Medical Imaging III SPECT Imaging PDF

Medical Imaging 3 PET Imaging R MURAT DEMIRER IŞIK ÜNİVERSİTESİ Main Principle FDG-PET EXPLOITS WHEN A RADIOACTIVE PET THEN PICKS UP THE FACT THAT ANALOG OF GLUCOSE THE RADIOACTIVITY, CANCER CELLS GROW IS ADMINISTERED (IN THEREBY IMAGING...

Medical Imaging 3 PET Imaging R MURAT DEMIRER IŞIK ÜNİVERSİTESİ Main Principle FDG-PET EXPLOITS WHEN A RADIOACTIVE PET THEN PICKS UP THE FACT THAT ANALOG OF GLUCOSE THE RADIOACTIVITY, CANCER CELLS GROW IS ADMINISTERED (IN THEREBY IMAGING QUICKLY AND ARE THE FORM OF 18FDG, CANCEROUS SITES. VERY ACTIVE OR GLUCOSE TAGGED CLINICIANS QUICKLY METABOLICALLY, AND WITH RADIOACTIVE SAW ITS VALUE FOR CONSEQUENTLY, THEY FLUORINE), TUMOR DETECTING CANCER ARE GLUCOSE CELLS METABOLIZE IT. AND HAVE SINCE GLUTTONS. MADE IT A STANDARD OF CARE FOR TRACKING CANCER PROGRESSION AND EVALUATING WHETHER A GIVEN TREATMENT IS HAVING THE DESIRED EFFECT MR compatible PET system located on patient table of Philips 3TAchieva whole body MR scanner PET Alzheimer PULSE SEQUENCES USED TO ASSESS EFFECT OF MR PULSE SEQUENCES ON PET SPATIAL RESOLUTION,SENSITIVITY AND COUNT RATE Nuclear medicine: Planar scintigraphy, SPECT and PET/CT  In nuclear medicine scans a very small amount, typically nanogrammes, of radioactive material called a radiotracer is injected intravenously into the patient.  The agent then accumulates in specific organs in the body. How much, how rapidly and where this uptake occurs are factors which can determine whether tissue is healthy or diseased and the presence of, for example, tumours technetium  (), an element which emits mono-energetic -rays at 140 keV Preprocessing algorithm program PET/CT Result of original interpolation method: (a) PET image only by pixel dimensions interpolation. (b) CT image of the same organs with (a) PET/CT Fusion   a poor SNR, SPECT  Low spatial resolution (~5–10 mm) and  slow image acquisition,  but extremely high sensitivity (being able to detect nanograms of injected radioactive material), and  very high specificity since there is no natural radioactivity from the body SPECT  Single photon emission computed tomography (SPECT), produces a series of contiguous two dimensional images of the distribution of the radiotracer using the same agents as planar scintigraphy  Decay of the radiotracer within the body produces -rays, a small percentage of which pass through the body (the vast majority are absorbed in the body).  A two-dimensional collimator (similar to the anti-scatter grid in X-ray imaging) is placed between the patient and the detector, so that only those c-rays which strike the gamma camera at a perpendicular angle are detected  A large, single scintillation crystal is used to convert the energy of the x-rays into light, which is in turn converted into an electrical signal by high-gain photomultiplier tubes (PMTs). Spatial information is encoded in the current produced by each PMT The operation of a nuclear medicine gamma camera SPECT brain scan using.  Chart of nuclides Black squares indicate stable nuclides. Yellow squares indicate radionuclide decaying by alpha decay. Pink squares indicate radionuclides decaying by negatron or β– decay. Blue squares indicate radionuclides decaying by either positron β+ decay or EC. White squares indicate other decay paths (e.g., spontaneous fission). (Chart generated from data from the National Nuclear Data Center, Brookhaven National Laboratory, Upton, NY, 2016.) Endocyte obtained exclusive global rights to develop and commercialize PSMA-617, a highly promising agent for certain types of prostate cancer, specifically those that Express PSMA PSMA is found in more than three-quarters of patients who have metastatic castration- resistant prostate tumors. This illustration shows the radioligand therapeutic 177lu-pSma-617 binding to a PSMA positive prostate cancer cell. (image courtesy of endocyte, inc.) Radioactivity and radiotracer half- life N nuclei of a particular radioactive isotope is defined as the decay constant and has units of Radioactivity is measured in units of curies (Ci), where one curie equals disintegrations per second millicuries (mCi), 1/1000 of a curie or disintegrations per second. A plot of the amount of two different radiotracers as a function of time. The thin line corresponds to a radiotracer with a shorter half-life than the one indicated by the thick line. The refers to the radiotracer with the shorter half-life. Effective Half-life (a) Schematic of how the parent (P) and daughter (D) radionuclides would be related as positioned on the chart of the nuclides for beta decay. (b) Energy level diagram for the decay of 99Mo. ITs (internal conversion) from the metastable to the ground state of Photoelectric interaction A photon comes in from the left, is absorbed by a K-shell electron, and provides enough energy Compton scattering (SPECT and PET) Interactions of radiation with matter The linear attenuation coefficient μ is dependent on the density of the material, the atomic number of the material, and the energy of the photons μ(ρ, Z, E) is the linear attenuation coefficient, which is a function of the density (ρ) and atomic number of the material (Z), and energy E of the photons  Energy spectrum Examples of energy spectra of 140 keV photon measured with NaI (Tl) detector. The spectrum to the left was measured with the source in air. The spectrum is characterized by a large photopeak and the Compton distribution. The photopeak represents all full energy absorptions in the detector of the 140 keV photons. visualize nuclear gamma rays emitted from radioactive tracers.  Two common techniques—single photon emission computed tomography (SPECT) and positron emission tomography (PET)—play important roles in the diagnosis.  However, they image a specific energy range of either X-rays or gamma rays; SPECT can image gamma rays with energies less than 300 keV with the use of the collimator, whereas PET can image positron emitters that emit 511-keV gamma rays. These lead to a limited number of radioactive tracers that can be imaged only with current SPECT and PET scanners. Properties of the ideal scintillator for PET PET Imaging: Probes and Principles  Radioisotopes suitable for PET imaging decay via the emission of positrons (β+-emission).  Once ejected from the nucleus, positrons travel through space, gradually loosing velocity, until they eventually annihilate through collision with a negatron (electron), producing two high- energy γ-rays(511 keV), released in almost opposite directions.  Surrounding PET detectors recognize coincidence photons, i.e., photons detected at the same time on opposite sides of the detector ring Isotopes The general tracer compartmental model. There are Q tissue compartments in this model.  The normalized kinetic dictionary. The green line shows theplasma input basis withw=0. The blue lines show the plasma inputbasis with different values ofw. PET Imaging INTRODUCTION TO SPECT IMAGING  Tomography provides the ability to re-create or reconstruct 3-D distributions from information collected along ray or line integrals through the object from many directions.  In emission tomography, the information acquired is related to the concentration of activity at each point along the ray as altered by attenuation, the inclusion of scattered photons, and other degradations. It takes two points or one point plus a direction to determine the path of a ray. SPECT Imaging Attenuation Correction  PET generally provides higher sensitivity as well as better temporal and spatial resolution than SPECT, although small animal SPECT systems can provide high-resolution imaging  Attenuation correction is performed to account for the greater probability that annihilation photons arising deep within an imaging subject will not reach the detectors than photons arising from more superficial structures. Attenuation correction can be performed using a transmission source which rotates around the imaging subject or from computed tomography (CT) or magnetic resonance imaging (MRI) data in the case of PET/CT and PET/MRI systems.  Partial volume effect Top: Effect of the blur introduced by the PSF of a SPECT imaging system. The blue structure spills in the red one, and conversely. Because the red structure is small in size with respect to the width of the PSF of the SPECT imaging system, the activity level in that red structure is underestimated (height of the solid red curve less than the height of the light red rectangular object) Bottom: Effect of voxel sampling, also known as tissue fraction effect. When a voxel includes two tissue types (left: dark blue with an activity concentration of 10 and white with an activity concentration of 1), the voxel value is a weighted average of the activity in each tissue (central picture). If a given tissue (dark blue) is always present with other tissues in a voxel, then the activity in that tissue cannot be easily recovered (bottom right).  Reconstructed SPECT image X is modeled as the convolution of the unblurred image I with the 3D PSF F of the imaging system, where ⊗ denotes the 3D convolution operator How to reduce partial volume to enhance the spatial resolution of the reconstructed images using deconvolution The Van Cittert deconvolution procedure consists in estimating I iteratively using Partial Volume effects Partial volume effects occur when an object (the small square) partially occupies the sensitive volume (the triangular region) of the imaging instrument (in space or in time). The arrow represents possible motion of the object during acquisition Partial Volume Correction  The ultimate goal of partial volume correction (PVC) is to reverse the effect of the system PSF in a PET or SPECT image and thereby restore the true activity distribution, qualitatively and quantitatively.  This can be done by deconvolution, either in the image domain or during iterative image reconstruction by incorporating the PSF in the system matrix. The first approach results in noise amplification, while the second approach has a superior noise performance.  This is because a larger number of measured data values are involved in the reconstruction of each voxel value, resulting in noise averaging. In both cases, however, the resulting images often suffer from so called ‘Gibbs artefacts’, corresponding to ringing in the vicinity of sharp boundaries, which is related to missing high frequency information. This could be caused by information loss during data acquisition due to limitations in the detector system design, or by insufficient sampling in the image domain by the use of too large voxels. The main advantage of these methods is that they depend on PET or SPECT data only, and no other data set is needed. This is, however, also a limitation. PET Motion Correction  The Fourier transform of the PSF is known as the modulation transfer function (MTF). The MTF contains the same information as the PSF, but expressed in the frequency domain rather than in the spatial domain.  The Fourier transform of a Gaussian function is also a Gaussian function, and the widths of the two functions have an inverse correlation: as the spatial domain  Gaussian becomes broader, the frequency domain Gaussian becomes narrower, and vice- versa Point spread functions with FWHM of 5 and 10 mm (left), and corresponding modulation transfer functions (right) Images of a point source 10 cm from the camera head acquired without (left) and with (right) a collimator in place. Without the collimator, the intensity of the photons emitted by the point source falls off according to the inverse-square law with distance. With the collimator in place, an image of the point source is obtained. Major components of a gamma camera Photons emitted from the patient will hit the collimator. If the direction of the photons aligns with the detector channels, they will reach the scintillation crystal. When the photons interact in the scintillation crystal, a light flash is produced. This light is collected by the array of PMTs. The signals from the PMTs will be processed by the electronics to determine the energy and the position of the photon interaction. If the energy of the interaction falls within the energy photomultiiplier tube (PMT) and its internal structures that include photocathode, several dynodes connected under cascade of increasing potential difference and positively charged (Image taken from www.physicsopenlab.org under the license of Creative Commons Attribution 4.0 International Properties of scintillation crystals used in gamma camera NaI(Tl) crystal  formed by adding a controlled amount of thallium to a pure sodium iodide crystal during growth. The addition of thallium makes the NaI crystal scintillate at room temperature since pure NaI crystal works at a low temperature under nitrogen cooling. S  ome precautions are taken into account during the design of the NaI(Tl) crystal. It must be sealed in an airtight enclosure, usually aluminum, to avoid exposure to air owing to its hygroscopic properties. Exposing the crystal to air can cause yellow spots, which can develop heterogeneous light transmission The signal TIME Q = ENERGY shape characteristics released from PMT and fed into preamplifer and amplifer circuitry. (From www. physicsopenlab.org under the license of Creative Commons Attribution 4.0 International Different shape, size, and number of channels of position- sensitive photomultiplier tubes. (Images are Courtesy Hamamatsu Photonics) Modern two-headed SPECT system shown in two different configurations of the detector heads (180° to the left and 90° to the right). Gamma camera imaging systems based on CZT detectors. (a) A diagram showing the rotating collimator confguration in D- SPECT model (Spectrum Dynamics) and (b) Discovery NM 530c showing the stationary collimator- (GE Healthcare) CZT Camera  CZT camera serves to convert the incident gamma photons directly into electronic signal through the generation of electron and hole pairs with number proportional to the incident photons. The generated electron–hole pair carries information about event position and energy. As energy conversion is effcient process and large amount of charge are created upon photon interaction with the detector, the energy resolution is much better than the standard gamma camera  Myocardial perfusion imaging  The CZT can handle large amount of photons per unit area (i.e., 106 photons/s mm2 ).  The superior system sensitivity do help in significantly reducing the imaging time without loss of image quality (a) Image of whole body bone scan. (b) Laplacian of (a). (c) Sharpened image obtained by adding (a) and (b). (d) Sobel gradient of image (a). (Original image courtesy of G.E. Medical Systems.) State-of-the-art SPECT/CT systems: (a) Symbia Intevo bold and (b) NM/CT 870 DR SPECT/CT. (Images are courtesy of Siemens Healthcare and GE Healthcare, respectively) Sobel Filters Gamma camera, SPECT and SPECT/CT system tests described in the present chapter with frequencies recommended by the IAEA, AAPM and EANM Source–detector confguration for the evaluation of gamma camera intrinsic uniformity Transmission image of the linearity mask without (left-hand figure) and with (right-hand figure) the linearity correction applied Correct energy (Eγ) peaking (left-hand figures) and the effect of incorrect peaking (right-hand images) energy (Eγ) peaking  Instability of the PMTs or variation (drift) in the high voltages applies to the PMTs can result in both poorer energy resolution and/or a shift in the position of the photopeak  Energy peaking should be checked daily for all radionuclides that are used clinically. For isotopes that emit low-energy photons such as 99mTc and 57Co, the centroid position of the photopeak should correspond within ±3% to the nominal photon energy for that isotope (140.5 keV for 99mTc and 122 keV for 57Co) Evaluation of whole-body scanning uniformity using a flood source