Comprehensive Notes on Medical Imaging Technologies

The Effect of Filters on Projections

• Filtering, back projection, and creating tomographic images are key steps.

Tomographic Image

• This is the image of a body slice after reconstruction.
• Slice Thickness: Determined by the fan beam's thickness (0.5-10 mm).
• Image Size: CT scan images are 512x512 pixels.
• Pixel Value: Each pixel shows how much a tissue absorbs compared to water.
• Modern CT: Can reconstruct images in any plane after initial reconstruction.

Pixel Value and Hounsfield Units

• Pixel value $x
  _{i,j}$ shows tissue absorption relative to water, measured in Hounsfield units (HU).
• HU Formula:
  $HU = <br /> \frac{\mu<br /> _{i,j} - \mu<br /> _{water}}{\mu<br /> _{water}} <br /> \times 1000$
• This scales the contrast with water.

Field of View (FOV)

• FOV is the visible area's diameter, based on the fan beam angle.
• Typical FOV:
  • 35 cm for chest, abdomen, pelvis.
  • 25 cm for head, neck.
• Pixel Dimensions (approximate):
  • 0.7 mm in body scans (35 cm / 512).
  • 0.5 mm in head/neck scans (25 cm / 512).
• Useful Field of View (UFOV)

Slip Ring Technology

• Allows continuous X-ray tube rotation in CT scanners since the 1990s.

Helical/Spiral/Volume CT

• Uses a 3rd generation CT scanner.
• Continuous X-ray tube rotation with table movement.
• The scanner moves in a helix around the patient.
• No scanning downtime.
• Requires slip-ring technology.
• Faster data acquisition from large areas.

Multi-Slice/Multi-Row Spiral CT

• Gets data for multiple slices per X-ray rotation using multi-detector arrays.

Evolution of Multi-Detector CT Scanners

• Lists the progression from 2-slice to 256-320 slice CT scanners with dates and manufacturers.

Contrast Media in CT Imaging

• Enhances tissue contrast, usually given intravenously or orally.
• Contains iodine to increase opacity.
• Used in many CT scans.

CT Examination with Contrast Medium

• Unenhanced images are taken first, then enhanced images after contrast.

Technological Evolution of CT Imaging

• Highlights significant advancements from the 1970s to the 2020s.

Single Photon Emission Computed Tomography (SPECT)

• SPECT uses a rotating gamma camera to take multiple images of radioactivity distribution.
• Creates cross-sectional images from these projections.
• Requires specific acquisition and reconstruction software.

SPECT Image Acquisition Parameters

• Involves setting the arc of rotation, number of images, image matrix, and acquisition time.

Typical SPECT Acquisition Values

• Outlines standard values for arc of projections, number of projections, acquisition time, and matrix size.
  • 180° for myocardial SPECT.
  • 360° for other SPECT exams.
• Number of projections:
  • 32 in 180° for myocardial SPECT.
  • 64 or 128 in 360° for other SPECT exams
• Acquisition time per projection:- 20-40 seconds.
• Matrix size:- 64x64 or 128x128.

Multiple-Head Gamma Cameras

• Saves time and improves image quality.

Reconstruction of Tomographic Images in SPECT

• Uses filtered back-projection or iterative methods.
• Can reconstruct images in different planes after transverse reconstruction.

Myocardium Perfusion SPECT Examination

• Includes vertical long axis slices, horizontal long axis slices, and short axis slices.

SPECT/CT Systems

• Combines a multi-slice CT system with a SPECT gamma camera.
• Provides attenuation/scatter correction using CT images.
• Offers both biochemical and anatomical information.

Basic Principles of Positron Emission Tomography (PET)

• Uses positron-emitting radioisotopes to map tissues.

Imaging Procedure in PET

1. Label organic molecules with a positron-emitting radioisotope.
2. Administer the labeled substance intravenously ($10^{13}-10^{15}$ molecules).
3. Tissues take up the substance, emit positrons, and annihilate, emitting gammas.
4. The PET scanner detects the gamma radiation.
5. Generate tomographic images showing positron/photon event concentration.

Positron-Electron Annihilation

• Describes momentum and energy conservation during annihilation.
• Before: System at rest (momentum ~ 0).
• After: Two identical photons move in opposite directions (total momentum = 0).
• Energy conservation: Before: 2 electrons with zero kinetic energy and rest energy of 511 keV each ($511 \text{ keV} = m
  e \times c^2$).
• After: 2 photons with energy 511 keV each.

Emission and Annihilation of e+ in PET Imaging

• Positrons lose energy, interact with electrons, and annihilate, producing detectable photons.

The Principle and Configuration of PET Scanner

• PET scanners use rings of detectors to detect annihilation events along lines of response (LOR).

How PET Images Are Produced

• Images are made from 300-600k LORs, showing radiopharmaceutical distribution.
• Reconstruction methods used: FBP or algebraic methods.

Radionuclides Used in PET Imaging

• Require a cyclotron for production due to short half-lives.
• Fluorodeoxyglucose (18F) (FDG) is commonly used because cancer cells take it up more due to high energy demands.

Cyclotron

• Cyclotrons are very high cost.

The Reconstructed PET Image

The Transition of PET to PET/CT Imaging

• PET/CT combines PET with multi-slice CT.
• PET-only scanners are becoming obsolete.

Benefits of PET/CT over PET

• Provides high-quality CT images and corrects PET images for better accuracy.
• Combines metabolic and anatomical information.

Radiobiology Basics

• Discusses radiation risk estimation after exposure.

Contents/Aims

• Explores how radiation affects living matter, why radiations are dangerous, and how risk is assessed.

Ionizing and Non-Ionizing Radiation

• Non-ionizing radiation cannot cause ionizations (e.g., radio waves).
• Ionizing radiation causes ionizations (e.g., X-rays, gamma rays).

Absorbed Dose from Ionizing Radiation

• Absorbed dose is energy deposited per mass (1 Gray (Gy) = 1 J/kg).
  • 1 mGy = 0.001 Gy = 1 mJ/kg
• 1 J = $6.2 \times 10^{18}$ eV and 1 mJ = $6.2 \times 10^{15}$ eV
• Effective dose is used for non-uniform radiation (1 Sievert (Sv). 1 mSv = 0.001 Sv)

The Electromagnetic (E/M) Spectrum

• Shows ionizing and non-ionizing radiation.

X-rays and Gamma Rays

• X-rays are E/M radiation produced by atom de-excitations and deceleration of charged particles (bremsstrahlung).
• Gamma rays are E/M radiation from nucleus de-excitations.
  • E/M radiation (photons)
  • Produced by Nucleus de-excitations
  • High ability to penetrate matter
  • Used in Nuclear Medicine

Why Are the Effects of Ionizing Radiation on Biological Tissue of Interest?

• Ionizing radiation is a harmful environmental factor used in diagnosis and treatment.

Ability of IR to Harm Living Matter

• A whole-body dose of 5 Gy can be lethal, while lower doses can cause cancer.

Sunlight and Ionizing Radiation

• Compares the energy from sunlight to ionizing radiation to illustrate the potential harm.

Vulnerability of Bio-Matter to Ionizing Radiation

• DNA is a sensitive target because it contains information for cell functioning.

Biological Effect of Ionizing Radiation

• Radiation-induced DNA damage can lead to cell malfunction, cancer, or genetic alterations.

Radiation Interaction with DNA Molecules

• Direct Action: Radiation directly damages DNA.
• Indirect Action: Radiation ionizes water, creating reactive species that damage DNA.

Physical Stage: Ionization of Water Molecules (duration $10^{-12}$ sec)

• Ionizing radiation interacts with water molecules leading to ionization.

Physicochemical Stage: Formation of Reactive Oxygen Species (duration $10^{-10}$ sec)

• Water ionization produces reactive oxygen species (ROS) that damage cells.

Biochemical Stage: Diffusion and Interaction with DNA (duration $10^{-6}$ sec)

• ROS diffuse and damage DNA, causing strand breaks and lesions.

Biological Stage (duration: few minutes to many years !!!)

• Cell repair mechanisms fix DNA damage, but errors can lead to cell death or adverse effects.

Macroscopic Biological Effects of Ionizing Radiation

• Effects are categorized into deterministic and stochastic effects.

Deterministic Effects

• Require a substantial number of cells to die.
• Immediate results.
• Dose threshold exists.
• Severity is proportional to dose.
• Examples: Erythema, hair loss, cataract.

Deterministic Effect Threshold Values

• Lists threshold doses for various deterministic effects.

Lethal Dose

• Lethal Dose 50% ($LD
  _{50}$) is the dose that causes a loss of biological activity in 50% of irradiated cells or organisms. Humans: 5 Gy

Stochastic Effects

• Can occur even with a single cell affected.
• Long-term effects.
• No dose threshold.
• Severity is independent of dose.
• Probability is proportional to dose.
• Examples: Carcinogenesis, genetic mutations.

Survival Fraction Curves

• Illustrate the relationship between radiation dose and cell survival.
• $SF = \frac{\text{Number of cells/organisms preserving a biological function}}{\text{Total number of cells exposed to radiation}}$

Radiosensitivity of Human Tissues

• Lists tissues by radiosensitivity level (high, moderate, low).

Factors Influencing the Biological Outcome of Exposure to Ionizing Radiation

• Biological, physical, and chemical factors affect radiation outcome.

Cell Cycle Phase and Radiosensitivity

• Cells are more sensitive in M and G2 phases, more resistant in the S phase.

Types of Radiation and Biological Effect

• Different radiation types have varying biological effects depending on ionization density.

Dose Fractionation and Biological Effectiveness

• Fractionating radiation reduces biological outcome due to repair mechanisms.

Oxygen Tension and Biological Effectiveness

• Oxygen presence increases radiosensitivity.

Natural Sources of Exposure to Radiation

• Cosmic, earth, and internal sources contribute to natural radiation exposure (2.7 mSv/yr).

Man-Made Sources of Exposure to Radiation

• Medical procedures, consumer items, and nuclear tests contribute to man-made exposure (1.9 mSv/yr).

Average Annual Radiation Dose per Person in Greece

• Total is approximately 4.6 mSv/yr.

Radiation Exposure From Medical Applications

• Contributes 30-40% to total annual exposure.

CT Examinations Contribute to 80% of Radiation Exposure

Radiation Protection Organizations

• ICRP develops radiation safety policies.
• Established in 1928
• Develops and disseminates policies and recommendations to ensure the safe use of radiation, prevent or minimize radiation-related risks, and manage exposure in medical, occupational, and public contexts.
• NCRP & NRPB are also mentioned.

Radiation Protection Principles

• Justification, optimization (ALARA), and dose limits.

Rationale of Establishing Dose Limits

• Ensures deterministic effects do not occur and reduces stochastic risks.

Recommended Dose Limits Over Time

• Lists the evolution of dose limits from 1900 to 2007.

Recommended Dose Limits (ICRP 2007)

• Provides specific dose limits for occupational and public exposure.
  

• Risk estimations are extrapolated from high doses due to lack of reliable low-dose data.

Nominal Cancer Incidence

• Difficult to quantify excess cancer incidence at low doses.

Radiation-Related Risks for Stochastic Effects (ICRP 2007)

• Lists cancer risk factors for adults and the whole population.
  

• Based on radiobiological data and individual characteristics.

Estimation of Stochastic Radiation Risk

• Cancer risk = Effective Dose x Risk factor.
• Hereditary effects risk = Dose to gonads x Risk factor.

Cancer Risks from Common Radiological Exams

• Lists cancer risks for common exams like thorax radiographs and CT scans.

Cancer Risks from Common Nuclear Medicine Exams

• Lists cancer risks for common nuclear medicine exams.

Annual Dose to Occupationally Exposed Workers

• Radiologists, operators, nursing staff of X-ray and CT unit

Cancer Risk Estimate for Occupationally Exposed Workers in UHoH

• Calculates cancer risk based on annual dose and risk factors ($40 \times 10^{-6}$).

Physics of Radiation Therapy

• Discusses cancer as a leading cause of death and the role of radiation therapy.

Anticancer Treatments

• Lists surgery, chemotherapy, immunotherapy, and radiotherapy.

Radiation Therapy

• Uses radiation to manage diseases.

Effective Management of Disease

• Balances tumor exposure with complications in normal tissues.

Radical vs. Palliative Radiation Therapy

• Radical therapy aims to eliminate the tumor.
• Palliative therapy aims to relieve symptoms.

Types of Radiation Therapy

• Brachytherapy: Placement of radioactive sources inside or near the tumor site.
• Gamma ray emitter with energy suitable for the intended tumor site.
• External radiotherapy: Radiation beams generated outside the patient’s body target the tumor site.

Properties of Radioactive Sources

• Includes suitable gamma ray energy, high specific activity, small size, and appropriate half-life.

Brachytherapy Techniques

• Surface molds, interstitial treatment, and intracavitary treatment.