: 1441-1446
• 21 Fatar M, Stroick M, Griebe M et al. Brain temperature during 340-kHz pulsed ultrasound insonation: a safety study for sonothrombolysis. Stroke 2006; 37: 1883-1887
• 22 Wu J, Cubberley F, Gormley G et al. Temperature rise generated by diagnostic ultrasound in a transcranial phantom. Ultrasound Med Biol 1995; 21: 561-568
• 23 Busto R, Dietrich WD, Globus MY et al. Small differences in intraischemic brain temperature critically determine the extent of ischemic neuronal injury. J Cereb Blood Flow Metab 1987; 7: 729-738
• 24 Dietrich WD, Busto R, Globus MY et al. Brain damage and temperature: cellular and molecular mechanisms. Advances in neurology 1996; 71: 177-194 ; discussion 194 – 177
• 25 Ginsberg MD, Busto R. Combating hyperthermia in acute stroke: a significant clinical concern. Stroke 1998; 29: 529-534
• 26 Meshorer A, Prionas SD, Fajardo LF et al. The effects of hyperthermia on normal mesenchymal tissues. Application of a histologic grading system. Arch Pathol Lab Med 1983; 107: 328-334
• 27 Dewhirst MW, Viglianti BL, Lora-Michiels M et al. Basic principles of thermal dosimetry and thermal thresholds for tissue damage from hyperthermia. Int J Hyperthermia 2003; 19: 267-294
• 28 Stoll AM, Greene LC. Relationship between pain and tissue damage due to thermal radiation. J Appl Physiol 1959; 14: 373-382
• 29 Horder MM, Barnett SB, Vella GJ et al. Ultrasound-induced temperature increase in guinea-pig fetal brain in utero: third-trimester gestation. Ultrasound Med Biol 1998; 24: 1501-1510
• 30 Hekkenberg RT BR. On the development of a method to measure the surface temperature of ultrasonic diagnostic transducers. Journal of Physics: Conference Series 2004; 1: 84-89