Summary Minor Biomedical Imaging - From Molecule to Man (LUMC)
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Course
Minor Biomedical Imaging
Institution
Universiteit Leiden (UL)
A structured summary of the lectures about imaging modalities (Ultrasound, X-ray, CT, PET, scintigraphy, SPECT, MRI, fluorescence) and their pros and cons. Including links to good sources for more reading material! Also a summary of the last weeks about 3D-lab, AR and AI. The document is in English...
1. Ultrasound
1.1 Physics i
• Ultrasound waves are produced by oscillations of piezoelectric crystals in a transducer,
and frequencies range from 1 to 15 MHz, sometimes up to 40 MHz.
o Higher frequencies are
absorbed/attenuated more
easily and are therefore not as
penetrating – but they do
provide more detail. They are
emitted by a linear transducer
(7-12 MHz) and used more for
superficial body structures,
whereas a convex transducer
emits low frequencies (3-6
MHz) that are used for deeper
structures.
Figure 1: Linear transducer (top) and convex transducer
o A doctor often starts with a low (bottom)
frequency convex transducer
before continuing with a high frequency linear transducer.
• The pulses of sound are sent from the transducer, propagate through different tissues,
and then return to the transducer as reflected echoes.
• Ultrasound waves are reflected at the surfaces between the tissues of different density,
the reflection being proportional to the difference in acoustic impedance (i.e. how much
resistence a sound wave encouters; Z = c * ρ).
o If there is no difference in a tissue or between tissues, no echoes are produced.
o If the difference in tissue density is very high, the sound is completely reflected,
resulting in acoustic shadowing. This happens behind bones and air.
▪ This is why in practice, a layer of gel is needed between the transducer
and the skin.
• The returned echoes are converted back into electrical impulses (alternating current) by
the transducer crystals and are further processed to form the ultrasound image
presented on the screen.
1.2 Making an ultrasound image ii
• Several modes of ultrasound are used in medical imaging.
o The most common type of ultrasound image is a B-mode (brightness mode)
image, which displays the acoustic impedance of a 2D cross-section of tissue,
scanned by a linear array of transducers.
iMore info: https://radiopaedia.org/articles/physical-principles-of-ultrasound-1
iiSources: https://en.wikipedia.org/wiki/Medical_ultrasound#Modes, https://radiopaedia.org/articles/beam-
focusing and https://radiopaedia.org/articles/beam-steering
3
, o A-mode (amplitude mode) simply plots the echo as a function of depth from a
single transducer.
o In M-mode (motion mode), pulses are emitted in quick succession (A- or B-mode)
which effectively results in recording a video over time. This can be used to
determine the velocity of specific organ structures, as the organ boundaries that
produce reflections move relative to the probe.
o The Doppler mode makes use of the Doppler effect in measuring and visualizing
blood flow.
• Beam focusing refers to creating a narrow
point in the cross-section of the ultrasound
beam called the focal point. It is at the focal
point where the lateral resolution of the
beam is the greatest.
o Before the focal point is the Fresnel
zone, where the ultrasound waves
Figure 2: Delay in activation of the piezoelectric
travel parallel (or converge; see elements can make the beam converge in the Fresnel
below). This zone always has a zone, creating a focal depth
length of L = r2/λ.
o Distal to this focal point is the
Fraunhofer zone, where the beams
diverge and become more diffused.
This happens under an angle of φ,
where sin(φ) = 0.61*λ/r.
o You can focus and steer your beam
(Figure 2 and 3) by changing the Figure 3: Delay in activation of the piezoelectric
elements can make the beam steer
activation delay between the
different piezoelectric elements. This changes the shape of the wavefront, and
can give you a specific focal depth.
• The resolution of an ultrasound image can be defined in a few ways.
o Axial resolution: to distinguish objects that are behind each other. If two
reflected waves follow each other so quickly that they are indistinguishable, and
they are 2*d apart from each other, then any object smaller than depth d can not
be seen on the sonogram.
▪ Higher frequency → shorter pulse length → higher spatial resolution
but less penetration depth.
o Lateral resolution: to distinguish objects that are lateral to each other. The
lateral resolution of ultrasound is 4x worse than axial resolution.
o Temporal resolution: to distinguish changes between successive images over
time (i.e. movement). Temporal resolution depends on the depth of field (i.e. the
pulse travel distance), the number of beamlines per field, and the number of
focal points (to limit beamline duplication).
4
,2. X-ray
2.1 Physics iii
• X-rays are a form of ionizing electro-magnetic
radiation. They are produced by an X-ray tube,
consisting of an envelope (often made of glass),
which contains a cathode and anode.
• A high tube voltage (20-200 kVp) makes electrons
want to accelerate from the negative cathode to
the positive anode.
o The electrons can only accelerate through
the vacuum after being released by a
tungsten/wolfram filament around the
cathode.
o Tube voltage = tube current *
belichtingstijd
• The produced electrons interact with the anode, Figure 4: The inside of an X-ray tube
thus producing X-rays. The X-rays produced
include Bremsstrahlung and the characteristic radiation for the anode element; see 2.3.
• X-rays interact with matter via different processes, which are fundamental in
understanding how an image is formed in a radiographic exam; see 2.4.
2.2 Making an X-ray image
• Filters are used to remove low energy X-rays from the beam spectrum, which are other-
wise generally absorbed by superficial structures of the body (adding to patient dose
and scatter) and would not contribute to image quality. The X-ray tube itself aleady
filters some of these low energy photons.
• To reduce scattered radiation reaching the detector (produced mainly by the Compton
effect) and to improve image contrast, anti-scatter grids are placed between patient and
the detector.
• There are multiple ways to finally produce an image of the attenuated X-ray photons.
o Conventional radiographs.
o Computed Radiography (CR). Latent image is ‘saved’ in a phosphor plate. Digital
images are read by laser scannning.
o Digital Radiography (DR) using flat panel detectors (FPD).
▪ Directly: X-ray → electral charge
▪ Indirectly: X-ray → light → electrical charge
▪ Indirect digital radiography can also be done by using a charge-coupled
device (CCD), that uses lenses to read the image.
o Photon Counting Detector (PCD).
iii More info: https://radiopaedia.org/articles/x-rays-1
5
, 2.3 Interaction of β-radiation
• Bremsstrahlung (remstraling) is produced when an
elektron decelerates after being deflected by an atomic
nucleus of the anode. The loss in kinetic energy is
converted to radiation/photons.
• Characteristic radiation (karakteristieke straling) occurs
when an inner electron in the anode is removed from the
nucleus, and an electron from an outer shell takes its
place, after which a photon is irradiated. The wavelength
of this photon is characteristic for the anode material. Figure 5: Bremsstrahlung
2.4 Interaction of γ-radiation
• The photoelectric effect occurs when an inner
electron in the anode is removed from the nucleus,
and an electron from an outer shell takes its place,
after which a photon is irradiated. The wavelength of
this photon is characteristic for the tissue.
• With the Compton effect, the photons energy is
distributed over removing an electron from the Figure 6: Photoelectric effect
nucleus and irradiating a new photon (a.k.a.
Compton quant) under an angle.
• Rayleigh/coherent/classical scattering (klassieke
verstrooiing) is an interaction with the entire
atom, and results in zero ionization and little
energy loss. It occurs primarily with low energy
radiation.
2.5 Biological effects of radiation Figure 7: Compton effect
• Effects on molecule level:
o Energy absorption, ionization, formation of radicals.
o Interaction with DNA (radiation-induced damage): base damage, single strand
damage, double strand damage.
• Effects on cellular level:
o Reproductive death.
o Mutations.
o Chromosomal aberrations.
• Effects on the organism:
o Diseases.
o Influence on development and growth.
o Hereditary/genetic effects. Radiation does not in new or unique genetic effects,
but rather increases the frequencies of mutations that already normally occur.
• 4.1: biologische effecten 1: equiv./eff./ dosis
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