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Imaging modalities

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Title: Imaging modalities


1
Imaging modalities
2
History
Energy Source The earliest medical images used
light to create photographs, either of gross
anatomic structures, or if a microscope was used,
of histological specimens. Light is still an
important source for creation of images. However,
visible light does not allow us to see inside
the body. X-rays were first discovered in 1895
by Wilhelm Conrad Roentgen, who was awarded the
1901 Nobel prize in physics for this
achievement. The discovery caused worldwide
excitement, especially in the field of medicine
by 1900, there already were several medical
radiological societies. Thus, the foundation was
laid for a new branch of medicine devoted to
imaging the structure and function of the body
3
X-Ray system
Principle of an X-ray system with image
intensifier. X rays impinging on the image
intensifier are transformed into a distribution
of electrons, which produces an amplified light
image on a smaller fluorescent screen after
acceleration. The image is observed by a
television camera and a film camera and can be
viewed on a computer screen and stored on a
CD-ROM or a PACS.
4
X-Ray tube
5
The X-rays are produced from electrons that have
been accelerated from in vacuum from the
cathode to the anode. Emission occurs when
filament is heated by passing current through
it. When the filament is hot enough, the
electrons obtain thermal energy sufficient to
overcome the energy binding the electron to the
metal of the filament. After accelerated they
will be stopped at a short distance. Most of the
electron energy will produce heat at the anode.
Some percentage will be converted to X-ray by
two main methods. Deceleration of charged
particle results in the emission of
electromagnetic field called Bremmstralung
radiation. These rays will have wide, continuous
distribution of energies with the maximum being
the total energy the electron had when reaching
the anode. The number of X-rays will be small at
higher energies and increased for lower energies.
6
Contrast enhancement
Principle of contrast enhancement (a) intensity
distribution along a line of an image (b) same
distribution after injection of the contrast
medium (c) intensity distribution after
subtraction (d) intensity distribution after
contrast enhancement.
7
Example of digital subtraction angiography (DSA)
of the bifurcation of the aorta
An initial image mask is obtained digitized and
stored Contrast medium is injected Number of
images are obtained. Mask is subtracted The
resulting image contains only the relevant
information The differences can be amplified so
the eye will be able to perceive the the blood
vessels. Quality of deteriorate due to
movements of the body can be corrected to some
extent.
8
Mammography
The mammogram is an X-ray shadowgram from a
quasi-point source irradiate the breast and the
transmitted X-rays are recorded by an image
receptor. A region of reduced transmission
corresponding to a structure of interest such as
a tumor, a calcification or normal
fibroglandular tissue. The imaging system must
have a sufficient spatial resolution to
delineate the edges of fine structures in the
breast. Structural detail small as 50 ?m must be
resolved adequately. Because the breast is
sensitive to ionization radiation, which at
least at high doses is known to cause breast
cancer, it is desirable to use the lowest
radiation dose compatible with excellent image
quality.
9
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10
Simplified computer model of the mammographic
image acquisition process For the simplified
case of monoenergetic x-rays of energy E, the
number of x-rays recorded in a fixed area of the
image is proportional to   In the
background and


The difference in x-ray transmission gives rise
to subject contrast which can be defined as
11
For monoenergetic x-rays and temporarily ignoring
scattered radiation
For a given image recording system (image
receptor) a proper exposure requires a specific
value of x-ray energy transmitted by the breast
and incident on the receptor, i.e. a specific
value of NB. The breast entrance skin exposure
required to produce an image is therefore
proportional to
12
What Can Diagnostic MammographyShow?
Diagnostic mammography may show that an
abnormality (lesion) has a high likelihood of
being benign (not cancer). For these, it
is common to ask the woman to return earlier than
usual for a recheck, usually in 6 months. A
diagnostic mammogram may show that
the abnormality is not worrisome at all and the
woman can then return to routine yearly screening
mammography. In some cases, patients with a cyst
(fluid filled pocket) or other abnormality will
also receive ultrasound imaging to obtain further
diagnostic information. Finally, the diagnostic
work-up may suggest that biopsy (tissue sampling)
is needed to tell whether or not the abnormality
is cancerous. A recommendation for biopsy does
not necessarily mean that the abnormality is
cancer. About 65 of all breast lesions that
are evaluated with biopsy are found to be benign
(non-cancerous) when evaluated under the
microscope.
13
What Abnormalities Does Mammography Detect and
Diagnose? Mammography is used to detect a
number of abnormalities, the two main ones being
calcifications and masses. Calcifications are
tiny mineral deposits within the breast tissue
that appear as small white spots on the films.
Calcifications are divided into two
categories, macrocalcifications and
microcalcifications. A mass is any group of cells
clustered together more densely than the
surrounding tissue. A cyst or fluid collection
may also appear as a mass on mammography. The
difference between a solid mass and a cyst can
often be shown with ultrasound.
14
Calcifications Macrocalcifications are
coarse (larger) calcium deposits that are often
associated with benign fibrocystic change or
with degenerative changes in the breasts, such as
aging of the breast arteries, old injuries,
or inflammation. Macrocalcification deposits are
associated with benign (noncancerous) conditions
and do not require a biopsy. Macrocalcifications
are found in about 50 of women over the age of
50. Microcalcifications are tiny (less than
1/50 of an inch) specks of calcium in the breast.
When many microcalcifications are seen in one
area, they are referred to as a cluster and may
indicate a small cancer. About half of the
cancers detected by mammography appear as a
cluster of microcalcifications.
Microcalcifications are the most common
mammographic sign of ductal carcinoma in situ
(meaning the cancer has not spread or invade
neighboring tissue). Almost 90 of cases of
ductal carcinoma in situ are associated with
microcalcifications. An area of
microcalcifications seen on a mammogram does not
always mean that cancer is present. The shape
and arrangement of microcalcifications help the
radiologist judge the likelihood of cancer
being present. In some cases, the
microcalcifications do not indicate a need for a
biopsy. Instead, a doctor may advise a follow-up
mammogram within 6 months. In other cases, the
microcalcifications are more suspicious and a
stereotactic biopsy is recommended. Only 17 of
calcifications requiring biopsy are cancerous.
15
Masses Another important change seen on a
mammogram is the presence of a mass, which may
occur with or without associated calcifications.
A mass is any group of cells clustered together
more densely than the surrounding tissue. Masses
can be due to many things, including cysts,
which are non-cancerous collections of fluid in
the breast. A cyst cannot be diagnosed by
physical exam alone nor can it be diagnosed by
mammography alone, although certain signs can
suggest the presence of a cyst or cysts. To
confirm that a mass is a cyst, either breast
ultrasound or aspiration with a needle is
required. If a mass is not a cyst, then further
imaging may be obtained. As with calcifications,
a mass can be caused by benign breast conditions
or By breast cancer. Some masses can be
monitored with periodic mammography while others
may require biopsy. The size, shape, and margins
(edges) of the mass help the radiologist in
evaluating the likelihood of cancer. Breast
ultrasound is often helpful. Prior mammograms
may help show that a mass is unchanged for many
years, indicating a benign condition and helping
to avoid unnecessary biopsy. Having prior
mammograms available to the radiologist, as
discussed above, is very important. Mammography
alone cannot prove that an abnormal area is
cancer although some abnormalities are very
characteristic of malignancy. If mammography
raises a significant suspicion of cancer, tissue
must be removed for examination under the
microscope to tell if it is cancer. This can be
done with one of several breast biopsy
techniques. Ductography, also know as a
Galactogram, is special type of contrast enhanced
mammography used for imaging the breast ducts.
Ductography can aid in diagnosing the cause of an
abnormal nippledischarge and is valuable in
diagnosing intraductal papillomas.
16
Digital Mammography
One of the most recent advances in x-ray
mammography is digital mammography. Digital
(computerized) mammography is similar to
standard mammography in that x-rays are used to
produce detailed images of the breast. Digital
mammography uses essentially the same mammography
system as conventional mammography, but the
system is equipped with a digital receptor and a
computer instead of a film cassette. Several
studies have demonstrated that digital
Mammography is at least as accurate as standard
mammography. Digital spot view mammography
allows faster and more accurate stereotactic
biopsy. This results in shorter examination
times and significantly improved patient comfort
and convenience since the time the patient must
remain still is much shorter. With digital
spot-view mammography, images are acquired
digitally and displayed immediately on the system
monitor. Spot-view digital systems have been
approved by the U.S. Food and Drug Administration
(FDA) for use in guiding breast biopsy.
Traditional stereotactic biopsy requires a
mammogram film be exposed, developed and then
reviewed, greatly increasing the time before the
breast biopsy can be completed. In addition to
spot-view digital mammography, the FDA has
recently approved a "full-field" digital
mammography system to screen for and diagnose
breast cancer. Currently, only hard copy
printouts of the digital mammographic images
maybe used by radiologists. With continued
improvements, the "full-field" mammography
systems may eventually replace traditional
mammography.
17
  • How Does Digital Mammography Differ From Standard
    Mammography?
  • In standard mammography, images are recorded on
    film using an x-ray cassette. The film is
  • viewed by the radiologist using a "light box" and
    then stored in a jacket in the facilitys
    archives.
  • With digital mammography, the breast image is
    captured using a special electronic
  • x-ray detector, which converts the image into a
    digital picture for review on a computer monitor.
  • The digital mammogram is then stored on a
    computer. With digital mammography, the
  • magnification, orientation, brightness, and
    contrast of the image may be altered after the
    exam is
  • completed to help the radiologist more clearly
    see certain areas.
  • Digital mammography provides many benefits over
    standard mammography equipment.
  • These benefits include
  • faster image acquisition
  • shorter exam time
  • easier image storage
  • physician manipulation of breast images for
    more accurate
  • detection of breast cancer
  • transmittal of images over phone lines or a
    computer network for remote consultation with
  • other physicians
  • Digital mammography has the potential to
    significantly reduce the amount of time required
    to

18
Many radiologists support digital mammography as
an effective tool to screen for breast cancer.
The contrast resolution of these devices is
inherently better, "In addition, the extra
features that digital mammography will
ultimately provide, such as telemammography,
tomosynthesis, and computer-aided diagnosis will
prove invaluable to patients and their
doctors, Telemammography (also called
teleradiology) allows radiologists to share
digital images via phone or network connection
for remote consultation with other
physicians tomosynthesis allows radiologists to
add or subtract digital mammography images using
a computer workstation for enhanced diagnostic
capability. Computer-aided detection (CAD) was
approved by the FDA in June 1999. CAD helps
radiologists more accurately detect breast cancer
by marking suspicious areas on digitized
mammograms. Promising Developments in Digital
Mammography The FDA has approved the first
"full-field" digital mammography scanner to
screen for and diagnose breast cancer in
February 2000. Before applying for FDA
certification, data was gathered from 662
patients at four institutions the University of
Colorado, the University of Massachusetts Medical
Center, Massachusetts General Hospital, and the
Hospital of the University of Pennsylvania. The
data compared hard copies of digital breast
images on film to conventional mammography films
finding that digital mammography is as effective
at detecting breast cancer as standard film
mammograms. A separate study revealed that the
digital mammography scanner showed a slight
advantage in the visibility of breast tissue at
the skin line.
19
Disadvantages to Digital Mammography While
digital mammography is quite promising, it still
has additional hurdles to undergo before it
replaces conventional mammography. Digital
mammography must provide higher detail
resolution (as standard mammography does)
become less expensive (currently several times
more costly than conventional mammography)
provide a method to efficiently compare digital
mammogram images with existing mammography
films on computer monitors Standard mammography
using film cassettes has the benefit of providing
very high detail resolution (image sharpness),
which is especially useful for imaging
microcalcifications (tiny calcium deposits) and
very small abnormalities that may indicate early
breast cancer. While full-field digital
mammography may lack the spatial resolution of
film, clinical trials have shown digital
mammography to be at least equivalent to standard
film screening mammography. This is because
digital mammography has the benefit of providing
improved contrast resolution, which may make
abnormalities easier to see. Various
manufacturers are trying to develop digital
mammography systems with detail resolution
equivalent to standard film mammography while
also providing the benefits of digital
mammography noted above. The high cost of digital
mammography is a major obstacle. Digital
mammography systems costs roughly four to five
times as much as standard mammography equipment.
Standard mammography systemsare currently
installed in over 10,000 locations across the
United States. It may take years for this current
equipment to be updated or replaced and for
digital mammography to become widespread after
its approval by the FDA.
20
Benign lesion - Fibroadenoma
21
Computerized Tomography
Imaging of a cross sectional slice of the body
using X-rays. Invented by Dr. G. N. Housfield in
1971. Received the Nobel prize in medicine in
1979. The method is constructing images from
large number of measurements of x-ray
transmission through the patient. The resulting
images are tomographic maps of the X-ray linear
attenuation coefficient.
22
First generation CT
23
4th generation CT Fan beam, stationary detectors.
24
Fifth generation CT (Image data are acquired in
as little as 50 mSEC).
25
CT (by Picker)
Spiral scan
Colonoscopy with spiral CT
26
Example of cross-sections through several parts
of the body skull, thorax, and abdomen,obtained
by computed tomography.
Visualization of the values of the attenuation
coefficients by way of gray values produces an
anatomic image.
27
The principle of CT
The intensity of the transmitted beam as a
function of the attenuation coefficient of the
pixels traversed. Upper part, the intensity after
crossing one volume element middle part, after
traversing n volume elements lower part, the
analog case.
28
Upper left, density distribution of a point
absorber along a line through the object lower
left, the resulting intensity profiles lower
right, the back- projection upper right,
reconstructed density distribution on a line
through the object.
29
Back projection method
Starts with the assumption that the absorbing
medium is uniformly distributed. with several
intensity profile we get a star-like
reconstructed image. By increasing the number of
angles, the intensity in the center decreases
and we get back projected image but less sharp.
Instead of showing one attenuation pixel, The
neighboring pixels are visible in the
reconstructed image as well. This blurring is
corrected with filtering techniques.
30
Some Mathematics
y
t
s
(x,y)
?
x
P(t,?) - Projection data into each detector
(Radon transform)
The relationship between the source position
(x,y) the projection angle ? and the position
of the detection on the 1D detector array is
given by
In 2D tomographic imaging, The 1D detector
rotates around the object.
31
The goal of image reconstruction is to solve the
inverse Radon transform. The solution is the
reconstructed image estimate of the object
distribution f(x,y). The measured projection
data is given by
I0 - intensity of the incident x-ray. ?(x,y) - 2D
attenuation coefficient ct - gain factor which
transforms x-ray intensity to detected
signals. Well write again the reconstruction
problem
With the goal to solve the for the attenuation
coefficient.
32
Image reconstruction algorithms from projections
Simple backprojection
Where ?j - the jth projection angle. m - number
of projection views. ?? - The angular spacing
between adjacent projections. This backprojected
image is a poor approximation of the true
object. It is equivalent to the true objection
object, blurred by a blurring function in the
form 1/r.
33
Filtered backprojection
  • Filter the measured projection data at different
    projection
  • angles with a special function.
  • Backproject the filtered projection data to form
    the
  • reconstructed image.
  • Filtering can be implemented in 2 ways, in the
    spatial domain, the filter operation is
  • equivalent to to convolving the measured
    projection data using a special convolving
  • function h(t)
  • More efficient multiplication will be in the
    spatial frequency domain.
  • FFT the measured projection data into the
    frequency domain
  • p(?,?)FT p(t, ?)
  • Multiply the the fourier transform projections
    with the special function.
  • Inverse Fourier transform the product p(?,?).

34
The solution of the inverse Radon transform
specifies the form of the special function. This
function is given below
Where ?X is the linear sampling interval and
sinc (z) sin(z)/z. The function h(x) consists
of a narrow central peak with high magnitude and
small negative side lobes. It removes the
blurring from the I/r function found in the
simple backprojected images. In the frequency
domain H(?) is given by H(?) ?rect(?) where
? is the ramp function and 1 ??0.5 rect
(?) 0 ?0.5 The rectangular function
rect(?) when the absolute value of ? is less
than the Nyquist frequency at 0.5 cycles per
pixel. Additional smoothing function may be
applied for noisy data.
35
Attenuation coefficients of several tissues
expressed in Hounsfield units.
36
Magnetic resonance imaging (MRI)
37
Joe
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39
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41
Magnetic resonance imaging exploits the existence
of induced nuclear magnetism in the patient.
Magnets with an odd number of photons or neutrons
possess a weak but observable nuclear magnetic
moment. Most commonly photons (H) are imaged,
although (13C, Phosphorous (P) sodium (Na) and
Fluorine (F) are also of significant interest.
The nuclear moments are normally randomly
oriented, but they align when placed in a strong
magnetic field (typically 0.2-1.5 T). The NMR
signal from a human is due predominantly to water
protons. Since these protons exists in identical
magnetic environments, they all resonate at the
same frequency. Hence the NMR signal is simply
proportional to the volume of the water. The key
innovation for MRI is to impose spatial
variation on the magnetic field to distinguish
spins by their location. Applying a magnetic
field gradient causes each region of the volume
to oscillate at a distinct frequency. The
primary contrast mechanisms exploit relaxation of
the magnetization are T1 and T2. Spin-lattice
relaxation T1 The exponential rate constant
describing the decay of the z component of
magnetization towards the equilibrium
magnetization. Typical values in the body are
between 300 and 3000 ms. Spin-Spin relaxation T2
The exponential rate contrast describing the
decay of the transverse components of
magnetization (Mx and My). MR images provide
excellent contrast between various forms of soft
tissues. For patients who have no ferromagnetic
foreign bodies within them, MRI scanning appears
to be perfectly safe and can be repeated as
often as necessary without danger. The NMR signal
is also not blocked by air like US and there is
no need for radioactive tracers as in the case of
nuclear medicine scanning. Typical imaging
studies range from 1 to 10 minutes but new fast
imaging techniques acquire images in less than
50 msec.
42
MRI by Picker
43
Spinal cord
Brain section
44
Functional MRI
fMRI is a technique that images intrinsic blood
signal change with magnetic Resonance
imagers. Changes in neuronal activity are
accompanied by focal changes in cerebral blood
flow (CBF), blood volume (CBV), blood oxygenation
and metabolism. These physiological changes can
be used to produce functional maps of mental
operations. There are two basic techniques
used 1 Saturation or inversion of incoming blood
signal to quantify absolute blood flow. By
focusing on blood flow change and not steady
state flow, it is possible to image brain visual
functions associated with quantitative perfusion
change. At this way common baseline artifact can
be subtracted. Measuring changes in blood
oxygenation during neuronal activity. The study
of changes in blood flow is done also with
injection of contrast agents (i.e.
gadolinium-DTPA). Two relaxation rates are
measured in fMRI T1 and T2 (represents the rate
of decay of MRI signal due to magnetic field
in-homogeneities and changes in used to measure
blood oxygenation change. T2 changes reflect
the interplay between changes in cerebral blood
flow, volume and oxygenation. As hemoglobin
becomes deoxygenated, it becomes more
paramagnetic than the surrounding tissue and
thus creates a magnetically inhomogeneous
environment.
45
A functional map (in color) in the cerebellum
during performance of a cognitive peg- board
puzzle task, overlaid on a T2-weighted axial
image in gray scale. The dentate nuclei appear
as dark crescent shapes at the middle of the
cerebellum due to iron deposits. fMRI images
were acquired by conventional T2-weighted FLASH
techniques with a spatial resolution of
1.25x1.25x8 mm3 and a temporal resolution of 8
seconds. Each color represents a 1 increment,
starting at 1. R, right cerebellum L, left
cerebellum. A left-handed subject used the left
hand to perform the task. Bilateral activation
in the dentate nuclei and cerebellar cortex was
observed. The activated area in the dentate
nuclei during performance of pegboard puzzle was
3-4 times greater than that seen during the
visually guided peg movements. (see details in
Kim et al., 1994b).
46
Whole brain functional imaging study during a
visuo-motor error detection and correction task.
Functional images were acquired by the
multi-slice single-shot EPI imaging technique
with spatial resolution of 3.1x3.1x5 and
temporal resolution of 3.5 seconds. The skull and
associated muscles were eliminated by image
segmentation. The 3-D image constructed from
multi-slice images was rendered by Voxel View
program (Vital Images, Fairfield, Iowa).The task
was to move a cursor from the central start box
onto a square target by moving a joystick. Eight
targets were arranged circumferentially at 450
angles and displaced radially at 200 around a
central start box. Activation (in color) is
observed at various brain areas. Top image
displays the brain as a 3-D solid object so that
only the cortical surface is seen. In the bottom
image, a posterior section was removed at the
level of the associative visual cortex to display
activation not visible from the surface (Kindly
provided by Jutta Ellermann, Jeol Seagal, and
Timothy Ebner).
47
Open MRI units
48
Nuclear Imaging
  • Use of G rays, Radionuclides and
    Radiopharmaceuticals in medical imaging.

49
Nuclear imaging looks at physiological processes
rather than at anatomical structures. In nuclear
imaging, short-lived radiopharmaceuticals
(radioactive drugs that emit gamma rays and that
are attracted to the organ of interest) are
injected into a patient's bloodstream (in
amounts of picomolar concentrations thus not
having any effect on the process being studied).
The half life of these materials is between few
minutes to weeks. The time course of the process
being studied and the radiation dose to the
target are considered. The nuclear camera then,
in effect, takes a time-exposure "photograph" of
the pharmaceutical as it enters and concentrates
in these tissues or organs. By tracing this
blood flow activity, the resulting nuclear
medicine image tells physicians about the
biological activity of the organ or the vascular
system that nourishes it. Nuclear Medicine has a
wide variety of uses, including the diagnosis of
cancer, studying heart disease, circulatory
problems, detecting kidney malfunction, and
other abnormalities in veins, tissues and organs.

50
Nuclear camera
51
Whole body nuclear image
52
SPECT (single photon emission computerized
tomography
SPECT is based on the conventional nuclear
imaging technique and tomographic reconstruction
methods.
53
The most important tool in nuclear medicine is
the scintillation camera (anger camera) based on
a large area (40 cm in diameter) NaI(Tl)
crystal.When a photon hits and interact with the
crystal, the scintillation generated and detected
by the area of PMTs. An electronic circuit
evaluates the relative signals from the PMTs and
determines the location of the signal.
Collimator
Y
Counts/pixel
X
54
Performance characteristics of Nuclear Imaging
Systems
Spatial resolution - A measure for the degree of
detail the final reconstructed image provides
and hence the size of lesions that might
potentially be detected. In other words how
fine the details are that can be
separated. Sensitivity, dead time - describes
how well the radioactive decays in a tracer
distribution are exploited to form image
counts. A source radiated isotropically into all
directions. The camera collects part that is
entering into its solid angle less the photon
which will impinge the collimators. Some of the
events are lost because the system is still
processing a previous event (dead time).
55
Signal to Noise ratio (SNR) - The relative
strength of the information and the noise. If
the lesion is small compared with the spatial
resolution the contrast is reduced because the
high lesion activity blurred into the
neighborhood by the detector response. Uniformit
y, Linearity - The image of an object should be
independent of its position in the field of
view. This is not true in real systems. This can
be assessed in calibration measurements to derive
correction factors. This reduces non-uniformity
from 10 to 3.
56
The conventional nuclear medicine imaging
process. Typical radionuclides used are 140 KeV
Tc-99m and 70 KeV photons from Tl-201.   The
gamma ray photons emitted from the
radiopharmaceutical penetrate through the
patient body and are detected by a set of
collimated radiation detectors. The emitted
photon experience interaction within the body by
the photoelectric effect which stops their
emergence from the body or compton scattering
which transfers part of the energy to free
electrons and the photon is scattered into a new
direction. These photons are also detected by
the camera and cause blurring of the image if
un-treated with image reconstruction and
processing tools.  
57
In SPECT projection data are acquired from
different views around the patient. Similar to
X-ray CT, image processing and reconstruction
methods are used to obtain transaxial or cross
sectional images from multiview projection data.
58
Camera based SPECT systems can be one of the
configurations below
59
SPECT Machine
60
Discrete geometry used for iterative
reconstruction methods
q
Pixel I Activity ai Intersected area fi
r
P(r,q)
Measured profile
Calculated ray sum
61
In 2-D tomographic imaging, the 1D detector array
rotates around the object distribution f(x,y)
and collects projection data from various
projection angles ?. The integral transform of
the object distribution to its projections is
given by       Which is called the Radon
transform. The goal of image reconstruction is
to solve the inverse Radon transform. The
solution is the constructed image estimate
f(x,y) of the object distribution f(x,y). The
measured projection data can be written as the
integral of radioactivity along the projection
rays.
62
 The measured projection data can be written as
the integral of radioactivity along the
projection rays. In SPECT attenuation
coefficient is not so important, so it can be
considered as constant in the body region under
inspection.   l(x,y) is the pathlength between
the point (x,y) and the edge of the attenuator
(or patients body) along the direction of the
projection ray. The image reconstruction problem
is further complicated by the non stationary
properties of the collimator detector and scatter
response functions and their dependence on the
size and composition of the patients body.
63
Brain and Liver Tomographic Reconstruction and 3D
Rendering
64
Positron emission tomography
PET enables physicians to assess chemical or
physiological changes related to metabolism.
Since the origins of many diseases are
biochemical in nature, these functional changes
often predate or exceed structural change in
tissue or organs. PET imaging utilizes a variety
of radiopharmaceuticals, called "tracers," to
obtain images. PET tracers mimic the natural
sugars, water, proteins, and oxygen found in our
bodies. These tracers are injected into a
patient and collect in various tissues and
organs. The PET system takes a time-exposure of
the tracer and generates a "photo" of cellular
biological activities. PET images can be used to
measure many processes, including sugar
metabolism, blood flow and perfusion,
receptor-ligand binding rates, oxygen
utilization and a long list of other vital
physiological activities.
65
PET TRACER PRODUCTION SYSTEMS
PET scanning uses artificial radioactive
tracers and radionuclides. Their lifetime is
usually rather short, thus they need to be
produced on site.
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Some examples of such materials
are   Radionuclide Half life Application Carbo
n-11 20.3 min Positron emitter for metabolism
studies Copper 64 12.8 hours clinical
diagnostic agent for cancer and metabolic
disorder Iodine 122 3.76 min Positron emitter
for blood flow study Iodine 131 8.1
days Diagnose thyroid disorders including
cancer Iron - 52 8.2 hours Iron tracer for PET
bone marrow imaging Nitrogen 13 9.9
min Positron emitter used as 13NH for
heart perfusion studies Strontium 85 64
days Study of bone formation metabolism Oxygen
15 123 sec Positron emitter used for blood
flow Technetium 99m 6 hours The most widely
used radiopharmaceutical In nuclear
medicine
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Radiopharmaceutical
O
EtOOC
COOEt
N
NH
99mTc
S
S
Application Brain perfusion
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PET has a million fold sensitivity advantage over
MRI in tracer study and its chemical
specificity, PET is used to study neuroreceptors
in the brain and other body tissues. It is
efficient in the nanomolar range where much of
the receptor proteins in the body. Clinical
studies include tumors of the brain, breast,
lung, lower GI tract. Additional study of
Alzheimers disease, Parkinsons disease,
epilepsy and coronary artery disease affecting
heart muscle metabolism and flow.
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Parkinsons disease study
PET studies has immeasurably added to the
understanding of oxygen utilization and
metabolic changes that accompany disease.
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Theory
PET imaging starts with the injection of
metabolically active tracer a biologic
molecule that carries with it a positron
emitting isotope. Over a few minutes the isotope
accumulates in an area of the body for which the
molecule has an affinity. i.e. glucose labeled
with 11C or glucose analogue labeled with 18F,
accumulates in the brain or tumors, where
glucose is used as the primary source of energy.
The radioactive nuclei then decay by positron
emission. In positron (positive electron) , a
nuclear proton changes into a positive electron
and a neutron. The atom maintains its atomic
mass but decreases its atomic number by 1. The
ejected positron combines with an electron
almost instantaneously, and these 2 particles
undergo the process of annihilation. The energy
associated with the masses of the positron and
electron particles is 12.022MeV in accordance
with EMC2 . This energy is divided equally
between 2 photons which fly away from one
another at 1800 angle. Each photon has an energy
of 511 keV. These high energy gamma rays emerge
from the body in opposite directions and
recorded simultaneously by pair of detectors.
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The annihilation event that gave rise to them
must have occurred somewhere along the line
connecting the detectors. Of course if one of the
photons is scattered, then the line of
coincidence will be incorrect. After 100,000 or
more annihilation events are detected, the
distribution of the positron-emitting tracer is
calculated by tomographic reconstruction
procedures. PET reconstructs a 2 dimensional
image from the one dimensional projections seen
at different angles. 3-D reconstructions can be
done using 2D projections from multiple angles.
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Resolution factors are
  • Detector crystal width
  • Anger logic
  • Photon noncolinarity
  • Positron range
  • Reconstruction algorithm

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Overall data flow during PET acquisition and
processing
Acquisition
Calibration data
Sinogram
Correction data
Counts/ray
Reconstruction
Image
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Whole body PET
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Study for cardiomyopathy
SA reconstructed slices
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Measurements
  • Blood volumes
  • Oxygen consumption
  • Perfusion
  • Glucose consumption

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Ultrasound Imaging
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Ultrasound operates much the same as sonar, using
high-frequency sound waves as its imaging
source. Ultrasound is the reflection of a sound
wave as it collides with the anatomy being
studied. The resulting pattern of that reflection
is converted into diagnostic information via a
hand-held transducer passed over the area being
imaged. First utilized some 50 years ago, this
medical technology's non-radioactive nature has
made it the modality of choice for ob-gyn
procedures in fact, it is most commonly
associated with fetal imaging. Advances in
ultrasound technology have resulted in
applications that extend far beyond fetal
imaging to include cardiac, vascular and breast
imaging, as well as cyst identification and
guidance of a variety of surgical and other
therapeutic procedures.
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Ultrasound examination
Convex 3.5 MHz For abdominal and OB/GYN studies
Ultrasound machine
Micro-convex 6.5MHz For transvaginal
and transrectal studies
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Ultrasound transducers generate acoustic waves by
converting magnetic, thermal, or electric
energy into mechanical energy. The most efficient
technique for medical ultrasound uses the
piezoelectric effect. Applying stress on a
crystal creates electrical potential and vise
versa. The transducer developed when linear
arrays were developed. To implement real time
imaging, rapid steer of the acoustic beam is
needed. Linear sequential arrays were designed
to electronically focus the beam in a
rectangular image region. Linear phased area
transducers were designed to electronically steer
and focus the beam at high speed in a sector
image format. The standard material fot medical
ultrasound for many years is the ferroelectric
ceramic lead-zirconate-titanate (PZT) it has a
high electromechanical conversion efficiency and
low intrinsic losses. The properties of the PZT
can be adjusted by modifying the ratio of
zirconium to titanium and introducing small
amounts of other substances, such as Tantalum.
PZT is mechanically strong and can operate at
temperatures up to 1000 C and its stable for a
long period of time. The disadvantage is high
acoustic impedance (Z30 Mrayls) compared with
body tissue (1.5 Mrayls). This is compensated
with acoustic matching layers. Other materials
are used as well (i.e. PVDF-Polyvinylidene
difluoride).
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Array transducers use the same principal as
acoustic lenses to focus an acoustic beam. In
both cases variable delays are applied across the
transducer aperture. Focusing and steering is
done by delayed excitation signals as follows
Excitation signals
The acoustic signal from all elements reach the
focal point at the same time. According to
Huygens principle the net acoustic signal is the
sum of all signals. For receiving an ultrasound
echo, the phase array works in reverse. The echo
from a receive focus is incident on each array
element at a different time interval. The
received signals are electronically delayed so
that the delayed add in phase for an echo
originating at the focal point.
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In the receive mode, the focal point can be
dynamically adjusted so that it coincides with
the range of returning echoes. After transmission
of acoustic pulse, the initial echoes return
from targets near the transducer. Therefore, the
scanner focuses the phase array on these
targets, located at the first focus. As echoes
return from from more distance targets, the
scanner focuses at a greater depth. Focal zones
are established with adequate depth of field so
targets are always in focus to receive. This
process is called dynamic receive focusing.
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Arrays can be configured as Linear sequential
array (512 elements) Curvilinear (convex)
arrays. Linear phased arrays. 1.5D arrays 2D
arrays.
2D array
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Ultrasound is a frequency over 20Khz which is the
maximum frequency heard. But Frequency and
wavelength (therefore resolution) are inversely
related so the lowest frequency used is 1 MHz.
Axial resolution is approximately wavelength so
at 1Mhz its 1.5 mm in most soft tissues. So one
must go to 1.5 MHz for 1 mm resolution.
Attenuation of ultrasound signals increases with
frequency in soft tissue and so tradeoff must be
made between penetration to a particular
application. Deep penetration like in cardiology
and Gynecology request 2-8 MHz and application
with shallow penetration like ophthalmology and
peripheral vascular use 20 MHz, Intra-arterial
uses 20-50 MHz and in ultrasonic microscopy for
inspection of structures within individual cells
go up to 200 MHz. High frequencies Good
resolution but small penetration. Low
frequencies Bad resolution but deep penetration.
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Two basic equations used in ultrasonic
imaging   Where d the one way distance of
an object that cause the echo t time delay
(for the round trip) c - speed of sound in tissue
(between 1450 and 1520 m/s) The other
equation    Where   S(t) - Received signal
strength. T(t) - Transmitted signal B(t) -
transducer properties A(t) - The attenuation of
signal path to and from the scatterer ?(t) - The
strength of the scatterer   In the frequency
domain it turns to be  
 
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Definition of terms A-mode - The original
display mode of ultrasound measurements, in which
the amplitude of the returned echoes along a
single line is displayed on an oscilloscope. B-mo
de (2-D) - The current display mode of choice.
This is produced by sweeping the transducer from
side to side and displaying the strength of the
returned echoes as bright spots in their
geometrically correct direction and
distance.   M-mode - Followed A mode by recording
the strength of the echoes as dark spots on
moving light sensitive paper. Object that move,
such as the heart cause standard patters of
motion to be displayed. And a lot of diagnostic
information such as valve closure rates, whether
valves opened and closed completely, and wall
thickness could be obtained from this mode.
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Transducer
Ribs
Chest wall
Heart in cross section (diastole-relaxation)
M-line
Heart in cross section (systole-contraction)
Ultrasound line of sight
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Ultrasound is also used for measurement of blood
flow in the blood vessels as shown below
The target is red blood cells in a smallest
region as possible. One type of system uses the
Doppler effect. The Doppler shift frequency is
equal to 2fcvc fc - transducer center
frequency v - velocity components of the blood
cells c - Speed of sound within tissue.
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Color flow mapping - A pseudo color velocity
display overlaid on a 2D gray scale image. Here
simultaneous amplitude and velocity information
is presented.
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Ultrasound contrast agents
Reflection of sound waves depend on the acoustic
impedance which are defined by its density and
the velocity of sound in the medium. Acoustic
impedances differences are very small between
soft tissues. Echofarnaceuticals (US Cas) have
been proposed to increase acoustic impedance
differences at tissue interfaces. Secondly to
increase the respective echo intensities. The
most effective principle by far that has emerged
is the diffraction of ultrasonic waves on gas
bubbles (microbubble containing solutions ) and
colloidal, sometimes temperature dependent
diphasic systems.
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Ultrasound contrast agents
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Fetus Ultrasound
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