Nuclear Medicine and Imaging Techniques Quiz

PET Imaging

• Positron Emission Tomography (PET) is a sophisticated imaging technique that takes advantage of the unique characteristics of cancer cells. These cells are known for their rapid growth and significantly elevated metabolic rates compared to normal cells. One of their key energy sources is glucose, which is critical for their survival and proliferation.

• To visualize these cancerous cells, radioactive glucose analogs, such as Fluorodeoxyglucose (FDG), are administered to the patient. This radioactive compound mimics glucose and is taken up by cells throughout the body; however, tumor cells exhibit a higher uptake of this analog due to their increased metabolic demands.

• As the tumor cells metabolize the radioactive glucose faster than the surrounding healthy tissue, PET scans can detect the accompanying radioactivity. This allows clinicians to create detailed images of the body's internal structures, highlighting the locations of cancerous growths. The precise imaging capabilities of PET make it a valuable tool in oncology, aiding in the diagnosis, staging, and monitoring of cancer.

• PET exploits the rapid growth and high metabolic activity of cancer cells, which are glucose-dependent. Radioactive glucose analogs (18FDG) are administered. Tumor cells metabolize this analog faster than normal cells.

• PET detects the radioactivity, thereby imaging cancerous sites. Clinicians have employed this for cancer detection, progression tracking, and treatment efficacy assessment.

• PET scans use MR compatible PET system located on patient table of Philips 3TAchieva whole body scanner,

• PET is used for Alzheimer's disease diagnosis. PET images illustrate metabolic differences in brain regions in normal, mild cognitive impairment and Alzheimer's disease patients.

• Various pulse sequences (GE, EPI, TSE) are used for PET/CT assessment, influencing spatial resolution, sensitivity and count rate.

• A nuclear medicine technique, planar scintigraphy, SPECT and PET/CT exploit radioactive material (a radiotracer) intravenously injected for image acquisition. Different uptakes highlight tissue healthiness.

Nuclear Medicine Techniques

• In nuclear medicine scans, a tiny amount of radioactive material (a radiotracer) is intravenously injected. This substance accumulates in specific organs.
• How much, the rapidity, and the area of uptake—all help determine tissue health or disease (tumors, etc.).
• Different elements (e.g., Technetium) with varying levels of emitted energy distinguish between normal and abnormal tissues.

Preprocessing Algorithm for PET/CT

• Original interpolation method involves pixel-based PET image dimension interpolation.
• Similar CT images of the same organs can be processed with the PET-based method.

PET/CT Fusion

• Fusion of PET and CT images, utilizing mathematical expressions to generate interpolated data points (i, j) which represent pixel values of the PET images (f(i, j)).
• The ratio of the physical distances between CT and PET images is represented by D.

SPECT (Single Photon Emission Computed Tomography)

• SPECT produces two-dimensional images of the radiotracer's distribution in the body using gamma rays.
• Gamma rays originate from the radiotracer decay within the body.
• A two-dimensional collimator is positioned between the patient and detector; it only detects gamma rays striking the camera perpendicularly.
• A large, single scintillation crystal changes x-ray energy to light and then into an electrical signal. The signal is read out by photomultiplier tubes (PMTs).
• Spatial information is encoded via the PMT signal for each point on the detector.

SPECT Brain Scan

• Radiotracer injected into the patient produces gamma rays (140 keV). In-situ scintillation converts them to light (415nm).
• A pulse height analyzer processes the light signals and uses a positioning network to create image displays.

Chart of Nuclides

• The chart displays nuclides exhibiting various types of radioactive decay (alpha, beta, electron capture etc.).
• Stable, unstable, beta-decay, electron capture/positron decay, alpha decay, and other decay types are categorized.

PSMA-617 for Prostate Cancer

• Endocyte holds exclusive rights to develop and commercialize this prostate cancer treatment, especially for castration-resistant metastatic prostate tumors.
• The illustrated radioligand therapeutic 177Lu-PSMA-617 binds to PSMA-positive prostate cancer cells.

Radioactivity and Radiotracer Half-Life

• Radioactivity in curries (Ci) or millicuries (mCi), denotes disintegrations per second.
• Radioactive isotope nuclei decay according to the equation: N(t) = N(0) exp(-λt)
• Half-life(1/2) is proportional to the decay constant. This relationship is described by 1/2 = ln 2 / λ

Effective Half-Life

• Effective decay depends on both physical decay and biological effects (biologic half life).
• Physical half-life and biological half-life can both affect the effective half-life. This calculation is crucial when understanding the decay rate of a radiotracer in a patient.

ITs (Internal Conversion)

• Internal conversion is the transfer of energy from a metastable nucleus to an orbital electron. This is exemplified through energy level diagrams depicting 99mTc decay and the associated energy transfer process.

Photoelectric Interactions

• A photon, traveling from left to right, interacts with a K-shell electron within an atom. The photon transfers enough energy to eject the electron.
• The remaining photon energy becomes the electron kinetic energy and the electron binding energy.

Compton Scattering

• Compton scattering is an interaction between gamma rays and atomic electrons. The incident gamma ray loses energy and changes direction. The scattered gamma ray interacts with the detector and contributes to the SPECT and PET image.

Interactions of Radiation with Matter

• The linear attenuation coefficient (μ) is material density (ρ) and atomic number (Z) dependent, as well as the photon energy.
• The relationship can be represented as: I(x) = I0e−μ(ρ, Z, E)x.

Energy Spectrum

• SPECT/CT systems measure energy spectra of photons emitted by radioactive elements like 140 keV, revealing a clear photopeak and Compton distribution in the energy spectra.

Visualization of Radiotracers

• SPECT and PET use gamma rays and positron-emitting radiotracers to visualize the distribution of radioactivity within the body. They image different energies, SPECT uses lower energy gamma rays and PET uses higher energy gamma rays. The differences in energy enable these imaging systems to view varying tissue types selectively.

Ideal Scintillator Properties for PET

• The ideal scintillator exhibits high gamma-ray detection efficiency, high coincidence timing accuracy, large crystal-to-readout-channel ratios, efficient light transport, and stability for consistent performance. Cost-effectiveness of manufacturing is also important.

PET Imaging: Probes and Principles

• Radioisotopes used in PET scans undergo positron emission.
• Positrons collide with electrons and produce two gamma rays traveling in opposite directions (511 keV).
• The PET detectors detect these simultaneous photons (coincidence).

Isotopes

• Commonly used isotopes, including 18F, ¹³N, ¹¹C, show short half-lives and are produced on-site or nearby. 68Ga and 82Rb are examples of isotopes with longer half-lives that are produced off-site.

Tracer Kinetic Model

• The normalized kinetic dictionary demonstrates how radioactivity concentration (Cr(t)) changes over time (t). It illustrates plasma input and varying values (w) for different compartments within the tissue compartment model.

PET Imaging

• Positron-emitting isotopes undergo decay.
• The emitted positrons interact with an electron and produce two 511 keV gamma rays moving in opposite directions.
• Surrounding detectors recognize the simultaneous emission of these photons.

Introduction to SPECT Imaging

• Tomography reconstructs 3D distributions using line integrals through many directions.
• SPECT captures information related to activity concentration, altered by attenuation, scattered photons, etc.

SPECT Imaging

• Radioisotopes decay and emit gamma rays.
• Emitted gamma rays pass through the patient and reach a scintillation crystal within the detector.
• Detected light flashes produce signals processed to ascertain energy and position.

Attenuation Correction

• Attenuation correction accounts for gamma photons' reduced likelihood of reaching the detector, due to interactions in deeper tissues.
• Attenuation correction techniques employ transmission sources or data from CT/MRI for correction.

Partial Volume Effect

• Partial volume effect from the point spread function (PSF) of the SPECT imaging system causes blurring.
• The activity level in a small regional structure is lower than the adjacent (larger) structure due to the effect of voxel sampling.
• The voxel sampling effect can also be called the tissue fraction effect.

Partial Volume Correction

• Partial volume correction (PVC) aims to reverse the influence of the PSF on image restoration.
• The procedure relies on deconvolution (in either image or iterative reconstruction domains using the PSF in the system matrix) that incorporates information on noise amplification.
• Additional methods may be helpful using alternative datasets to increase the accuracy of reconstruction.

PET Motion Correction

• PET motion correction involves using tracking data from an external device to account for patient movement during acquisition and adjust data accordingly.
• Corrections can occur in list mode data or sinograms.

Modulation Transfer Function (MTF)

• The MTF is the Fourier transform of the PSF (point spread function).
• It is equivalent to the PSF, however, it is expressed in the frequency domain (rather than the spatial domain). The width of the MTF function is inversely proportional to the width of the PSF function in the spatial domain.
• A Gaussian frequency spectrum results in a Gaussian spatial domain function.

Point Spread Functions (PSFs)

• PSFs are characterized by their full-width at half-maximum (FWHM)—related to resolution on images.

Images of a Point Source

• Images acquired without a collimator show the inverse-square law decay of intensity from the point source with increasing distance.
• The collimator in place results in better image formation.

Major Components of a Gamma Camera

• Gamma rays emitted from a patient pass through a collimator to a scintillator crystal.
• The resulting light flash generates electrical signals from photomultiplier tubes (PMTs).
• The PMT signals provide data to compute energy and position.

Photomultiplier Tube (PMT)

• PMTs comprise a photocathode and several dynodes.
• Electrons are released from the photocathode, and multiple interactions occur with dynodes, increasing the signal amplitude (gain).
• The output voltage is directly proportional to the energy of the impinging photon, enabling the measurement.

Scintillation Crystals

• NaI(Tl) and CsI(Tl) crystals are frequently utilized.
• The choice of crystal is determined by factors such as density, decay time, photon yield, refractive index, and hygroscopic properties.
• The high photon yield of CsI(Tl) is offset by some of its hygroscopic properties. Thus, proper sealing is critically important for CsI(Tl).

Nal (TI) Crystal Formulations

• Nal (TI) crystal formulation entails controlled thallium additions to pure sodium iodide during crystal growth at low temperatures.
• This is performed under nitrogen to avoid hygroscopic contamination and issues with crystal uniformity.

Signal-Time Energy Relationships

• PMT signals reveal that decay and rise times are related to energy.
• Preamplifiers and shaping amplifiers process these signals for peak amplitude measurement, that is correlated to energy.

Position Sensitive PMTs

• Position-sensitive photomultiplier tubes (PMTs) are used in SPECT and PET imaging to identify the precise location of gamma ray interactions on the scintillator and thereby establish position relations on the final image.

Modern Two-Headed SPECT Systems

• Modern SPECT systems use two detectors at 180° and 90° angles to provide better image precision and quicker acquisition.

CZT Camera

• The CZT camera directly converts gamma photons into electronic signals.
• The high sensitivity of CZT cameras lets them handle enormous photon amounts per unit area.

Bone Scan Images

• Various processes (Laplacian, Sobel) are used to sharpen images of various types of bone scans to enhance contrast.

Mathematical Modeling in Image Processing

• Mathematical modeling, using convolution with the PSF (point spread function), is essential to understand the formation of reconstructed images (X).
• There is an iterative deconvolution approach for reduction in partial volume effect to enhance spatial resolution.

Gamma Camera Testing Standards

• Standardized testing protocols (used by IAEA, AAPM, and EANM) for evaluating various gamma camera characteristics (such as peaking, uniformity, resolution, etc.) are critical in guaranteeing reliability of measurements.

Source-Detector Configurations for Gamma Camera Intrinsic Uniformity

• Source-detector configuration for examining and standardizing intrinsic uniformity in gamma cameras involves the use of various source/detector arrangements.
• The configuration will need to be designed with appropriate geometry, distance, and collimating elements for accurate measurement.

Integral Uniformity (IU) Measurement

• IU is a measure used to quantify the global pixel count variations across the entire image.
• It accounts for the largest variances in pixel count values, within the field-of-view (FOV), and between the central and peripheral regions.

Linearity Mask Transmission Image

• Transmission images of linear objects (without and with corrections) are used to quantify the linearity of the system.
• This measures the impact that the corrections have on the linearity of the system.

Energy Peaking Quality Measures

• Daily checks of energy peaking are necessary for radionuclide use and should match within ±3% of the expected nominal energy.
• In using 99mTc and 57Co, these criteria are used.

Whole-Body Scanning Uniformity Evaluation

• Evaluating whole-body acquisition uniformity using a flood source (e.g., 57Co) involves a specific scanning technique consisting of a ramped up-and-down detector motion with constant travel.