Introduction to Medical Imaging SPECT, Introduction to Medical Imaging SPECT, PET and Lesion Detection Guy Gilboa Course

Topics Nuclear Imaging ◦ SPECT - Single-Photon Emission Computed Tomography ◦ PET – Positron Emission Tomography

Nuclear imaging uses low doses of radioactive substances linked to compounds used by the body's cells or compounds that attach to tumor cells. Using special detection equipment, the radioactive substances can be traced in the body to see where and when they concentrate. Two major instruments of nuclear imaging used for cancer and cardiac imaging are PET and SPECT. SPECT+CT data Taken from imaging/molecular-imaging-and-nuclear-medicine/ imaging/molecular-imaging-and-nuclear-medicine/

SPECT - Single-Photon Emission Computed Tomography SPECT scan monitors level of biological activity. Emissions from the radionuclide indicate amounts of blood flow in the capillaries of the imaged regions. SPECT imaging is performed by using a gamma camera to acquire multiple projections, from multiple angles, similar to X-ray based CT. 3D reconstruction algorithms are used. The spatial resolution of SPECT is ~6-10mm.

Gamma camera GE SPECT Gamma camera After it’s inventor Hal Anger (invented the gamma camera in 1957). Today photon-counting technologies (based on CZT for instance) are also used for advanced nuclear imaging.

Radiotracers Radio tracers should be with: Half life of several hours High enough energy (over 100keV) High uptake at the targeted organs compared to the rest of the body

Technetium Technetium is the most commonly used radiotracer. 6 hours half life 140keV gamma-ray energy Can be bonded with multiple pharmaceutical agents for various applications.

SPECT Acquisition The gamma camera is rotated around the patient. Projections are acquired typically every 3–6 degrees. Each projection takes about15–20 seconds is typical. Total scan time is15–20 minutes. Multi-headed gamma cameras can provide accelerated acquisition.

Spatial resolution Reconstructed images typically have resolutions of 64×64 or 128×128 pixels. The number of projections acquired is chosen to be approximately equal to the width of the resulting images. In general, the resulting reconstructed images will be of lower resolution, have increased noise than planar images, and be susceptible to artifacts.

Problems and artifacts Motion artifacts as scanning is time consuming. Uneven distribution. A very intense area of activity (e.g., the bladder) can cause extensive streaking of the images and obscure neighboring areas of activity. FBP is more prone to streaking artifacts. Iterative reconstruction is less sensitive and can also correct for attenuation and depth dependent blurring.

Attenuation correction Attenuation of the gamma rays within the patient can lead to significant underestimation of activity in deep tissues, compared to superficial tissues. Approximate correction is possible, based on relative position of the activity. However, optimal correction is obtained with measured attenuation values. Modern SPECT equipment is available with an integrated CT scanner. CT images are an attenuation map of the tissues, used to correct the SPECT reconstruction algorithm. It also provides a CT image with additional anatomical information.

Oncology Tumor and lesion detection

Myocardial perfusion Myocardial perfusion imaging (MPI) is a form of functional cardiac imaging, used for the diagnosis of ischemic heart disease. The underlying principle is, under conditions of stress, diseased myocardium receives less blood flow than normal myocardium.

MPI example Arrows indicate a small perfusion defect on the backside of the heart (visible only on the stress images), showing ischemia in this region of the heart wall. Philipp A Kaufmann, MD, and Oliver Gämperli, MD, University Hospital Zurich. SPECT imaging performed after stress. The upper row shows short-axis slices after pharmacologic stress; the lower row shows the same slices when the body is at rest In the myocardial perfusion scans using Tc-99m, the radiopharmaceuticals 99mTc- tetrofosmin or 99mTc-sestamibi are used.

PET – Positron Emission Tomography A more advanced higher resolution nuclear imaging technology.

PET principle Detects pairs of gamma rays (at 511keV) emitted indirectly by a positron-emitting radioactive tracer. 3D images of tracer concentration within the body are constructed using tomography.

PET general scheme

Electron–positron annihilation Occurs when an electron (e − ) and a positron (e+, the electron's antiparticle) collide. The result of the collision is the annihilation of the electron and positron, and the creation of gamma ray photons. FDG = Fluorodeoxyglucose

How the test is performed The tracer is given through a vein (IV), most often on the inside of the elbow. The tracer travels through the blood and collects in organs and tissues. The patient needs to wait as the tracer is absorbed by the body. This takes about 1 hour. The patient lies on a bed which slides slowly, similar to other imaging modalities. The patient should lie still during test. Too much movement can blur images and cause errors. Scan time can take between 30 minutes to 2 hours, depends on the part of body being scanned.

PET-CT Today often PET is conducted on a PET-CT scanner. Both scans are performed on the patient during the same session, in the same machine. Areas of abnormality on the PET imaging can be well correlated with anatomy on the CT images. PET CT

FDG Tracer The molecule most commonly used as radiotracer is fluorodeoxyglucose (FDG), an analogue of glucose that is labeled with fluorine- 18. Half life-time is ~ 110 minutes. FDG is used in essentially all scans for oncology and most scans in neurology, used in ~95% of the scans. Other tracers (e.g. based on carbon-11, nitrogen- 13 or oxygen-15), with different properties, can also be used. Active research is conducted on that.

Production of FDG Due to the short half-lives of FDG it is usually produced using a cyclotron in close proximity to the PET imaging facility. Cyclotron – a particle accelerator in which charged particles accelerate outwards from the center along a spiral path.

SPECT vs. PET SPECT is similar to PET in its use of radioactive tracer material and detection of gamma rays. Differences: ◦ The tracer used in SPECT emits gamma radiation that is measured directly, whereas in PET the chain of reaction causes two gamma photons to be emitted in opposite directions. ◦ In PET we get more radiation event localization information, higher spatial resolution. ◦ SPECT scans are significantly less expensive than PET. In part because they are able to use longer-lived more easily-obtained radioisotopes than PET.

Nuclear imaging vs other modalities High sensitivity Intrinsic functional information Poor SNR (esp. SPECT) Low resolution (6-10mm SPECT, 3-6mm PET) Slow image acquisition (15-20 min)

PET-CT combined image

PET-CT cancer therapy monitoring

Lung cancer Taken from

Liver

Lymph nodes near neck

PET Brain Scans PET scans can detect the decline in glucose metabolism associated with decreased cognitive function, particularly in the temporal and parietal lobes located on the sides and the back of the brain, the regions associated with memory formation and language.

Detecting Alzheimer's disease using new tracers New radioactive tracer PiB detects beta-amyloid plaques, which are found in the brains of Alzheimer patients.

Other abnormalities detected