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?

HOW IS THE IMAGE FORMED?

SPECT: SINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY Philips SKYLight gamma camera Picker CT scanner

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 SCATTER ATTENUATION PARTIAL VOLUME EFFECT DEAD TIME PIXELS 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 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 Ventilation Perfusion Anatomy

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 3 rd leading cause of cancer death Liver metastases present late and have very poor prognosis (survival ~ 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?