# Basic Physics of Nuclear Medicine/Nuclear Medicine Imaging Systems

!`<span style="color:white;">`{=html}`</span>`{=html}

Topics we have covered in this wikibook have included radioactivity, the
interaction of gamma-rays with matter and radiation detection. The main
reason for following this pathway was to bring us to the subject of this
chapter: nuclear medicine imaging systems. These are devices which
produce pictures of the distribution of radioactive material following
administration to a patient.

The radioactivity is generally administered to the patient in the form
of a radiopharmaceutical -- the term
**radiotracer** is also used. This follows some physiological pathway to
accumulate for a short period of time in some part of the body. A good
example is ^99m^Tc-tin colloid which following intravenous injection
accumulates mainly in the patient\'s liver. The substance emits
gamma-rays while it is in the patient\'s liver and we can produce an
image of its distribution using a nuclear medicine imaging system. This
image can tell us whether the function of the liver is normal or
abnormal or if sections of it are damaged from some form of disease.

Different radiopharmaceuticals are used to produce images from almost
every region of the body:

  Part of the Body     Example Radiotracer
  -------------------- -------------------------------------------------------------------
  Brain                ^99m^Tc-HMPAO_exametazime "wikilink")
  Thyroid              Na^99m^TcO~4~
  Lung (Ventilation)   ^133^Xe gas
  Lung (Perfusion)     ^99m^Tc-MAA_albumin_aggregated "wikilink")
  Liver                ^99m^Tc-Tin (or Sulphur) Colloid
  Spleen               ^99m^Tc-Damaged Red Blood Cells
  Pancreas             ^75^Se-Selenomethionine
  Kidneys              ^99m^Tc-DMSA

Note that the form of information obtained using this imaging method is
mainly related to the physiological functioning of an organ as opposed
to the mainly anatomical information which is obtained using X-ray
imaging systems. Nuclear medicine therefore provides a different
perspective on a disease condition and generates additional information
to that obtained from X-ray images. Our purpose here is to concentrate
on the imaging systems used to produce the images.

Early forms of imaging system used in this field consisted of a
radiation detector (a scintillation detector for example) which was
scanned slowly over a region of the patient in order to measure the
radiation intensity emitted from individual points within the region.
One such device was called the Rectilinear
Scanner. Such imaging systems have
been replaced since the 1970s by more sophisticated devices which
produce images much more rapidly. The most common of these modern
devices is called the Gamma Camera and we
will consider its construction and mode of operation below. A review of
recent developments in this technology for cardiac applications can be
found in Slomka et al (2009)[^1].

## Gamma Camera

The basic design of the most common type of gamma camera used today was
developed by an American physicist, Hal Anger
and is therefore sometimes called the Anger Camera. It consists of a
large diameter NaI(Tl) scintillation crystal which is viewed by a large
number of photomultiplier tubes.

A block diagram of the basic components of a gamma camera is shown
below:

!Block diagram of a gamma
camera

The crystal and PM Tubes are housed in a cylindrical shaped housing
commonly called the **camera head** and a cross-sectional view of this
is shown in the figure. The crystal can be between about 25 cm and 40 cm
in diameter and about 1 cm thick. The diameter is dependent on the
application of the device. For example a 25 cm diameter crystal might be
used for a camera designed for cardiac applications while a larger 40 cm
crystal would be used for producing images of the lungs. The thickness
of the crystal is chosen so that it provides good detection for the 140
keV gamma-rays emitted from ^99m^Tc -- which is the most common
radioisotope used today.

Scintillations produced in the crystal are detected by a large number of
PM tubes which are arranged in a two-dimensional array. There are
typically between 37 and 91 PM tubes in modern gamma cameras. The output
voltages generated by these PM tubes are fed to a position circuit which
produces four output signals called ±X and ±Y. These position signals
contain information about where the scintillations were produced within
the crystal. In the most basic gamma camera design they are fed to a
cathode ray oscilloscope (**CRO**). We will
describe the operation of the CRO in more detail below.

Before we do so we should note that the position signals also contain
information about the intensity of each scintillation. This intensity
information can be derived from the position signals by feeding them to
a summation circuit (marked ∑ in the figure) which adds up the four
position signals to generate a voltage pulse which represents the
intensity of a scintillation. This voltage pulse is commonly called the
**Z-pulse** which, following pulse height analysis, (**PHA**) is fed as
the **unblank pulse** to the CRO.

So we end up with four position signals and an unblank pulse sent to the
CRO. Let us briefly review the operation of a CRO before we continue.
The core of a CRO consists of an evacuated tube with an electron gun at
one end and a phosphor-coated screen at the other end. The electron gun
generates an electron beam which is directed at the screen and the
screen emits light at those points struck by the electron beam. The
position of the electron beam can be controlled by vertical and
horizontal deflection plates and with the appropriate voltages fed to
these plates the electron beam can be positioned at any point on the
screen. The normal mode of operation of an oscilloscope is for the
electron beam to remain switched on. In the case of the gamma camera the
electron beam of the CRO is normally switched off -- it is said to be
**blanked**.

When an unblank pulse is generated by the PHA circuit the electron beam
of the CRO is switched on for a brief period of time so as to display a
flash of light on the screen. In other words the voltage pulse from the
PHA circuit is used to unblank the electron beam of the CRO.

So where does this flash of light occur on the screen of the CRO? The
position of the flash of light is dictated by the ±X and ±Y signals
generated by the position circuit. These signals as you might have
guessed are fed to the deflection plates of the CRO so as to cause the
unblanked electron beam to strike the screen at a point related to where
the scintillation was originally produced in the NaI(Tl) crystal.
Simple!

The gamma camera can therefore be considered to be a sophisticated
arrangement of electronic circuits used to translate the position of a
flash of light in a scintillation crystal to a flash of light at a
related point on the screen of an oscilloscope. In addition the use of a
pulse height analyser in the circuitry allows us to translate the
scintillations related only to photoelectric events in the crystal by
rejecting all voltage pulses except those occurring within the photopeak
of the gamma-ray energy spectrum.

Let us **summarise** where we have got to before we proceed. A
radiopharmaceutical is administered to the patient and it accumulates in
the organ of interest. Gamma-rays are emitted in all directions from the
organ and those heading in the direction of the gamma camera enter the
crystal and produce scintillations (note that there is a device in front
of the crystal called a **collimator** which we will discuss later). The
scintillations are detected by an array of PM tubes whose outputs are
fed to a position circuit which generates four voltage pulses related to
the position of a scintillation within the crystal. These voltage pulses
are fed to the deflection circuitry of the CRO. They are also fed to a
summation circuit whose output (the Z-pulse) is fed to the PHA and the
output of the PHA is used to switch on (that is, unblank) the electron
beam of the CRO. A flash of light appears on the screen of the CRO at a
point related to where the scintillation occurred within the NaI(Tl)
crystal. An image of the distribution of the radiopharmaceutical within
the organ is therefore formed on the screen of the CRO when the
gamma-rays emitted from the organ are detected by the crystal.

What we have described above is the operation of a fairly traditional
gamma camera. Modern designs are a good deal more complex but the basic
design has remained much the same as has been described. One area where
major design improvements have occurred is the area of image formation
and display. The most basic approach to image formation is to photograph
the screen of the CRO over a period of time to allow integration of the
light flashes to form an image on photographic
film. A stage up from this is to use a
storage oscilloscope which allows each
flash of light to remain on the screen for a reasonable period of time.

The most modern approach is to feed the position and energy signals into
the memory circuitry of a computer for storage. The memory contents can
therefore be displayed on a computer monitor and can also be manipulated
(that is **processed**) in many ways. For example various colours can be
used to represent different concentrations of a radiopharmaceutical
within an organ.

The use of digital image
processing is now widespread in
nuclear medicine in that it can be used to rapidly and conveniently
control image acquisition and display as well as to analyse an image or
sequences of images, to annotate images with the patient\'s name and
examination details, to store the images for subsequent retrieval and to
communicate the image data to other computers over a
network.

The essential elements of a modern gamma camera are shown in the next
figure. Gamma rays emitted by the patient pass through the collimator
and are detected within the camera head, which generates data related to
the location of scintillations in the crystal as well as to the energy
of the gamma rays. This data is then processed on-the-fly by electronic
hardware which corrects for technical factors such as spatial linearity,
PM tube drift and energy response so as to produce an imaging system
with a spatially-uniform sensitivity and distortion-free performance.

A multichannel analyzer (MCA) is used to display the energy spectrum of
gamma rays which interact inside the crystal. Since these gamma rays
originate from within the patient, some of them will have an energy
lower than the photopeak as a result of being scattered as they travel
through the patient\'s tissues -- and by other components such as the
patient table and structures of the imaging system. Some of these
scattering events may involve just glancing interactions with free
electrons, so that the gamma rays lose only a small amount of energy.
These gamma rays may have an energy just below that of the photopeak so
that their spectrum merges with the photopeak. The photopeak for a gamma
camera imaging a patient therefore contains information from
spatially-correlated, unattenuated gamma rays (which is the information
we want) and from spatially-uncorrelated, scattered gamma rays. The
scattered gamma rays act like a variable background within the true
photopeak data and the effect is that of a background haze in gamma
camera images.

!Essential elements of a modern gamma camera. MCA: Multi-Channel
Analyzer{width="400"}

While scatter may not be a significant problem in planar scintigraphy,
it has a strong bearing on the fidelity of quantitative information
derived from gamma camera images and is a vital consideration for
accurate image reconstruction in emission tomography. It is the
unattenuated gamma rays (also called the
`<b>`{=html}primary`</b>`{=html} radiation) that contain the desired
information, because of their direct dependence on radioactivity.

The scatter situation is illustrated in more detail in the figure below,
which shows estimates of the primary and scatter spectra for ^99m^Tc in
patient imaging conditions. Such spectral estimates can be generated
using Monte Carlo methods. It is seen
in the figure that the energy of the scattered radiation forms a broad
band, similar to the Compton
Smear
described previously, which merges into and contributes substantially to
the detected photopeak. The detected photopeak is therefore an
overestimate of the primary radiation. The extent of this overestimate
is likely to be dependent on the specific imaging situation because of
the different thicknesses of tissues involved. It is clear however that
the scatter contribution within the detected photopeak needs to be
accounted for if an accurate measure of radioactivity is required.

!Detected gamma ray energy spectrum for ^99m^Tc (green) with estimates
of the scatter (blue) and primary (red)
components. with estimates of the scatter (blue) and primary (red) components."){width="400"}

One method of compensating for the scatter contribution is illustrated
in the figure below and involves using data from a lower energy window
as an estimate for subtraction from the photopeak, i.e.

```{=html}
<div class="center">
```
Primary Counts =Photopeak Window Counts- k (Scatter Window Counts)

```{=html}
</div>
```
where k is a scaling factor to account for the extent of the scatter
contribution. This approach to scatter compensation is referred to as
the `<b>`{=html}Dual-Energy Window`</b>`{=html} (DEW) method. It can be
implemented in practice by acquiring two images, one for each energy
window, and subtracting a fraction (k) of the scatter image from the
photopeak image.\
\
For the spectrum shown above, it can be seen that the scaling factor, k,
is about 0.5, but it should be appreciated that its exact value is
dependent on the scattering conditions. Gamma cameras which use the DEW
method therefore generally provide the capability of adjusting k for
different imaging situations. Some systems use a narrower scatter window
than that illustrated, e.g. 114-126 keV, with a consequent increase in k
to about 1.0, for instance.\
\
A host of other methods of scatter compensation have also been
developed. These include more complex forms of energy analysis such as
the `<b>`{=html}Dual-Photopeak`</b>`{=html} and the
`<b>`{=html}Triple-Energy Window`</b>`{=html} techniques, as well as
approaches based on deconvolution and models of photon attenuation. An
excellent review of these developments is provided in Zaidi & Koral
(2004).

!Gamma ray energy spectrum for ^99m^Tc, with energy discrimination
settings of 92-126 keV for scatter estimation (blue) and of 126-154 keV,
centred on 140 keV, for the photopeak
(red). and of 126-154 keV, centred on 140 keV, for the photopeak (red)."){width="400"}

Some photographs of gamma cameras and related devices are shown below:

  ----------------------------------------------------------------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  !A single-headed gamma camera.{width="160"}                                                              !Another single-headed gamma camera.{width="160"}   !The NaI crystal of a gamma camera.{width="160"}                          !The cathode ray oscilloscope (CRO) of a gamma camera. of a gamma camera."){width="160"}
  !The image processing system of a gamma camera.{width="160"}   !A dual-headed gamma camera.{width="160"}            !Another view of a dual-headed gamma camera.{width="160"}   !The image acquisition and processing console of a dual-headed gamma camera.{width="160"}
  ----------------------------------------------------------------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

We will continue with our description of the gamma camera by considering
the construction and purpose of the collimator.

## Collimation

The collimator is a device which is attached
to the front of the gamma camera head. It functions something like a
lens used in a photographic camera but this analogy is not quite correct
because it is rather difficult to focus gamma-rays. Nevertheless in its
simplest form it is used to block out all gamma rays which are heading
towards the crystal except those which are travelling at right angles to
the plane of the crystal:

!Diagram of parallel-hole collimator attached to a crystal of a gamma
camera. Obliquely incident gamma-rays are absorbed by the
septa.{width="360"}

The figure illustrates a magnified view of a **parallel-hole
collimator** attached to a crystal. The collimator simply consists of a
large number of small holes drilled in a lead plate. Notice that
gamma-rays entering at an angle to the crystal get absorbed by the lead
and that only those entering along the direction of the holes get
through to cause scintillations in the crystal. If the collimator was
not in place these obliquely incident gamma-rays would blur the images
produced by the gamma camera. In other words the images would not be
very clear.

Most gamma cameras have a number of collimators which can be fitted
depending on the examination. The basic design of these collimators is
the same except that they vary in terms of the diameter of each hole,
the depth of each hole and the thickness of lead between each hole
(commonly called the **septum thickness**). The choice of a specific
collimator is dependent on the amount of radiation absorption that
occurs (which influences the **sensitivity** of the gamma camera), and
the clarity of images (that is the **spatial resolution**) it produces.
Unfortunately these two factors are inversely related in that the use of
a collimator which produces images of good spatial resolution generally
implies that the instrument is not very sensitive to radiation.

Other collimator designs beside the parallel hole type are also in use.
For example a **diverging** hole collimator produces a minified image
and **converging** hole and **pin-hole** collimators produce a magnified
image. The pin-hole collimator is illustrated in the following figure:

!Diagram of a pin-hole collimator illustrating the inversion of
acquired
images.{width="360"}

It is typically a cone-shaped device with its walls made from lead. A
cross-section through this cone is shown in the figure. It operates in a
similar fashion to a pin-hole photographic
camera and produces an inverted image of
an object -- an arrow is used in the figure to illustrate this
inversion. This type of collimator has been found useful for imaging
small objects such as the thyroid gland.

## Example Images

A representative selection of nuclear medicine images is shown below:

  ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  !A SPECT slice of the distribution of ^99m^Tc Ceretec within a patient\'s brain.{width="120"}                                                                                                 !A SPECT slice through a patient\'s liver.{width="120"}   !Images from a patient\'s bone scan.{width="120"}                                                                                                                                                                 !A PET slice of a patient\'s brain, with a region of interest drawn to indicate the skin surface.{width="120"}   !Images from a ventilation (V) and perfusion (Q) scan of a patient\'s lungs. and perfusion (Q) scan of a patient's lungs."){width="120"}
  !A series of planar images acquired every 10 seconds during a renogram of a patient with a stone blocking their right kidney.{width="120"}   !Selected images from a renogram series.{width="120"}      !A graphical display showing the number of counts in each kidney versus time for a renogram.{width="120"}   !A SPECT slice of a patient\'s heart.{width="120"}                                                                                                                         !A blood pool study covering the whole body of a patient.{width="120"}
  !A series from a SPECT study of a patient\'s brain.{width="120"}                                                                                                                                                                                        !Images from a SPECT study of a patient\'s heart.{width="120"}    !A thyroid uptake study.{width="120"}                                                                                                                                                                                !A gastric-emptying study evaluating a patient\'s digestive system.{width="120"}                                                                  !A ^201^Tl study of the whole body of a patient.{width="120"}
  ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

## Emission Tomography

The form of imaging which we have been describing is called **Planar
Imaging**. It produces a two-dimensional image of a three-dimensional
object. As a result images contain no depth information and some details
can be superimposed on top of each other and obscured or partially
obscured as a result. Note that this is also a feature of conventional
X-ray imaging.

The usual way of trying to overcome this limitation is to take at least
two views of the patient, one from the front and one from the side for
example. So in chest radiography a posterio-anterior (PA) and a lateral
view can be taken. And in a nuclear medicine liver scan an
antero-posterior (AP) and lateral scan are acquired.

This limitation of planar X-ray imaging was overcome by the development
of the CAT Scanner about 1970 or thereabouts.
CAT stands for Computerized Axial Tomography or Computer Assisted
Tomography and today the term is often shortened to Computed Tomography
or CT scanning (the term tomography comes from the Greek word
`<i>`{=html}tomos`</i>`{=html} meaning **slice**). Irrespective of its
exact name the technique allows images of slices through the body to be
produced using a computer. It does this in essence by taking X-ray
images at a number of angles around the patient. These slice images show
the third dimension which is missing from planar images and thus
eliminate the problem of superimposed details. Furthermore images of a
number of successive slices through a region of the patient can be
stacked on top of each other using the computer to produce a
three-dimensional
image_reconstruction "wikilink").
Clearly CT scanning is a very powerful imaging technique relative to
planar imaging.

The equivalent nuclear medicine imaging technique is called **Emission
Computed Tomography**. We will consider two implementations of this
technique below.

#### Single Photon Emission Computed Tomography (SPECT)

:   This
    SPECT
    technique uses a gamma camera to record images at a series of angles
    around the patient. These images are then subjected to a form of
    digital image processing called Image
    Reconstruction in order to
    compute images of slices through the patient.

```{=html}
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```

:   The Back Projection
    reconstruction process is illustrated below. Let us assume for
    simplicity that the slice through the patient actually consists of a
    2x2 voxel array with the radioactivity in each voxel given by
    A1\...A4:

!Illustration of the acquisition of four projections around the
patient,
P1\...P4{width="320"}

:   The first projection, P1, is imaged from the right and the second
    projection, P2, from the right oblique and so on. The back
    projection process involves firstly adding the projections to each
    other as shown below:

!Illustration of the back projection computation
process.{width="320"}

:   and then normalising the summed (or superimposed) projections to
    generate an estimate of the radioactivity in each voxel. Since this
    process can generate streaking artefacts in reconstructed images,
    the projections are generally filtered prior to back projection, as
    described in a later
    chapter,
    with the overall process referred to as Filtered Back Projection
    (FBP):

!Illustration of the filtered back projection computation
process.{width="320"}

:   An alternative image reconstruction technique is called Iterative
    Reconstruction, a successive
    approximation technique. The Maximum-Likelihood
    Expectation-Maximisation (ML-EM) algorithm is widely applied where a
    division process is used to compare the actual and estimated
    projections, as shown below:

!Illustration of the Maximum-Likelihood Expectation-Maximisation
(ML-EM)
algorithm. algorithm."){width="480"}

:   One cycle of data through this processing chain is referred to as
    one `<b>`{=html}iteration`</b>`{=html}. Sixteen or more iterations
    can be required in order to generate an adequate reconstruction and,
    as a result, computation times can be rather long. The
    Ordered-Subsets
    Expectation-Maximisation
    (OS-EM) algorithm can be used to substantially reduce the
    computation time by utilising a limited number of projections
    (called `<b>`{=html}subsets`</b>`{=html}) in a sequential fashion
    within the iterative process. Noise generated during the
    reconstruction process can be reduced, for example, using a Gaussian
    filter built into the reconstruction calculations or applied as a
    post-filter:

!Illustration of an Iterative Reconstruction
process.{width="480"}

:   Images generated using different iterations, subsets and filtration
    settings can be found in an online
    book.

```{=html}
<!-- -->
```

:   A comparison of these image reconstruction techniques is shown below
    for a slice through a ventilation scan of a patient\'s lungs:

![](fbpiRComp.jpg "fbpiRComp.jpg"){width="480"}

:   The gamma camera is typiclly rotated around the patient in order to
    acquire the images. Modern gamma cameras which are designed
    specifically for SPECT scanning can consist of two camera heads
    mounted parallel to each other with the patient in between. The time
    required to produce images is therefore reduced by a factor of about
    two. In addition some SPECT gamma cameras designed for brain
    scanning have three camera heads mounted in a triangular
    arrangement.

```{=html}
<!-- -->
```

:   A wide variety of strategies can be used for the acquisition and
    processing of SPECT images.

#### Positron Emission Tomography (PET)

:   You will remember from chapter
    2
    that positrons can be emitted from radioactive nuclei which have too
    many neutrons for stability. You will also remember that positrons
    do not last for very long in matter since they will quickly
    encounter an electron and a process called **annihilation** results.
    In the process the positron and electron vanish and their energy is
    converted into two gamma-rays which are emitted at roughly 180^o^
    degrees to each other. The emission is often referred to as two
    **back-to-back** gamma-rays and they each have a discrete energy of
    0.51 MeV.

```{=html}
<!-- -->
```

:   So if we administer a positron-emitting radiopharmaceutical to a
    patient an emitted positrons can annihilate with a nearby electron
    and two gamma-rays will be emitted in opposite directions. These
    gamma-rays can be detected using a ring of radiation detectors
    encircling the patient and tomographic images can be generated using
    a computer system. The detectors are typically specialised
    scintillation devices which are optimised for detection of the 0.51
    MeV gamma-rays. This ring of detectors, associated apparatus and
    computer system are called a PET
    Scanner:

![](petDiag2.jpg "petDiag2.jpg"){width="320"}

:   The locations of positron decays within the patient are highlighted
    by the solid circles in the above diagram. In addition only a few
    detectors are shown in the diagram for reasons of clarity. Each
    detector around the ring is operated in coincidence with a bank of
    opposing detectors and the annihilation gamma-rays thus detected are
    used to build up a single profile.

```{=html}
<!-- -->
```

:   It has also been found that gamma cameras fitted with thick crystals
    and special collimators can be used for PET scanning.

```{=html}
<!-- -->
```

:   The radioisotopes used for PET scanning include ^11^C, ^13^N, ^15^O
    and ^18^F. These isotopes are usually produced using an instrument
    called a cyclotron. In addition these
    isotopes have relatively short half lives. PET scanning therefore
    needs a cyclotron and associated radiopharmaceutical production
    facilities located close by. We will consider cyclotrons in the
    next
    chapter
    of this wikibook.

```{=html}
<!-- -->
```

:   Standardized Uptake Value (**SUV**) is a semi-quantitative index
    used in PET to express the uptake of a radiopharmaceutical in a
    region of interest of a patient\'s scan. Its typically calculated as
    the ratio of the radioactivity in the region to the injected dose,
    corrected for body weight. It should be noted that the SUV is
    influenced by several major sources of variability and it therefore
    should not be used as a quantitative measure.

```{=html}
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```

:   A number of photographs of a PET scanner are shown below:

  -------------------------------------------------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------
  !The detectors and associated electronic circuitry.{width="200"}   !The scanner itself -- the detectors are under the covering panel.{width="200"}
  !Another view of the detectors.{width="200"}                                           !The image processing computer.{width="200"}
  -------------------------------------------------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------

Images reconstructed using different settings of subsets and iterations
for iterative reconstruction[^2] are shown below:
!centre\|frame\|Images reconstructed using different subsets/iterations
settings within an iterative reconstruction algorithm (VPFXS). GF:
Gaussian
filtered. GF: Gaussian filtered")

## References

```{=html}
<references />
```
## External links

-   Centre for Positron Emission
    Tomography at the Austin &
    Repatriation Medical Centre, Melbourne with sections on what PET is,
    current facilities, projects & research and a PET image library


[^1]: Slomka PJ, Patton JA, Berman DS & Germano G, 2009. Advances in
    technical aspects of myocardial perfusion SPECT
    imaging. Journal of
    Nuclear Cardiology, 16(2), 255--76.

[^2]: Maher KP, 2016. Iterations, Subsets & Gaussian Filtration, 3rd
    Edition
    (Bookemon.com\]
