8
7.
Which one of the following statements is TRUE about a single
pulse of ultrasound from a transducer?
A. It contains a range of frequencies.
B. It contains sound at the nominal frequency of the
transducer only.
C. It contains sound at the center frequency of the trans-
ducer only.
D. The shorter the pulse the narrower the bandwidth.
E. Sound energy is continuously transmitted.
Question Type
:
No Look
Keywords:
true, single pulse
Central concept:
The relationship between frequency and
time is reciprocal. Short periods of time
represent large frequency ranges. This is one
reason why a backing material is
used for a
transducer. The backing material shortens the
ring time (pulse duration), thereby increasing
the bandwidth (range of frequencies over which
a transducer can operate). A short pulse also
creates a short spatial pulse length, improving
longitudinal resolution.
Now find the choice which matches your answer.
Choice A is true.
Check other choices to be certain there
is no better choice.
Choice B
is not true. (Note that choices A and B are mutu-
ally exclusive.)
Choice C
is not true. (Note that A and C are also mutually
exclusive)
Choice D
is the opposite of reality. Shorter time implies
greater frequency range.
Choice E
is a ridiculous answer to complete the required
number of choices.
8.
Increasing the gain of pulse echo instruments results in higher
echoes displayed in A-mode. This is due to
A. increased amount of sound emitted by the transducer
B. increased amount of sound reflected
C. increased efficiency of transducer conversion of sound
into electricity
D. increased amplification in the receiver
E. decreased amplification in the receiver
Question Type:
No Look
Keywords:
increasing gain, higher echoes in A- mode
Central concept:
Increasing the receiver gain increases the
amount of amplification of the signal after
it has already returned from the body. In-
creasing receiver gain increases the display
brightness.
Now find the choice which matches your answer.
Choice A
describes an increase in transmit power, not
receive gain.
Choice B
describes an increase in reflected energy. Receiver
gain occurs after the signal is received from the
patient, making this choice ridiculous.
Choice C
is completely unrelated. Receiver gain does not
affect the efficiency of a transducer design.
Choice D
is true.
Choice E
is false. (Note that choices D and E are mutually
exclusive and related, so one of these
two answers would have to be correct.)
9.
A sound wave leaves its source and travels through air. The
speed of sound in air is 330 m/sec. One second later, an echo
returns to the source. At what distance from the source is the
reflector that produced the echo?
A. 1540 meters
B. 770 meters
C. 660 meters
D. 330 meters
E. 165 meters
Question Type:
No Look
Keywords:
330m/sec, one second at what distance is the
reflector
Central concept:
This problem is a simple application of the
distance equation. The main reason people
will get this problemwrong is they will either
ignore or miscalculate the roundtrip effect of
ultrasound, which is a reflective mode. You
must be careful to account for the roundtrip
effect.
Solve the problem:
1 second * 330 m/sec = 330 meters
Sound can travel a total of 330 meters in one second,
or 165 meters to a reflector and 165 meters back from
the reflector.
Now find the choice which matches your answer.
Choice A
is wrong. Can you guess why they give a choice
of 1540 meters?
Choice B
is wrong. Note the factor of two difference from
choice A!
Choice C
is wrong. Note the miscalculated factor of two!
Choice D
is wrong. The roundtrip effect was ignored!
Choice E is correct.
(
Note: After looking at the choices, you should notice that many of the
choices are just multiples of two times another choice. If, when you
