The Safety of Diagnostic Ultrasound

[Guest guest]

Although diagnostic ultrasound (used to non-invasively examine heart, foetus

or muscles) generally has been shown to be very safe, some studies reveal

that this may not necessarily be true. Examine the following studies, for

example:

Ultrasound Med Biol 2002 Nov-Dec;28(11-12):1535-46

Microbubbles induce renal hemorrhage when exposed to diagnostic ultrasound in

anesthetized rats.

Wible JH, Galen KP, Wojdyla JK, MS, Klibanov AL, Brandenburger GH.

The generation of ultrasound (US) bioeffects using a clinical imaging system

is controversial. We tested the hypothesis that the presence of microbubbles

in the US field of a medical imager induces biologic effects. Both kidneys of

anesthetized rats were insonified for 5 min using a medical imaging system

after the administration of microbubbles. One kidney was insonified using a

continuous mode (30 Hz) and the opposite kidney was insonified using an

intermittent (1 Hz) technique. The microbubbles were exposed to three

different transducer frequencies and four transducer output powers. After

insonification, the animals were euthanized, the kidneys were removed and

their gross appearance scored under " blinded " conditions using a defined

scale.

After the administration of microbubbles, US imaging of the kidney caused

hemorrhage in the renal tissue. The severity and area of hemorrhage increased

with an increase in the transducer power and a decrease in the transducer

frequency. Intermittent insonification in the presence of microbubbles

produced a greater degree of renal hemorrhage than continuous imaging

techniques.

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Ultrasound Med Biol 2002 Oct;28(10):1349-64

Spontaneous homogeneous nucleation, inertial cavitation and the safety of

diagnostic ultrasound.

Church CC.

Gas bubbles of sufficient size to serve as cavitation nuclei may form

spontaneously in tissue in regions of very low interfacial tension. In the

absence of an acoustic wave or other mechanical stress, such nuclei will

quickly dissolve and disappear from the medium. Under the influence of an

acoustic wave, however, these microbubbles may grow to many times their

initial size and then collapse violently, a process known as inertial

cavitation.

In this work, the in vivo energetics and dynamics of the

nucleation-cavitation process were modeled by treating tissue as a

homogeneous fluid. The assumption of a viscosity of 10(-3) Pa s (i.e., that

of water) resulted in the lowest acoustic rarefactional pressure threshold

for nucleation-cavitation events, approximately 4.0 MPa, which was

essentially frequency-independent over the range 1 to 15 MHz. The

rarefactional pressure threshold for a viscosity of 5 x 10(-3) Pa s (that of

blood) also was approximately 4.0 MPa at 1 MHz, but the threshold for this

higher viscosity increased nearly linearly with frequency above

approximately 5 MHz, never being more than approximately 0.2 MPa below the

equivalent derated peak rarefactional pressure calculated assuming MI = 1.9,

the current USFDA guideline.

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Any more references or comments?

Dr Mel C Siff

Denver, USA

http://groups.yahoo.com/group/Supertraining/