32.1 Diagnostics and Medical Imaging

Medical imaging techniques revolutionize disease diagnosis and treatment. Radiopharmaceuticals target specific areas, emitting radiation detected by specialized cameras. This allows doctors to visualize organ function and detect abnormalities, aiding in early detection and treatment planning.

SPECT and PET imaging provide detailed 3D views of bodily processes. These techniques, along with CT, MRI, ultrasound, and X-rays, offer a comprehensive toolkit for medical professionals. Each method has unique strengths, helping doctors choose the best approach for each patient's needs.

Medical Imaging Techniques

Radiopharmaceuticals in medical imaging

• Radioactive substances administered to patients target specific organs, tissues, or physiological processes
• Emit gamma radiation detected externally by gamma camera or scanner creates images showing distribution and concentration in the body
• Different radiopharmaceuticals used for various diagnostic purposes
  • Technetium-99m ($^{99m}$Tc) commonly used for bone scans, cardiac imaging, and brain imaging
  • Iodine-131 ($^{131}$I) used for thyroid imaging and treatment
  • Fluorine-18 ($^{18}$F) used in positron emission tomography (PET) scans
• Allow for functional and molecular imaging provide information about physiological processes and biochemical pathways help diagnose and monitor diseases (cancer, heart disease, neurological disorders)
• Contrast agents may be used to enhance visibility of specific structures or tissues

SPECT vs PET imaging techniques

• Nuclear medicine imaging techniques use radiopharmaceuticals
• SPECT imaging
  • Uses radiopharmaceuticals that emit single gamma photons
  • Gamma camera rotates around patient to acquire multiple 2D images from different angles reconstructed to create 3D representation of radiopharmaceutical distribution
  • Provides functional information about organs and tissues
  • Lower spatial resolution compared to PET
• PET imaging
  • Uses radiopharmaceuticals that emit positrons (anti-electrons) which annihilate with electrons producing two gamma photons traveling in opposite directions
  • Gamma photons detected by ring of detectors surrounding patient
  • Coincidence detection of gamma photons allows precise localization of radiopharmaceutical
  • Provides functional and molecular information with higher spatial resolution than SPECT
  • Commonly used with fluorine-18 fluorodeoxyglucose ($^{18}$F-FDG) to measure glucose metabolism in tissues
• Both techniques valuable for diagnosing and monitoring various diseases
  • SPECT more widely available and less expensive than PET
  • PET offers higher sensitivity and spatial resolution useful for early disease detection and treatment monitoring

Additional Imaging Techniques

• Computed tomography (CT): Uses X-rays to create detailed cross-sectional images of the body
• Magnetic resonance imaging (MRI): Utilizes strong magnetic fields and radio waves to produce high-resolution images of soft tissues
• Ultrasound: Employs high-frequency sound waves to create real-time images of internal organs and structures
• X-ray: Produces two-dimensional images of dense structures like bones and can detect certain lung conditions

Medical Testing Techniques

Radioimmunoassay for medical testing

• Highly sensitive and specific technique for measuring substances in biological samples based on antigen-antibody binding
• RIA procedure
  1. Known quantity of radioactively labeled antigen (tracer) added to sample
  2. Unlabeled antigen from sample competes with labeled antigen for binding sites on limited amount of specific antibody
  3. As concentration of unlabeled antigen increases, less labeled antigen binds to antibody
  4. Amount of bound labeled antigen measured using gamma counter
  5. Standard curve generated using known concentrations of antigen to determine concentration in sample
• Used to measure wide range of substances
  • Hormones (insulin, thyroid hormones, steroid hormones)
  • Drugs and their metabolites
  • Tumor markers (prostate-specific antigen, alpha-fetoprotein)
  • Infectious agents (hepatitis B surface antigen)
• Advantages
  • High sensitivity allows detection of substances in very low concentrations
  • High specificity minimizes cross-reactivity with other substances
  • Ability to measure substances in various biological fluids (blood, urine, saliva)
• Widely used in clinical diagnostics and research, although other techniques (enzyme-linked immunosorbent assay ELISA, chemiluminescent immunoassay CLIA) have become more common in recent years
• Can be complemented by other diagnostic procedures such as biopsy for definitive diagnosis
• Endoscopy may be used for visual examination and sample collection in certain cases