Download physics in nuclear medicine

PHYSICS IN NUCLEAR MEDICINE:
QUANTITAITVE SPECT AND
CLINICAL APPLICATIONS
Kathy Willowson
Department of Nuclear Medicine, Royal North Shore Hospital
University of Sydney, Institute of Medical Physics
WHAT IS NUCLEAR MEDICINE?
• Nuclear medicine is a diagnostic imaging tool
• Nuclear medicine gives us FUNCTIONAL data
• Based on the TRACER PRINCIPLE: radioactive
compounds participate in a biological process the same
way as non-radioactive substances. Since these
radioactive materials can be detected by their emission
of gamma rays, these materials can be used to follow
physiological processes within the body
• Earliest study in humans: 1927 – Blumgart and Weiss
measured blood flow
WHAT IS NUCLEAR MEDICINE?
WHAT IS NUCLEAR MEDICINE?
WHAT IS NUCLEAR MEDICINE?
HOW IS THE IMAGE FORMED?
SPECT: SINGLE PHOTON EMISSION
COMPUTED TOMOGRAPHY
Picker CT scanner
Philips SKYLight gamma camera
SPECT: SINGLE PHOTON EMISSION
COMPUTED TOMOGRAPHY
The goal of SPECT is to produce an image that accurately
represents the distribution of radioactivity inside the body
at the time of scanning
BUT…
Photons must travel from inside the body to be detected
externally
SPECT: SINGLE PHOTON EMISSION
COMPUTED TOMOGRAPHY
ATTENUATION: A beam of photons traveling through a
material can be absorbed, scattered or transmitted.
Absorption + scatter = attenuation
The probability that a beam of photons will interact with a
material and be attenuated is determined by the initial
energy of the photons and the composition and thickness
of the absorber
SPECT: SINGLE PHOTON EMISSION
COMPUTED TOMOGRAPHY
ATTENUATION: The degree of attenuation in SPECT images
depends on the thickness and density of tissue that the
gamma rays must traverse in order to be detected
SPECT: SINGLE PHOTON EMISSION
COMPUTED TOMOGRAPHY
SCATTER: Scatter accounts for ~30-40% of photons that
make up a SPECT image
If not corrected for scatter = loss in contrast and spatial
resolution
Scatter also depends on source depth and material density
SPECT: SINGLE PHOTON EMISSION
COMPUTED TOMOGRAPHY
ATTENUATION
SCATTER
DEAD TIME
PIXELS
PARTIAL VOLUME
EFFECT
kBq/mL
CT BASED QUANTITATIVE SPECT
• Quantitative SPECT = SPECT data in units of absolute
activity (kBq/ml)
• Can use anatomical data from CT to tell us about material
density of every voxel in our image – correct for scatter
and attenuation etc.
• The camera sensitivity factor can be measured in
experiments and tells us how many counts our camera will
measure for every unit of radioactivity in the source –
converts our image into Bq
CLINICAL EXAMPLE OF QUANTITATIVE
SPECT: EVALUATING LUNG CANCER
• Patients suffering from lung cancer can
only undergo surgery if the remaining
lung will have adequate function
• In the past, post-resection function has been estimated from CT
alone, or by assuming rectangular “lobes” of equal function
weight
CLINICAL EXAMPLE OF QUANTITATIVE
SPECT: EVALUATING LUNG CANCER
Ventilation
Perfusion
Anatomy
• SPECT ventilation/perfusion imaging and qSPECT analysis
allows us to evaluate function of lungs before surgery on a lobar
by lobar basis
• Allows accurate estimates of loss in lung function following
surgery that correlate with post-treatment respiratory tests
CLINICAL EXAMPLE OF QUANTITATIVE
SPECT: ASSESSING BRAIN TUMOURS
• Malignant glioma has a poor prognosis and requires fast /
aggressive treatment – surgery and radio/chemo therapy
• CT and MRI have limited use due to inability to differentiate
between scarring/necrosis and disease recurrence
• SPECT brain studies can be an early identifier of disease
CLINICAL EXAMPLE OF QUANTITATIVE
SPECT: ASSESSING BRAIN TUMOURS
• In PET – the Standard Uptake
Value (SUV) is a quantitative
measure that is used to monitor
patients and predict survival /
response to therapy
• Can qSPECT play a similar role in
SPECT?
CLINICAL EXAMPLE OF QUANTITATIVE
SPECT: LIVER CANCER
• The liver is the 2nd most common site of metastases
(after lymph system) and 3rd leading cause of cancer death
• Liver metastases present late and have very poor
prognosis (survival ~ 12-24 months)
• Less than 25% of patients are eligible for surgery
• Remaining treatment options are limited and are not
associated with great success – particularly if there are
multiple/large metastases
CLINICAL EXAMPLE OF QUANTITATIVE
SPECT: LIVER CANCER
• SIR-SPHERES: microspheres (diam ~
40µ) labelled with Y-90 (β emitter)
• Millions of SIR-Spheres injected into
hepatic artery and become lodged in
microvasculature of the tumour = large,
localised radiation dose
• Some spheres end up in the lungs or
healthy liver – highly sensitive to
radiation so therapy cannot be done if a
large number of spheres will do this
CLINICAL EXAMPLE OF QUANTITATIVE
SPECT: LIVER CANCER
• Using qSPECT in work-up
stages for therapy to derive
estimates of radiation dose to
healthy organs and tumour
• Therapy tailored specifically
to the individual
• Improve survival time and
quality of life
PHYSICS IN NUCLEAR MEDICINE
• Physics plays a big role in many parts of nuclear medicine:
• Modeling radiation transport (incl monte carlo sim)
• Detector design
• Reconstruction algorithms
• Image analysis tools
• Modeling radiation dose
• Improving data acquisition (eg. patient motion)
…
WHAT IS NUCLEAR MEDICINE?