Nuclear Medicine Study Guide (Notes)

Routes of Administration

  • Description: Radiopharmaceuticals are administered by the route that best ensures the tracer reaches the target organ and produces diagnostically useful localization. The chosen route aims to optimize tracer delivery to the target system.
  • IV (intravenous) – most common route:
    • Provides rapid systemic distribution to organs such as bone, kidneys, heart, and liver (common targets in nuclear medicine).
    • Key Points: rapid distribution; broad applicability across systems.
  • Oral – for GI tract imaging or thyroid imaging:
    • Used for gastric emptying studies and radioiodine thyroid uptake imaging.
    • Key Points: GI tract imaging; thyroid uptake imaging.
  • Inhalation – for ventilation studies:
    • Xe-133 gas used for ventilation imaging; Tc-99m DTPA aerosol used for ventilation studies.
    • Key Points: ventilation assessment; gas or aerosol delivery.
  • Intrathecal – injection into cerebrospinal fluid:
    • Used for cisternography or shunt patency studies.
    • Key Points: direct CSF delivery; spinal canal/CSF-focused studies.
  • Subcutaneous or intradermal – lymphatic-focused imaging:
    • Used for lymphoscintigraphy and sentinel node mapping.
    • Key Points: lymphatic drainage visualization; sentinel node procedures.
  • Direct instillation – into cavities:
    • Examples include bladder instillation for cystography or joints for arthrography.
    • Key Points: cavity-focused localization (bladder, joints).
  • Practical takeaway:
    • The route is chosen to optimize tracer delivery to the target organ/system for diagnostically useful localization.

Mechanisms of Localization

  • Definition: Localization mechanisms describe physiologic or biochemical processes that determine where a radiopharmaceutical accumulates.
  • Examples of mechanisms:
    • Capillary blockade (physical trapping): Tc-99m MAA localizes in the lungs by blocking small capillaries.
    • Phagocytosis (cellular processing): Tc-99m sulfur colloid is phagocytosed by Kupffer cells in the liver and by macrophages in the spleen.
    • Active transport (transporter-mediated uptake): Iodide is taken into the thyroid via active transport; pertechnetate uptake in salivary glands.
    • Cell sequestration: Heat-damaged red blood cells localize in the spleen due to sequestration.
    • Diffusion: Xe-133 distributes by diffusion within the lungs.
    • Chemisorption: Tc-99m MDP binds chemisorptively to hydroxyapatite in bone.
    • Compartmental localization: DTPA distributes in CSF or urinary tract (compartments).
    • Receptor binding: In-111 octreotide binds to somatostatin receptors.
  • Key Points:
    • Capillary blockade: Tc-99m MAA in lung perfusion.
    • Phagocytosis: Tc-99m sulfur colloid in liver/spleen.
    • Active transport: Iodide in thyroid; pertechnetate in salivary glands.
    • Cell sequestration: Heat-damaged RBCs in spleen.
    • Diffusion: Xe-133 in lungs.
    • Chemisorption: Tc-99m MDP binding to bone mineral.
    • Compartmental localization: DTPA in CSF or urinary tract.
    • Receptor binding: In-111 octreotide binding to somatostatin receptors.
  • Short Answer:
    • Radiopharmaceuticals localize by mechanisms such as phagocytosis, active transport, sequestration, diffusion, chemisorption, compartmental distribution, and receptor binding.

Advantages and Disadvantages of SPECT vs PET

  • SPECT (Single Photon Emission Computed Tomography):

    • Availability: More widely available and cost-effective.
    • Isotopes: Uses longer-lived isotopes such as Tc-99m and I-123, which are easily produced with generators.
    • Resolution: Typically 8−12 mm8-12\ \mathrm{mm}
    • Sensitivity/Quantification: Lower sensitivity and limited quantitative capabilities compared to PET.

  • PET (Positron Emission Tomography):

    • Sensitivity/Resolution: Higher sensitivity and resolution, typically 4−6 mm4-6\ \mathrm{mm}, enabling quantitative analysis such as standardized uptake values (SUVs).
    • Isotopes: Uses short-lived isotopes like F-18 and C-11, often requiring a cyclotron.
    • Cost/Accessibility: More expensive and less widely available.

  • Key Points:

    • SPECT: Wider access, cheaper, longer-lived isotopes, resolution 8−12 mm8-12\ \mathrm{mm}.
    • PET: Higher sensitivity/resolution, quantitative SUVs, short-lived isotopes, more expensive, less accessible.

  • Short Answer:

    • SPECT is cheaper and widely available; PET offers higher resolution and quantification but is more expensive and less accessible.

  • Advantages and Disadvantages of SPECT vs Planar Imaging

    • Planar imaging: Two-dimensional projections; fast; good for surveys (e.g., whole-body bone scans, thyroid imaging).
    • Limitations: Overlap of structures reduces contrast and can obscure pathology.
    • SPECT: Three-dimensional reconstructions; better lesion localization and contrast; longer acquisition; requires patient stillness.

  • Key Points:

    • Planar: Quick, useful for surveys; limited by overlap/contrast.
    • SPECT: 3D imaging with improved localization/contrast; longer acquisition; requires stillness.

  • Short Answer:

    • Planar is fast and good for surveys; SPECT provides 3D detail and better localization but is slower and requires more cooperation.

Identify/Differentiate Each Type of Planar Imaging Technique

  • Planar imaging variations by purpose:

    • Static imaging: A single snapshot (e.g., thyroid scan or bone spot view).
    • Dynamic imaging: Sequential frames over time to visualize tracer movement (e.g., renal renogram; HIDA bile transit).
    • Whole-body imaging: anterior and posterior surveys of the entire body (e.g., bone scan, MIBG scan).
    • Gated imaging: ECG-synchronized data collection at specific cardiac cycle points (e.g., MUGA scans).

  • Key Points:

    • Static: Single snapshot (thyroid, bone spot views).
    • Dynamic: Sequential flow imaging (renogram, HIDA).
    • Whole body: Survey scans (bone scan, MIBG scan).
    • Gated: ECG-synchronized (MUGA, gated SPECT).

  • Short Answer:

    • Planar imaging can be static, dynamic, whole-body, or gated depending on the study purpose.

  • Things That Affect the Deposition of Radiopharmaceuticals

    • Influencing factors:
    • Physiological conditions: Blood flow and perfusion determine tracer delivery.
    • Organ function and clearance: Determines retention and excretion.
    • Uptake determinants: Receptor density and transporter activity.
    • Disease states: Tumors, ischemia, obstruction, infection alter deposition patterns.
    • Patient preparation: Fasting, hydration, medications influence distribution.
    • Competing substrates: For example, glucose competes with FDG in uptake.
    • Technical factors: Dose, route, and timing after injection affect image quality.

  • Key Points:

    • Blood flow and organ perfusion.
    • Organ function and clearance mechanisms.
    • Receptor density or transporter activity.
    • Pathology (tumors, ischemia, infection, obstruction).
    • Patient preparation (fasting, hydration, meds).
    • Competing substances (glucose vs FDG).
    • Technical factors (dose, route, timing).

  • Short Answer:

    • Tracer deposition is influenced by physiology, pathology, tracer chemistry, patient preparation, and technical choices.

Clinical Indications, Contraindications, and Warnings

  • Indications: Reasons to perform the study (diagnosis, staging, monitoring).
  • Contraindications: Situations in which the study should not be performed due to risk outweighing benefit.
  • Warnings/Precautions: Conditions requiring caution or adjustments but not absolute prohibition.
  • Key Points:
    • Indications: Diagnosis, staging, monitoring.
    • Contraindications: When not to perform.
    • Warnings/Precautions: Risks requiring caution.
  • Short Answer:
    • Indications = when to perform; contraindications = when not to; warnings = proceed with caution.

Absolute vs Relative Contraindications

  • Absolute contraindication:

    • A study must never be performed because risks always outweigh benefits (e.g., therapeutic I-131 in pregnancy).

  • Relative contraindication:

    • A study may be considered if benefits outweigh risks, with precautions (e.g., renogram in renal impairment, provided precautions are taken).

  • Key Points:

    • Absolute contraindication: never perform (e.g., I-131 therapy in pregnancy).
    • Relative contraindication: may perform if benefits outweigh risks (e.g., renal impairment before renogram).

  • Short Answer:

    • Absolute = never perform; relative = may perform if benefits outweigh risks.

  • Pharmaceutical vs Radiopharmaceuticals vs Radiochemicals

  • Definitions:

    • Pharmaceutical: A drug that lacks radioactivity.
    • Diagnostic radiopharmaceutical (tracer): A radioactive compound given in very small doses to monitor physiology or metabolism via imaging.
    • Therapeutic radiopharmaceutical: Delivers cytotoxic radiation in higher doses to treat disease (e.g., I-131 for thyroid ablation; Lu-177 for neuroendocrine tumors).
    • Radiochemical: The radionuclide in its chemical form (e.g., Tc-99m pertechnetate) before attachment to a targeting molecule.

  • Key Points:

    • Pharmaceutical: Drug without radioactivity.
    • Diagnostic radiopharmaceutical (tracer): Small radioactive dose for imaging.
    • Therapeutic radiopharmaceutical: Higher dose for treatment.
    • Radiochemical: Chemical form of the radionuclide itself.

  • Short Answer:

    • Pharmaceutical = drug; tracer = diagnostic agent; therapeutic radiopharmaceutical = treatment; radiochemical = radionuclide chemical form.

Standard Views in Planar Imaging

  • Planar imaging views are chosen to reduce overlap and clarify anatomy.

    • Bone scans: Whole-body anterior and posterior views; spot views (lateral skull, lateral spine, oblique ribs).
    • Renal imaging: Posterior views for native kidneys; anterior views for transplanted kidneys.
    • HIDA scans: Anterior projections; RAO (right anterior oblique) view to separate gallbladder from bowel.
    • Thyroid scans: Anterior and oblique views to show both lobes.
    • MUGA studies: LAO (left anterior oblique) projection to separate left ventricle from right ventricle.
    • Lung scans: Typically eight views (AP, PA, both laterals, and four obliques) to evaluate ventilation or perfusion.

  • Key Points:

    • Bone: Whole body AP/PA; lateral skull/spine; oblique ribs.
    • Renal: Posterior (native), anterior (transplant).
    • HIDA: Anterior + RAO to separate gallbladder from bowel.
    • Thyroid: Anterior + oblique views.
    • MUGA: LAO to separate LV from RV.
    • Lungs: Eight views (AP, PA, both laterals, obliques).

  • Short Answer:

    • Appropriate views are selected to highlight anatomy: posterior for kidneys, LAO for MUGA, RAO for HIDA, anterior/obliques for thyroid, and eight views for lungs.

  • Image Quality Factors

  • Technical factors affecting image quality:

    • Matrix size:
    • Larger matrix (e.g., 512×512512\times512) yields smaller pixels (higher resolution) but requires longer acquisition and greater storage.
    • Smaller matrix (e.g., 64×6464\times64 or 128×128128\times128) collects faster but has larger pixels (lower resolution).
    • Pixel number: More pixels yield sharper images; fewer pixels appear blocky.
    • Photon counts: Higher counts produce smoother, more diagnostic images; low counts yield grainy images.
    • Patient motion: Causes blurring, artifacts, or false pathology.
    • Pixel depth (bit depth): Determines shades of gray; higher depth improves contrast resolution.
    • SPECT angles: Too few angles lead to streak artifacts and poor reconstructions (typical range: 60−12060-120 projections).
    • Noise sources: Low photon counts, scatter, random events, detector electronics, or patient attenuation.

  • Key Points:

    • Matrix size: Large = detail, slower; small = faster, less detail.
    • Pixel number: More = sharp, fewer = blocky.
    • Photon counts: Higher = smooth, lower = noisy.
    • Motion: Causes blur, artifacts, or false pathology.
    • Pixel depth: High = better contrast, low = poorer contrast.
    • SPECT angles: Too few = streak artifacts, poor resolution.
    • Noise: From low counts, scatter, randoms, electronics, attenuation.

  • Short Answer:

    • Image quality depends on matrix size, pixel count/depth, photon counts, motion, and SPECT angles; noise and artifacts reduce accuracy.

  • Labeling Images

  • Importance: Correct labeling ensures accurate interpretation and legal compliance.

  • Required elements on every image:

    • Patient identifiers (name, date of birth, or ID).
    • Date and time of the exam.
    • Study type and body part imaged.
    • Projection/view and side markers.
    • Radiopharmaceutical information (type, dose, route, timing/post-injection time).
    • Institution/technologist identifiers as required.

  • Purpose: Prevent errors, support correct diagnosis, and satisfy medical record standards.

  • Key Points:

    • Patient identifiers.
    • Date/time of exam.
    • Study type and body part.
    • Projection/view and side markers.
    • Radiopharmaceutical (type, dose, route, timing).
    • Institution/technologist identifiers.

  • Short Answer:

    • Images must be labeled with patient info, date/time, study type, projection/view, and radiopharmaceutical details.

  • Multi-Head vs Single-Head Gamma Cameras

  • Detector configurations:

    • Single-head cameras: Adequate for planar imaging; slower and less sensitive; not ideal for SPECT.
    • Multi-head cameras (dual/triple head): Faster acquisition; data collected from multiple detectors simultaneously; improved sensitivity, resolution, and image quality; reduces patient motion artifacts.

  • Clinical impact:

    • Multi-head cameras enable shorter scan times and better image quality, especially important for SPECT.

  • Short Answer:

    • Single-head cameras work for planar studies; multi-head systems are faster, more sensitive, and provide better resolution for SPECT.

  • Collimator Types

  • Collimators determine which photons reach the detector, balancing resolution and sensitivity.

  • Common types:

    • Parallel-hole collimators: General-purpose; energy-specific options include LEHR (low-energy high-resolution), ME (medium-energy), HE (high-energy).
    • Pinhole collimators: Magnify small organs (e.g., thyroid) but reduce sensitivity.
    • Converging collimators: Magnify a small area by narrowing the field of view.
    • Diverging collimators: Expand coverage for larger patients/fields.
    • High-energy collimators: Required for isotopes with high-energy photons (e.g., I-131) to prevent septal penetration and scatter.

  • Key Points:

    • Parallel-hole: General purpose; energy-appropriate (LEHR, ME, HE).
    • Pinhole: Small-organ detail (thyroid) with higher resolution but lower sensitivity.
    • Converging/diverging: Adjust field of view.
    • High-energy: Necessary for high-energy isotopes like I-131.

  • Short Answer:

    • Collimators balance resolution and sensitivity; pinhole improves thyroid detail; high-energy collimators suit I-131 imaging; parallel-hole is standard for Tc-99m imaging.

  • CT vs Gamma Camera Exams

  • CT (Computed Tomography):

    • Uses X-rays to produce high-resolution anatomical images; excellent for structural detail in bones, organs, and vessels.

  • Gamma camera imaging:

    • Detects gamma photons from radiopharmaceuticals; provides physiologic/functional information with lower spatial resolution.

  • Hybrid imaging:

    • SPECT/CT or PET/CT combines anatomical and functional data in one study, improving localization and diagnostic accuracy.

  • Key Points:

    • CT: Anatomy; high-resolution structural information.
    • Gamma camera: Physiology/metabolism; functional information with lower resolution.
    • Hybrid imaging: Integrates anatomy and function for better localization.

  • Short Answer:

    • CT provides anatomy; gamma cameras provide physiology; hybrid imaging combines both.

  • Grayscale vs Contrast

  • Image display concepts:

    • Grayscale: The full range of shades from black to white; determined by pixel depth.
    • Contrast: The difference between adjacent shades; affects lesion visibility.

  • Balance and interpretation:

    • Adequate grayscale ensures smooth intensity transitions.
    • Appropriate contrast highlights clinically significant differences.

  • Key Points:

    • Grayscale: Range of shades; dimensionality set by pixel depth.
    • Contrast: Difference between adjacent shades; emphasizes pathology.

  • Short Answer:

    • Grayscale is the full range of shades; contrast is the difference that makes structures/pathology visible.