Nuclear Medicine & Radiopharmaceuticals Quiz

Nuclear Medicine Imaging

• Radiopharmaceuticals are substances used in nuclear medicine to image organ or tissue function.
• Nuclear medicine imaging uses radiopharmaceuticals to map organ-specific functions.
• CT/X-ray is used for structural or anatomical imaging - showing anatomy.
• PET/SPECT (Nuclear Medicine) is used for functional or physiological imaging - showing physiological processes.
• Structural imaging shows tissue/organ types and positions (e.g., bones, heart, muscles).
• Functional imaging detects biological changes (e.g., cancerous or benign tumors).

Nuclear Medicine Imaging Process

• Planar imaging uses a gamma camera to capture 2D images.
  • Static planar captures single images.
  • Dynamic planar records images over time to show changes (e.g., blood flow).
• SPECT uses rotating gamma cameras to create 3D images.
• PET uses fixed rings of detectors to capture detailed 3D images of functional processes.

Radiopharmaceuticals

• Purpose: To create images in nuclear medicine.
• Administration: Injected into the patient's vein.
• Components: A compound with specific biological properties combined with a radioactive isotope (e.g., 18F, 99mTc).
• Function: To study specific body functions like metabolism and cell differentiation.

Radiopharmaceutical Preparation

• Preparation: Biologically useful compound with a specific purpose.
• Targeting: Aims at a single target organ.
• Labeling: Should be simple and quick.
• Reaction time: Quick reactions are needed.
• Reconstitution: Stable for use after reconstitution.

Radiopharmaceutical Application in Patients

• No toxic/allergic reactions.
• Good shelf-life.
• Stable in-vivo - minimizes free isotopes in the body.
• Pathology (disease) seen as increased activity - detectable by increased activity in areas of concern.
• Multiple patients per vial are possible.

Commonly Used Radiopharmaceuticals (Tracers)

• 99mTc: Most commonly used tracer in nuclear medicine, with a gamma energy of 140 keV.

Planar Imaging

• Captures images of activity in the body projected onto a 2D plane.
• Uses a gamma camera with a collimator to detect gamma events.
• Nal(TI) scintillation detectors capture signals.
• Photomultiplier tubes convert signals to images.

Static Planar Imaging

• Conventional method for tracking tracer distribution.
• Views: Anterior, posterior, lateral, and oblique projections.
• Diagnostic and therapeutic information: Blockages, increased/decreased tracer uptake, response to treatment.
• Matrix size: 64x64, 128x128, or 256x256
• Pixel noise: Low pixel density results in a noisy image. Pixel size is related to FOV and matrix size, similar to CT.

Contrast Limit and Matrix Size

• Contrast Limit: Measures visible contrast differences relative to the background.
• Higher contrast limits = noisier image = harder to detect low-contrast structures.
• Proportional to pixel noise.

Static Planar Imaging Challenges

• Overlapping activity reduces image contrast.
• Circulating blood activity adds background noise.

SPECT (Single Photon Emission Computed Tomography)

• SPECT Tracers: Special radiopharmaceuticals for detailed organ function information.
• SPECT Imaging System: Uses rotating gamma cameras for 3D images.
• SPECT Image Quality: Image clarity and accuracy depends on the tracer, equipment, and patient condition.
• Production of SPECT Tracers: 99mTc is a common isotope, made from 99Mo in generators.

99mTc Radiopharmaceuticals

• MIBI (Tc-99m-Sestamibi): Used in myocardial perfusion imaging to diagnose myocardial ischemia or infarction.
• MDP (Tc-99m-methylene diphosphonate): Used for bone scans, diagnosing bone disorders.
• PERT (Tc-99m-pertechnetate): Used for thyroid imaging.
• ECD (Tc-99m-ethylene cysteine diethylester): Used in neuroimaging (blood flow/neural activity assessment).

SPECT Imaging (Machine Performance and Image Quality)

• Resolution: SPECT has lower resolution than planar imaging.
• Contrast: SPECT has better contrast than planar.
• Resolution Non-Uniformity: Resolution varies in depth.
• Emission vs Scattered Events: Images consist of useful and unwanted events.
• Attenuation: Patient's body affects image quality.

SPECT Advantages

• Improved contrast
• High sensitivity
• Multi-planar reconstruction
• Dual-energy studies

SPECT Disadvantages

• Non-uniform sensitivity
• Slow
• Low photon density
• Variable resolution
• Accurate attenuation correction needed

PET (Positron Emission Tomography)

• PET Technique: Imaging metabolic or functional processes in the body.
• PET Tracers: Radioactive substances for metabolic activity observation.
• PET Imaging Principles: Detects radiation from positron-emitting tracers.
• PET Imaging System: Detectors capture tracer signals.
• PET Image Quality: High-resolution images, useful for detecting diseases like cancer.

PET Advantages

• Superior resolution.
• Quantification of activity.
• Accurate attenuation correction.

PET Applications

• Clinical and research settings.
• Assessing metabolic activity and disease.

PET Tracer Annihilation Process

• Positrons annihilate with electrons, forming positronium.

Common PET Tracer: 18F-FDG (Fluorodeoxyglucose)

• Analog of glucose.
• Similar structure to glucose but not metabolized.
• Traps in cells, marker of glucose metabolism.
• Widely used.

PET Imaging Principles (Event Location)

• Distinguishing Opposed Gamma Photons: Identify 180-degree gamma photons.
• Angle of Travel: Determine the path of detected photons.
• Reconstruction: Create an image showing positron-emitting activity distribution.

PET Detection System

• Detectors consist of scintillator crystal arrays.
• Interaction of photons with crystals creates visible light.
• Photomultiplier tubes convert light to electronic signals.

PET Image Reconstruction

• Sinogram: Organizes coincidence events.
• Reconstruction Methods: Filtered back projection or Ordered subset estimation maximization (OSEM).

Image Data Corrections in PET

• Attenuation: Accounts for energy loss of 511-keV photons in the body
• Scattering: Accounts for scattering of 511-keV photons in the body.
• Normalization: Corrects for detector response variations.
• Electronic dead time: Accounts for detector processing times.

Spatial Resolution in PET

• Factors affecting Resolution: Intrinsic resolution, photon scattering, positron travel distance, and emitted photon direction.

Sensitivity of PET Imaging

• FDG-PET: Most sensitive diagnostic technique.
• 18F-FDG Imaging helps to diagnose and manage tumors (site, malignancy, scar tissue, therapy effectiveness, and spread of cancer).

Combined Imaging Systems

• Combines anatomical and functional imaging (e.g., PET/CT) to map body processes onto anatomical structures.
• CT scanner in front, PET system in back.

Machine Performance and Image Quality

• Resolution/Contrast: SPECT, has worse resolution compared to planar but has better contrast.
• Non-uniformity/Emission vs Scattered Events/Attenuation: Image quality is affected by body attenuation (e.g., dense tissue reduces image clarity).
• Other issues: Cupping effect, slow processing.

Clinical Applications of PET

• Oncology: Detecting cancer, assessing tumor size, grade, necrotic tissue, and distinguishing recurrent tumors.
• Cardiology: Evaluating heart muscle viability after a heart attack and determining coronary artery disease, determining myocardiac viability.
• Neurology: Used in diagnosing and differentiating dementia types; assessing cerebral blood volume and blood flow.
• Psychiatry: Detecting functional changes in brain biochemistry linked to behavioral disorders.
• Epilepsy: Identifying brain regions related to seizures.
• Cerebrovascular disease: Assessing conditions like transient ischemic attacks.