Radiology: Biological Effects & Radiation Interaction

• Radiology serves to assess the risk of radiological and interventional procedures to patients and staff.
• Radiologists educate patients and staff on these risks, especially concerning pregnant patients.

Biological Effects of Radiation

• Stochastic effects' probability increases with radiation dose, lacking a threshold.
• The risk increases with dose.
• Low-dose radiation poses a main health risk, with cell modification as the predominant effect.
• Examples include radiation-induced cancer and hereditary effects.
• Non-stochastic/deterministic effects exhibit increasing severity with dose.
• There is a threshold dose below which no effect is observed.
• Cell killing is the predominant effect, relevant at higher exposures but rare in diagnostic radiology.
• Examples include cataracts, skin erythema (redness), and fibrosis.

Interaction of Radiation with Tissue

• X-rays/γ-rays interacting with tissue produce energetic electrons.
• Secondary electrons transfer kinetic energy through excitation, ionization, and thermal heating.
• This process leads to many low-energy electrons.
• Electrons then interact with critical targets, causing damage directly.
• Electrons produce free radicals (H+ and OH-) that interact with critical targets, causing indirect interaction.
• Biological damage mainly stems from free radicals.

Radiation-Induced DNA Damages

• Radiation induces various base damages, including base damage, deletion, substitution, and hydrogen bond disruption.
• Base deletion ranges from single/few nucleotides to entire genes or surrounding genes.
• Base substitution involves swapping nucleotides during DNA replication.

Strand Breaks

• Strand breaks are more relevant at high doses.
• Single and double strand breaks can occur, as well as complex damage.
• Damage difficult to repair may lead to cell death, which can be important in radiotherapy.
• 1Gy, low linear energy transfer (LET) usually produces >1000 base damages, 100 SSB, and 40 DSB.

DNA Repair Mechanisms

• DNA damages can lead to mutation.
• Mutation rate is low because of DNA repair mechanisms.
• Examples of mechanisms include direct repair of damaged nucleotide, nucleotide excision repair, SSB and DSB repair.
• Apoptosis occurs if there are catastrophic mutations, leading to cell death and elimination of damaged genetic material.

Factors Affecting Biological Damage

• Amount of energy deposited per unit length determines biological consequences.

Linear Energy Transfer

• High LET radiation includes alpha particles, heavy ions, and low-energy neutrons.
• Low LET radiation includes x-rays, γ-rays, and electrons.
• High LET radiation produces more DSB and complex damages and is more difficult to repair.

Cell Cycle and Biological Damage

• Cell cycle phases: M, G1, S, G2
• Radiological sensitivity of cell varies depending on the cell cycle phase.
• Mitosis (M) and RNA synthesis (G2) are most sensitive phases, while pre-DNA synthesis (G1) and DNA synthesis (S) are less sensitive.
• The length of the average cycle in human cells is 24 hours.

Cell Type Radiosensitivity

• Cells vary in sensitivity to radiation.
• Sensitivity depends on the degree of differentiation (mitotic rate).
• Law of Bergonié & Tribondeau states rapidly dividing cells are highly susceptible to radiation.
• Radiosensitivity of cells is directly proportional to mitotic rate and inversely proportional to degree of differentiation
• Radiosensitive cells have a high mitotic rate and are undifferentiated (i.e., non-specialized).

Organ-Specific Responses to Radiation Exposure

Skin

• High-dose and image-guided interventional procedures as well as CT scans (if not done correctly) and radiotherapy can cause effects on the skin.
• Common effects include erythema and acute radiation dermatosis (skin defect).
• temporary hair loss can occur approximately 3 weeks after 3-6 Gy dose, but a cutaneous radiation syndrome (radiation-induced skin injuries can be severe following high doses.

Early Transient Erythema and Main Erythema

• Early transient erythema occurs with acute doses of 2Gy+ within hours with effect typically fading soon.
• Main erythema reappears 2 weeks after an initial high dose or repeated low-dose exposures.

Late Erythema Grades

• Late erythema occurs between 8 and 52 weeks after exposure.
• Late Ertethema grades are based on a scale out of 4, where 1 is least severe.

Organ Specific Male and Female Reaction

• Reproductive organs are very sensitive to radiation.
• Male Effects include reduced fertility, and temporary and permanent sterility.
  • Thresholds for temp.: ~150mGy
  • Thresholds for perm.: (6Gy)
  • Length of temporary sterility depends on dose and recovery time (<3 years after doses of 2Gy)
  • Acute dose of 150mGy may cause sperm count and motility to be reduced occurring after 6 weeks.
• Ova within ovarian follicles are sensitive to radiation.
• Radiosensitivity - Most Radiosensitive: intermediate follicles, Large (mature) follicles, Least Radiosensitive: small follicles
• Sterility is age dependent; Before puberty or 40 years old + have a threshold of ~10Gy to ~2Gy respectively.

Eyes and Cataracts

• The lens of eyes have radiosensitive cells, and no removal system for damaged cells → accumulate to cause vision-impairing cataracts.
• Usually occurs posteriorly (unlike senile cataracts).
• Dose threshold for Acute is 2Gy, and for Chronic it is 5Gy.
• Occupational limit: 20mSv (previously 150mSv/y).

Radiation Safety Measures

Collimation, Voltage and Filtration

• Collimation helps limit area exposed to primary x-ray beam while decreasing volume of tissue irradiated.
• This decreases scatter and thus improves image quality, accomplished by using tight collimation.
• Using higher kVp reduces skin dose, and less absorption in patient will results to a reduced need for mAs to achieve same exposure.
• Beam filtration removes lower energy (longer wavelength) x-rays and can decrease dose.
• In abdominal exams, increasing filtration from 2.5mm Al to 3.5mm Al can reduces of up to 30%.

Source to Skin Distance

• Increase source to skin distance (SSD) can decrease entrance dose.
• For mobile radiographic and fluoroscopic equipment, SSD should be at least 30cm
• For fixed radiographic and fluoroscopic equipment, it should be more than 45cm
• Increasing SSD decreases entrance dose and reduces tube leakage.

Fluoroscopy "Ten Commandments"

• Maintain proper distance to optimise image quality as patient size increases.
• Use minimal tube current (mA) and maximise tube kilovoltage (kVp) to achieve good image quality.
• Position the image intensifier close to the patient to optimise image quality and reduce dose.
• Use the lowest clinically acceptable magnification mode and avoid grids for smaller patients.

Radiation Effects in Utero

Pre-Implantation Stage

• Days 0-9 are "Pre-implantation stage" where the embryo is extremely radiosensitive, and radiation damage → prenatal death (all or nothing).
• Because the embryo has few cells, implantation can cease and lead to embryo death.
• At this stage with high cell repair, and if only a few cells are damaged → repaired/replaced → normal embryo development or congenital abnormalities are very low.

Organogenesis

• Weeks 2-8 are when major internal organs are formed that can be effected to high radiation doses → malformation of organs
• Increased incidence of microcephaly (small head size), CNS abnormalities and growth retardation observed in Atomic Bomb survivors
• Threshold is at ~150mGy+, which equivalent to more than 5 pelvic CT scans or 100 conventional x-rays.

Foetal Growth

• Weeks 8+ are in the foetal growth stage, No significant risk of radiation induced prenatal death or malformation
• Probability of mental retardation and permanent growth retardation of at dose level (threshold: ~1000mGy).
• Doses above ~150mGy → significant increase in risk
• Investigations do not result in foetal dose greater than 50mGy → risk of abnormalities small
• Therapeutic abortion is not advisable based on radiation risk alone.

Recommendations

• Foetal dose of 150mGy are more likely (6%) chance of mental retardation, while (<3%) increase risk of cancer and (<15%) small head size and lower IQ reduction.

Fluoroscopy Use

• Used to capture of dynamic sequency (up to 30 FPS) of x-ray images in real-time with high temporal resolution (i.e., 10 min procedure → 18k images)
• Clinically used for swallowing, meals and enemas to diagnostic and treatment of heart disease to digital subtraction angiography (DSA) to diagnose blockages in blood vessels and interventional radiology to insert/unblock stents.

Image Intensifier (II)

• Used to produce each image with 1/1000th of dose used in diagnostic radiography → very low mA.
• Composed of vacuum housing to keep air out, an input layer to convert x-rays into light, and an electron optics system to accelerate and focus electrons.
• The output phosphor converts accelerated electrons into a visible.
• The input phosphor then converts incident x-rays → visible light and is made of Cesium iodine (TI).

Photocathode with Types of Gain

• Converts light → electrons and with thin layer of antimony and alkali metals emits electrons via photo electric effect.
• Electrons from photocathode accelerate to anode by large electric field (~30,000V).
• Types of Gain includes Electrons accelerate from photocathode towards anode with acceleration causing kinetic energy of electrons.
• Minification increases gain and concentrates electrons released from input phosphor focused on small output phosphor (BG) while total brightness gain. Total brightness (BG) = electronic gain x minification gain (usually ~2500-7000).

Artefacts and Alternative Devices

• Pincushion artefacts is when output screen of the X-Ray appears flat.
• S-distortion artefacts is when the Spatial wrapping of image in S-shape throughout image where stray magnetic fields (e.g., MRI) influences.
• Vignetting artefacts appear as the Center of output phosphor appears brighter than periphery due to reduced number of electrons impacting that location.
• Flat panel fluoroscopy uses TFT panels in replacement of the II, lens assembly and camera system to directly records real-time fluoroscopy image.
• Uses either direct (Selenium semiconductor) incidents or Indirect (Photodiode with Phoshor) incidents.

Automatic Exposure Rate Control:

• Instead of manually manipulateing of exposure factors (kV and mA) to alter brightness brightness depending on density is replaced by rate is electronically and automatically.
• The automatic sensor measures x-ray intensity at the detector to feedback changes so that required to maintain adequate x-ray intensity.

Fluoroscopy Modes and Operation

• Continuous mode can used continuously used analogue systems with consistent production of an continuous x-ray beam (typically 0.5-6mA) or produce burst of xray when needed.
• Pulsed mode is usually for extremity use only, and has decreased Exposure time to achieve reduced blurring.
• Also, Dose rate falls approximately proportional to pulse rate, and varied pulse rate

Fluoro Magnification and Frame Measurements

• Magnification can allow to focus on smaller area on the input to be displayed on FOV by altering voltage within the machine
• Last frame can be acquired (instead of blank monitor) allowing for review.
• Can be further enhanced by review acquired before the sequence was stopped to average frames.

Fluoroscopy Dose Rates and Quality Measures

• Fluoroscopy provides excellent temporal resolution but is often noisy (attributed to low mA.
• It can however be reduced temporal resolution to lower quantum noise while averaging images to further achieve such benefits.
• Kerma air product (KAP)/dose area product (DAP measure units mGy.cm^2 is a useful metric of accumulated dose useful with skin doses.
• Cumulative dose (C dose) or reference point air kerma is measured in (Unit mGy) which is Representative of patient's skin and can approximate through the operation parameters.

Contrast Resolution

• Measured by counting the number of details and number of bar patterns resolved in image.
• Spatial of smallest object that system can detect through test phantoms with measures (lines/mm) affect resolution for: -System limitations (detector, FOV) -Geometry (focal spot) -Scattered rays