CIE A'level Physics 9702 A2: Medical Physics Exam Questions and Answers
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Course
Medical Physics
Institution
Medical Physics
xplain the main principles behind the GENERATION of ULTRASOUND to obtain diagnostic information about internal structure. - ANSWER-~ Use piezo-electric crystal
~ two sides coated with silver act as electrodes
~ centres of (+) & (-) charge do not coincident.
~ p.d. across crystal causes crystal...
CIE A'level Physics 9702 A2: Medical Physics
Exam Questions and Answers
Explain the main principles behind the GENERATION of ULTRASOUND to obtain
diagnostic information about internal structure. - ANSWER-~ Use piezo-electric crystal
~ two sides coated with silver act as electrodes
~ centres of (+) & (-) charge do not coincident.
~ p.d. across crystal causes crystal to change shape
~ alternating voltage applied across crystal causes crystal to vibrate.
~ so that it vibrates at resonant frequency
Ultrasound to obtain diagnostic information about structures within the body - ANSWER-
~ Pulses of ultrasound directed into body
~ reflected at boundary between tissues
~ reflected pulse is detected and processed
~ time for return of echo given information on depth
~ Intensity of reflection gives information on tissue structures
~ gel used to minimize reflection at skin
~ degree of reflection depends upon impedance of 2 media at boundary
Functions of two magnetic fields in a magnetic resonance (MR) scanner - ANSWER-~
strong uniform magnetic field aligns nuclei; or gives rise to Larmor frequency.
~ Non-uniform magnetic field enable nuclie to be located or change the Larmor
frequency
Principles of the use of magnetic resonance to obtain diagnostic information about
internal body structures - ANSWER-~ strong/large uniform magnetic field
~ nuclei rotate about field direction radio frequency pulse at Larmor frequency
~ causes resonance/nuclei absorb energy on relaxation/de-excitation
~ nuclei emit r.f. pulse
~ pulse detected and processed
~ non-uniform field superposed on uniform field allows position of resonating nuclei to
be determined allows for location of detection to be changed
sharpness in X-ray imaging - ANSWER-ease with which edges of structures can be
seen
contrast in x-ray imaging - ANSWER-difference in degree of blackening between
structures
explain why a continuous spectrum of wavelengths is produced in a spectrum of x-ray -
ANSWER-~ e.m. radiation/photon is produced whenever a charge is accelerated
~ wavelength depends on magnitude of acceleration
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