Transcript 02 Medical_Physics - TheWorldaccordingtoHughes

Medical imaging
X-ray scan
Define the terms attenuation
coefficient and half-value thickness.
The intensity I is the amount of energy per unit area
in the beam.
X is the distance travelled through the material.
The attenuation coefficient describes the extent to
which the intensity of an energy beam is reduced as
it passes through a specific material.
When used in the context of X-rays or Gamma-rays,
it is represented using the symbol μ, and measured
in cm-1.
Lead has a high attenuation coefficient.
The half-value thickness is the thickness
of the material that reduces the intensity
of the X-rays by 50%
Derive the relation between
attenuation coefficient and half-value thickness.
Staring with
As the intensity drops to 50%
Taking logs
Solve problems using the equation
I = I0 e− μx .
Describe X-ray detection, recording
and display techniques.
Students should be aware of photographic film, enhancement, electronic
detection and display.
Photographic plates are sensitive (the efficiency of X-Ray film to absorb x-ray
photons is only » 1%) to X-rays, they provide a means of recording the image, but
require a lot of exposure (to the patient), so intensifying screens were devised
(scintilator and photomultiplier tube). They allow a lower dose to the patient, because
the screens take the X-ray information and intensify it so that it can be recorded on
film positioned next to the intensifying screen.
Scintillators
Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to a
visible photon; an electronic detector can be built by adding a photomultiplier tube.
These detectors are called “scintilators", filmscreens or “scintilation counters". The
main advantage of using these is that an adequate image can be obtained while
subjecting the patient to a much lower dose of X-rays.
Scintillators
Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to
a visible photon; an electronic detector can be built by adding a
photomultiplier tube. These detectors are called “scintilators", filmscreens or
“scintilation counters". The main advantage of using these is that an
adequate image can be obtained while subjecting the patient to a much lower
dose of X-rays.
Photostimulable phosphors (PSPs) (Electronic detection)
An common method is the use of photostimulated luminescence (PSL), pioneered
by Fuji in the 1980s. In modern hospitals a photostimulable phosphor plate (PSP
plate) is used in place of the photographic plate. After the plate is x-rayed, excited
electrons in the phosphor material remain "trapped" in "colour centres" in the crystal
lattice until stimulated by a laser beam passed over the plate surface. The light
given off during laser stimulation is collected by a photomultiplier tube and the
resulting signal is converted into a digital image by computer technology, which
gives this process its common name, computed radiography (also referred to as
digital radiography). The PSP plate can be reused, and existing X-ray equipment
requires no modification to use them.
Image plates have the following advantages.
1. dynamic range larger than 5 orders of magnitude in x-ray dose,
2. lower limit of useful dose compared to the x-ray film,
3. reusability,
4. no wet chemical processing,
5. images are available in digital form,
6. image processing and pattern recognition possible, and
7. simple data storage on optical or digital media.
Disadvantages of image plates compared to the conventional x-ray film are
1. a poorer spatial resolution due to light scattering at the storage phosphor
grains during the readout process.
How X Rays Work
X Rays (continued)
X-rays: x-radiography
How do we obtain a clear x-ray
image?
X rays are scattered or absorbed as they pass through the body.
If the energy (tube voltage) of the x rays is too high, then all the x –
rays get through (penetration) and there is no contrast on the image.
If the x-ray energy is too low, then no x-rays reach the x-ray plate.
The choice of tube voltage (energy) depends upon the type of tissue
and the thickness of the tissue.
X ray contrast
• Structures in the body like bones are very dense and contain
elements such as calcium that have a high atomic number. This
makes bone absorb a high proportion of the x-rays. Soft tissues like
fat and muscle allow more x-rays to pass though. The body casts an
x-ray shadow onto the film. Where the x-rays have passed though
bone, the film is less exposed so it looks white; where they have not
passed though anything the film is exposed and turns black; and
where the x-rays have passed through soft tissues the film has
different levels of grey.
• In order to make some parts of the body show up better, contrast
media with a high atomic number can be used. This can be a
'barium meal', where the patient drinks a liquid containing barium
(atomic number 56) which makes the digestive tract show up clearly
on x-rays, or the patient can have an injection of iodine (atomic
number 53) which makes the blood vessels stand out (this is called
angiography).
Explain standard X-ray imaging
techniques used in medicine.
Students should appreciate the causes of loss of sharpness and of contrast in
X-ray imaging. They should be familiar with techniques for improving
sharpness and contrast.
The general relationship
between scattering angle
and size of the object is,
Angle = (object size)-1
This will cause the edge of
objects to lose sharpness
X-ray Sharpness
A quantitative measure of the loss of edge detail which is due to geometric
properties of the object and imaging system and not due to image noise or
X-ray scatter. It is usually expressed as the width of the band of changing
density or brightness arising from a sudden change in the intensity of the
radiation incident on the film or fluorescent screen. From this definition it
can be understood that unsharpness and resolution are different concepts.
It is possible for an edge to be "spread" by one of many factors, and at the
same time for two such edges to be resolved in the image. The factors
which contribute to the total image unsharpness include geometric
unsharpness, movement unsharpness, absorption unsharpness, image
receptor unsharpness, and parallax unsharpness. The various unsharpness
factors all contribute to the observed unsharpness of structures in an
image. However, the quantitative manner in which the factors combine is in
general complicated and is not completely understood. It is known from
observation that the total unsharpness is not the direct sum of the
contributing factors. In general, it appears that the total image unsharpness
is dominated by the unsharpness of the largest individual factor.
X-ray Sharpness
• Unsharpness is introduced by
1.A wide source (parallax)
2.A moving patient
IB Questions
Questions 7,11, 19b,c from review pack.
(b) An X-ray beam that consists of photons of the same energy is used to
image a possible bone fracture in the leg of a patient. At this photon energy
attenuation coefficient of bone = 0.62 cm–1
attenuation coefficient of tissue = 0.12 cm–1.
In passing through the leg, the X-rays effectively encounter a thickness of
tissue equal to 14 cm and thickness of bone equal to 8.0 cm.
Use the above data to explain why X-rays of this energy are suitable for
imaging a possible leg fracture.
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(b) intensity after passing through bone = I0e-0.62x8.0
= 7x10-3I0
intensity after passing through tissue =0.19I0
reduction by bone much greater than by tissue so good contrast
between bone and tissue;
Outline the principles of computed
tomography (CT).
A CT scan (sometimes called computed axial tomography, or a CAT
scan) uses x-rays.
In a CT scan the patient lies on a table and is moved though a
doughnut-shaped machine. It creates images that are slices through
the patient.
It does this by moving the x-ray tube and detector in a circle taking xray images of the slice from all angles around the body.
A computer then processes these images to produce a cross
sectional image (a picture of a slice through the body).
CT scans are useful as they can show a range of very different tissue
types clearly: lung tissue, bone, soft tissue and blood vessels.
CT Scan
X-rays: Computed tomography image (CT
scan)
Second
metatarsal bone
(the bone that
David Beckham and
Wayne Rooney broke!)
X-rays: Computed tomography image (CT
scan)
Ultrasound
Ultrasound scan
Ultrasound
Ultrasound imaging: What does it look
like?
Ultrasound imaging: development of a
pregnancy
24 weeks
8 weeks gestation (out of a 40 week pregnancy)
18 weeks
Describe the principles of the generation and the
detection of ultrasound using piezoelectric
crystals.
When the ultrasound beam is
generated, an electrical signal
is sent to the crystal, and
converted to mechanical
vibrations.
Naturally-occurring crystals
Cane sugar, quartz, topaz, bone,
enamel, dentine and tendons.
When the reflected signal is
received, the mechanical
vibrations are converted back
to electrical signals and sent to
the computer for processing
into an image.
Define acoustic impedance (Z) as the product of
the density (ρ) of a substance and the speed of
sound (c) in that substance.
Z = ρc
The acoustic impedance of the eardrum, for instance, corresponds
well with that of the auditory canal, guaranteeing maximum
efficiency of energy transfer, but it does not correspond well with
air. The pinna may be described as an impedance matching device
between the air and the auditory canal. Likewise, the flared end of
a trumpet results in less energy being lost by being reflected back
down the tube of the instrument.
Solve problems involving acoustic
impedance.
Students should understand the use of a gel on the surface of the skin.
Z = ρc
Z1
Z2
Gel is generally necessary because the
acoustic impedance mismatch between air
and the body is large. Without gel, nearly all
of the energy is reflected and very little is
transmitted into the body.
Outline the differences between
A-scans and B-scans.
B-scan
A-scan
An A-scan, is routine type of diagnostic test used in
ophthalmology. The A-scan provides data on the
length of the eye. (i.e. is 1 dimensional)
A B-scan, is a diagnostic test used in ophthalmology
to produce a two-dimensional, cross-sectional view
of the eye.
Ultrasound imaging: A-scan How does it
work?
• An ultrasound element acts like a bat.
• Emit ultrasound and detect echoes
• Map out boundary of object
Ultrasound imaging: B-scan How does it
work?
• Now put many elements together to make a
probe and create an image
How do the scans work?
A-mode: A-mode is the simplest type of ultrasound. A single
transducer scans a line through the body with the echoes
plotted on screen as a function of depth.
B-mode: In B-mode ultrasound, a linear array of transducers
simultaneously scans a plane through the body that can be
viewed as a two-dimensional image on screen.
M-mode: M stands for motion. In m-mode a rapid sequence
of B-mode scans whose images follow each other in
sequence on screen enables doctors to see and measure
range of motion, as the organ boundaries that produce
reflections move relative to the probe.
Identify factors that affect the
choice of diagnostic frequency.
Students should appreciate that attenuation and resolution are dependent on
frequency.
High frequency gives you high
resolution but high attenuation i.e.
limited depth penetration.
Low frequencies are less
absorbed (attenuated),
therefore can penetrate
deeply. But have lower
resolution.
Tsokos
Page 710 Q’s 1-8.
IB Q 2 from the review pack.
NMR and lasers
Hyperlink to video
MRI Scan
Hyperlink
Outline the basic principles of nuclear
magnetic resonance (NMR) imaging.
Students need only give a simple qualitative description of the principle,
including the use of a non-uniform magnetic field in conjunction with the
large uniform field.
Magnetic Field
When a person is lying in the magnetic
field of the MRI scanner the nuclei of the
hydrogen atoms in their body line up,
like compass needles in the Earth's
magnetic field, either pointing in the
direction of the field or opposite to it.
The hydrogen nuclei (protons) don’t stay
still though, but move like a spinning top
around the direction of the magnetic
field.
How MRI works
1.The body is mainly composed of water molecules which
each contain two hydrogen nuclei or protons. When a
person goes inside the powerful magnetic field of the
scanner these protons align with the direction of the field.
2. A second radio frequency electromagnetic field is then
briefly turned on causing the protons to absorb some of
its energy. When this field is turned off the protons
release this energy at a radio frequency which can be
detected by the scanner.
3. The position of protons in the body can be determined by
applying additional magnetic fields during the scan which
allows an image of the body to be built up. These are
created by turning gradients coils on and off which
creates the knocking sounds heard during an MR scan.
Proton nuclear magnetic resonance (NMR) detects the presence of hydrogen nuclei
(protons) by subjecting them to a large magnetic field to align the nuclear spins, then
exciting the spins with properly tuned radio frequency (RF) radiation, and then
detecting weak radio frequency radiation from them as they “relax" from this
magnetic interaction. The frequency of this proton "signal" is proportional to the
magnetic field to which they are subjected during this relaxation process. In the
medical application known as Magnetic Resonance Imaging (MRI), an image of a
cross-section of tissue can be made by producing a well-calibrated magnetic field
gradient across the tissue so that a certain value of magnetic field can be associated
with a given location in the tissue. Since the proton signal frequency is proportional
to that magnetic field, a given proton signal frequency can be assigned to a location
in the tissue. This provides the information to map the tissue in terms of the protons
present there. Since the proton density varies with the type of tissue, a certain
amount of contrast is achieved to image the organs and other tissue variations in the
subject tissue.
Since the MRI uses proton NMR, it images the concentration of protons. Many of
those protons are the protons in water, so MRI is particularly well suited for the
imaging of soft tissue,
It is estimated that about 80% of the body's atoms are hydrogen atoms, so most
parts of the body have an abundance of sources for the hydrogen NMR signals
which make up the magnetic resonance image.
Describe examples of the use of lasers
in clinical diagnosis and therapy.
Applications such as the use in pulse oximetry and in endoscopes should be
discussed. Students should be familiar with the use of a laser as a scalpel
and as a coagulator.
Infrared: Pulse oximetry
Heart rate:81 bpm
Blood oxygenation: 99%
Pulse oximetry
Pulse oximetry is a simple non-invasive method of monitoring the percentage of
haemoglobin (Hb) which is saturated with oxygen. The pulse oximeter consists
of a probe attached to the patient's finger or ear lobe which is linked to a
computerised unit.
The principle of pulse oximetry is based on the red and infrared light absorption
characteristics of oxygenated and deoxygenated haemoglobin. Oxygenated
haemoglobin absorbs more infrared light and allows more red light to pass
through. Deoxygenated (or reduced) haemoglobin absorbs more red light and
allows more infrared light to pass through.
Visible: Endoscopy
Visible: Endoscopy
Visible: Endoscopy
This is the
endoscope coming
out of the oesophagus
Parasitic
Worm!
The stomach wall has
relapsed back
into the oesophagus.
This is a hernia.