Medical Design Briefs - October 2024 - 16

Medical Imaging
Ultrasound Transducer Array
T/R Switch
Signal = Ultrasound Waves
Power Amplifier
DAC
Low Noise Amplifer
ADC
Fig. 1 - Ultrasound diagram.
This technology operates on the principles
of nuclear magnetic resonance
(NMR) (see Figure 2).
An MRI machine generates a magnetic
field that typically ranges in
strength from 1.5 to 3 T. The field varies
across the sample to form a gradient,
and detection algorithms determine
precisely where the signal is
coming from. The machine emits RF
pulses that disturb the gradient, and
the coils in the MRI machine detect
the rate at which it returns to its original
state once the RF pulse is turned
off (i.e., relaxation). That data is collected
and processed to create detailed
images based on relaxation times and
signal intensities from different tissues.
MRI functionality relies on its magnet
system, gradient coils, RF system,
and control computer. The superconducting
magnet generates the magnetic
field and maintains superconductivity
with the help of cryogenics. Gradient
coils create the variable magnetic field
that enables image creation; amplifiers
power the coils and determine the
strength and timing of gradients.
RF coils transmit RF pulses to disrupt
the gradient and detect returning signals
during relaxation. Meanwhile, an
RF transmitter and receiver support
their function by generating, detecting,
and amplifying the RF signals for further
analysis. A control computer is responsible
for both managing these operations
and storing image data.
Optical Computed Tomography
The principle behind optical computed
tomography (OCT) is white light interferometry,
also known as low- coherence inMagnet
Gradient
Coils
terferometry. This measurement technique
requires an interferometer (e.g.,
Michaelson Interferometer) to make precise
distance measurements. OCT leverages
these measurements to form precise
maps of surface details. For example,
OCT makes it possible for cardiologists to
image the microstructures of coronary arteries
at a higher resolution than other
existing imaging modalities (e.g., intravascular
ultrasound, x-ray angiography) (see
Figure 3).
The system traditionally includes a
low-coherence light source, interferometry,
a scanning system, high-sensitivity
detectors, and image reconstruction
algorithms; however, there are a
variety of scanning techniques available
depending on speed and resolution
requirements.
RF Coils (RX and TX)

Sample
RF Matching
Signal = RF
Power Amplifier

Mechanical scanning, more common
in time-domain optical coherence tomography
(TD-OCT), involves physically
moving the mirrors in an interferometer
setup to change the path length
of the light. Mirror adjustments make it
possible to measure optical path differences.
The other approach to scanning
involves altering the wavelength of the
light source without mechanical movements.
This method is made possible by
highly sensitive tunable lasers, and it's
more common in Fourier-domain optical
coherence tomography (FD-OCT).
Low Noise Amplifer
RF Source
X-Ray and Computed
Tomography (CT)
ADC
DAC
Fig. 2 - MRI machine diagram.
16
Both x-ray and computed tomography
(CT) rely on x-rays to create images of
internal body structures by leveraging radiation.
In x-ray imaging, x-rays pass
through the body, and the system meawww.medicaldesignbriefs.com
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Medical Design Briefs - October 2024

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