Medical
Physics: Ultrasound
- extended reading exercise
Ultrasound
B-Scan giving 2D sectional image of foetus
Bright
area indicates large echo amplitude
Sound
waves are propagated at a speed of approximately 1540 m/sec in soft
tissues.
The
thickness, size and location of various soft tissue structures in relation
to the origin of the ultrasound beam are calculated at any point in
time using an 'echo technique'.
The
strength of the reflected sound wave depends on the difference in "acoustic
impedance" between adjacent structures. The acoustic impedance of a
tissue is related to its density; the greater the difference
in acoustic impedance between
two adjacent tissues the more reflective will be their boundary. Higher
frequencyultrasound waves have a longer near field and less divergence
in the far field; they permit better resolution
of small structures. More energy however is absorbed and scattered
by the soft tissues so that higher frequencies have less penetrating
ability.
Conversely, a transducer producing lower frequencies
will provide greater depth of penetration but less well defined images.
Focusing and aperture control technology are often employed to narrow
the beam along it's entire path to achieve maximum right-and-left (lateral)
resolution. The transducer of a real-time scanner typically contains
over 300 crystals( piezo electric devices) arranged in a row where each
emit and receives an ultrasound beam in rapid succession to form a sweep.
The part of the abdomen under the probe is "swept" about 30 times (frames)
a second and a moving picture (a real-time picture) will be formed (not
unlike the principle in a movie projector). Beam density and dynamic
range control technologies are further being incorporated into each
scanner's design to optimize the resultant image.
Pregnancy
problems can be detected and general progress quantitatively monitored
by using A-scans for accurate measurements and B-scans for general development.
A
B-Scan produces a sectional 2-D image. Each point on the monitor represents
echo amplitude by 'grey-scale' representation. The brighter the point
on the screen the 'louder' the echo. This is used to identify the part
of the foetus to be measured. A multiple array of transducers is used to gain the information that builds up into the 2-d representation, whereas an A-scan probe only needs a single transducer..
A
simple ultrasound probe with a curved transducer array is ideal for
obstetric use as it has no moving parts, is comparatively inexpensive
and can be used by relatively unskilled operators. A frequency of between
1-3 MHz is suitable for abdominal imaging as low frequency produces
low resolution imaging and increasing attenuation occurs at very high
frequency.
Accurate
distances within the eye can be measured using a type of probe designed
especially for ophthalmic use. The scanner is either placed in direct
contact with the eye or via a water bath (less risk of damage to eye
surface) it therefore needs to be small and the transducer head suited
to the curvature of the eye. It is used in A-scan mode with a frequency
of 8-13MHz
The anatomical structure within the kidney can be
viewed using a common curvilinear (see below) probe with a frequency
of 5MHz in B-scan mode. The higher the frequency the better the resolution
of the image - 5 MHz will give detail of structure to within 1mm. A
simple scan technique is fine for this application.
The
function of the heart valve is best viewed using a trans-oesophageal
probe with a frequency of 2.0-5.0 MHz. M-Scan complemented by a B-scan.
Without this specialized probe a simple sector scan or compound scan
between the ribs would be necessary to avoid interference of them.
Doppler imaging would allow blood flow into and out of the valve to
be monitored and efficiency and any leakage can be assessed.
Patient undergoing a kidney scan B-scan of liver and kidney
B
and M mode images of the heart at the level of the mitral valve
M-mode image of the heart
NB:
M-mode is not on the syllabus neither is the Doppler equation - they
are in these notes to extend your background knowledge. Calculations
will be restricted to simple echo or simple Doppler equation applications.
Useful
Websites to visit:
http://www.ob-ultrasound.net/