Diagnostic ultrasound ( PDFDrive )

36 PART I Physics
Pressure
Pulse repetition period
p +
25
TISSUE ATTENUATION
Instantaneous
intensity
Time
p –
Pulse length
(temporal duration)
TP
PA
FIG. 2.1 Pressure and Intensity Parameters Measured in Medical
Ultrasound. The variables are deined as follows: p + , peak positive
pressure in waveform; p − , peak rarefactional pressure in waveform; PA,
pulse average; TA, temporal average; TP, temporal peak.
Temporal Considerations
TA
he ultrasound power is the temporal rate at which ultrasound
energy is produced. herefore controlling how ultrasound is
produced in time seems a reasonable method for limiting its
efects.
Ultrasound can be produced in bursts rather than continuously.
Ultrasound imaging systems operate on the principle of pulseecho,
in which a burst of ultrasound is emitted, followed by a
quiescent period listening for echoes to return. his pulsed wave
ultrasound may be swept through the image plane numerous
times during an imaging sequence. On the other hand, ultrasound
may be transmitted in a continuous wave (CW) mode, in which
the ultrasound transmission is not interrupted. he temporal
peak intensity refers to the largest intensity at any time during
ultrasound exposure (Fig. 2.1). he pulse average intensity is
the average value over the duration of the ultrasound pulse. he
temporal average intensity is the average over the entire pulse
repetition period (elapsed time between onset of ultrasound
bursts). he duty factor is deined as the fraction of time the
ultrasound ield is “on.” With signiicant time “of” between pulses
(small duty factor), the temporal average value will be signiicantly
smaller. For example, a duty factor of 10% will reduce the temporal
average intensity by a factor of 10 compared with the pulse average.
he time-averaged quantities are the variables most related to
the potential for thermal bioefects. Combining temporal and
spatial information results in common terms such as the spatial
peak, temporal average intensity (I SPTA ) and spatial average,
temporal average intensity (I SATA ).
he overall duration, or dwell time, of the ultrasound exposure
to a particular tissue is important because longer exposure of
Attenuation (dB/cm-MHz)
20
15
10
5
0
Amniotic
fluid
Blood
Brain
Liver
Muscle
Fat
Tissue Type
Tendon
Skin
Cartilage
Infant skull
Skull
FIG. 2.2 Tissue Attenuation. Values for types of human tissue at
body temperature. (Data from Duck FA, Starritt HC, Anderson SP. A
survey of the acoustic output of ultrasonic Doppler equipment. Clin Phys
Physiol Meas. 1987;8[1]:39-49. 47 )
the tissue may increase the risk of bioefects. he motion of the
scanhead during an examination reduces the dwell time within
a particular region of the body and can minimize the potential
for bioefects of ultrasound. herefore performing an eicient
scan, spending only the time required for diagnosis, is a simple
way to reduce exposure.
Tissue Type
Numerous physical and biologic parameters control heating of
tissues. Absorption is normally the dominant contributor to
attenuation in sot tissue. he attenuation coeicient is the
attenuation per unit length of sound travel and is usually given
in decibels per centimeters-megahertz (dB/cm-MHz). he
attenuation typically increases with increasing ultrasound frequency.
he attenuation ranges from a negligible amount for
luids (e.g., amniotic luid, blood, urine) to the highest value
for bone, with some variation among diferent sot tissue types
(Fig. 2.2).
Another important factor is the body’s ability to cool tissue
through blood perfusion. Well-perfused tissue will more efectively
regulate its temperature by carrying away the excess heat produced
by ultrasound. he exception is when heat is deposited too rapidly,
as in therapeutic thermal ablation. 1
Bone and sot tissue are two speciic areas of interest based
on the diferences in heating phenomena. Bone has high attenuation
of incident acoustic energy. In examinations during
pregnancy, calciied bone is typically subjected to ultrasound,
as in measurement of the biparietal diameter (BPD) of the skull.
Fetal bone contains increasing degrees of mineralization as
gestation progresses, thereby increasing risk of localized heating.
Special heating situations relevant to obstetric ultrasound
examinations may also occur in sot tissue, where overlying
structures provide little attenuation of the ield, such as the
luid-illed amniotic sac.