Comprehensive Radiologic Technology and Human Biology Study Guide

Fundamentals of Human Anatomy and Physiology

• The Cell: The basic structural and functional unit of the human body. Cells aggregate to form tissues, which in turn form organs and systems.

• Cellular Organelles:

  • Mitochondria: Known as the "powerhouse of the cell." They generate adenosine triphosphate (ATP) through oxidative phosphorylation and aerobic respiration.

  • Ribosomes: Primarily involved in protein synthesis, which is essential for cell survival and structural integrity.

  • Nucleus: Controls metabolism, growth, and repair. It contains chromatin (DNA), which carries genetic information and controls heredity and cell division. Note: Mature red blood cells lack a nucleus to maximize space for hemoglobin.

  • Golgi Apparatus: Functions as the "post office" of the cell, responsible for packaging, modifying, and secreting proteins.

  • Lysosomes: Known as "suicide bags," they contain autolytic digestive enzymes that destroy damaged cell components.

  • Smooth Endoplasmic Reticulum: Participates in lipid and steroid synthesis and detoxification.

• Hematology and Immunity:

  • Blood pH: The normal range is $7.35–7.45$ (slightly alkaline). Deviations lead to acidosis or alkalosis.

  • Red Blood Cells (RBCs): Main function is to transport oxygen via hemoglobin. Their normal lifespan is approximately $120\,\text{days}$. In adult males, normal hemoglobin levels are $13–17\,\text{g/dL}$.

  • White Blood Cells (WBCs): Responsible for immunity and body defense. The process by which they move out of blood vessels into infection sites is called diapedesis.

  • Platelets: Primary role is in blood clotting (hemostasis) by forming a platelet plug.

• Organ Systems:

  • Integumentary System: Skin is the largest organ. It consists of the epidermis (outer layer, contains melanocytes), dermis (contains blood vessels, nerves, and glands), and hypodermis (fat storage and insulation).

  • Skeletal System: The adult skull has $22\,\text{bones}$. The clavicle is the collar bone. The stapes is the smallest bone (middle ear). The femur is the longest and strongest bone. The axial skeleton primarily protects the CNS.

  • Nervous System: The functional unit is the neuron. The cerebrum is the largest part of the brain (higher mental functions). The cerebellum coordinates balance and coordination. The hypothalamus regulates body temperature.

  • Endocrine System: The pituitary gland (located in the sella turcica) is the "master gland." Insulin is secreted by the pancreas. Thyroxine (from the thyroid) regulates basal metabolic rate (BMR).

  • Digestive System: The jejunum is the major site for nutrient absorption. The liver is the largest gland. The gallbladder stores and concentrates bile.

Diagnostic Imaging Principles: X-Ray and Radiography

• X-ray Production:

  • Anode Heel Effect: Beam intensity is greater on the cathode side than the anode side. This is more pronounced with small anode angles, short source-to-image distances (SID), and large field sizes.

  • Line-Focus Principle: By angling the anode target, the effective focal spot is made smaller than the actual focal spot, improving spatial resolution while maintaining heat capacity.

  • Filament Current: Controls the number of electrons (tube current/mA), which determines the quantity of X-ray photons.

  • Tube Voltage (kVp): Determines the kinetic energy of electrons and the quality (penetrability) of the beam.

• X-ray Interactions with Matter:

  • Photoelectric Effect: Predominates at low energies and high atomic number tissues (like bone). It is responsible for image contrast but contributes most to patient dose.

  • Compton Scattering: Predominates in the diagnostic range ($30–150\,\text{keV}$). It is the main source of image-degrading scatter and occupational exposure.

• Computed Radiography (CR) vs. Digital Radiography (DR):

  • CR: Uses photostimulable phosphor (PSP) plates (usually barium fluorohalide). The latent image is stored as trapped electrons. Readout is performed by a laser scanning the plate.

  • DR: Uses flat-panel detectors. Direct conversion (amorphous selenium) converts X-rays directly to charge. Indirect conversion (cesium iodide) uses a scintillator to convert X-rays to light, then to charge via a thin-film transistor (TFT) array.

• Image Quality Factors:

  • Grid: A device with lead strips used to absorb scatter radiation and improve contrast. Grid Ratio: height of lead strips divided by the width of the interspace.

  • Inverse Square Law: Intensity is inversely proportional to the square of the distance: $I \propto \frac{1}{d^2}$. Doubling the distance reduces exposure to one-fourth.

Computed Tomography (CT) and Magnetic Resonance Imaging (MRI)

• Computed Tomography:

  • CT Number (Hounsfield Units): Standardized based on water ($0\,\text{HU}$) and air ($-1000\,\text{HU}$).

  • Pitch: The ratio of table travel per gantry rotation to the total beam collimation. Pitch > 1.0 reduces patient dose but may decrease longitudinal resolution.

  • Iterative Reconstruction: Modern method that reduces image noise and allows for lower radiation doses compared to Filtered Back Projection (FBP).

• Magnetic Resonance Imaging:

  • Basic Principle: Based on the interaction between a strong external magnetic field and the nuclei of hydrogen atoms (protons).

  • Relaxation Times:

    • T1 (Longitudinal Recovery): Time for $63\%$ recovery. T1-weighted images show fat as bright (good for anatomy).

    • T2 (Transverse Decay): Time for $63\%$ decay. T2-weighted images show fluid/CSF/edema as bright (good for pathology).

  • Sequences: FLAIR (suppresses CSF to show lesions), STIR (suppresses fat to show marrow edema), DWI (highly sensitive for acute ischemic stroke).

Specialized Modalities and Procedures

• Ultrasound (USG):

  • Works on the Piezoelectric Effect: conversion of electrical energy into mechanical vibrations (sound) and vice versa.

  • Acoustic Impedance ($Z$): $Z = \rho \times c$ (density $\times$ velocity). Mismatch at interfaces causes reflection.

  • Frequency Range: Diagnostic range is $2–15\,\text{MHz}$. Higher frequency improves resolution but decreases penetration depth.

• Mammography:

  • Uses low $kVp$ ($25–30$) to enhance soft tissue contrast (photoelectric effect).

  • Anode material is usually Molybdenum to produce characteristic X-rays ideal for breast tissue.

• Fluoroscopy and Angiography:

  • Image Intensifier: Converts X-rays to light (input phosphor: Cesium Iodide), then to electrons (photocathode), and finally back to a bright light image (output phosphor).

  • Digital Subtraction Angiography (DSA): Subtracts a pre-contrast "mask" image from post-contrast images to visualize vessels without bone or soft tissue overlap.

  • Seldinger Technique: Technique for safe vascular access using a needle, guidewire, and catheter.

Radiation Protection and Safety

• Dose Units:

  • Gray (Gy): Absorbed dose (energy per unit mass).

  • Sievert (Sv): Effective dose (biological risk, considers radiation type and tissue sensitivity).

• ICRP Recommendations:

  • Occupational Limit: $20\,\text{mSv/year}$ averaged over 5 years.

  • Pregnant Workers: Dose to the fetus during the remainder of the pregnancy should not exceed $1\,\text{mSv}$.

• ALARA Principle: Keeping radiation exposure As Low As Reasonably Achievable through the use of Time (minimize), Distance (maximize), and Shielding (lead aprons, at least $0.25\,\text{mm\,Pb}$ equivalent).

• Biological Effects:

  • Stochastic: Probability increases with dose (e.g., cancer, genetic effects). No threshold energy.

  • Deterministic: Severity increases with dose (e.g., cataract, skin erythema). Has a threshold dose.