Biological Effects of Ionising Radiation
Radiation Dose Several quantities are used to describe radiation dose: exposure, absorbed dose & effective dose Absorbed Dose
The SI unit for ~ is the gray (Gy) 1Gy = 1J/kg
The older unit of absorbed dose is the rad (radiation absorbed dose) 1Gy = 100 rad
Both units measure energy absorbed in the material (tissue) Equivalent Dose (H); Sv
The equivalent dose HT is given by
Where:
- WR is the radiation weighting factor, which reflects the relative biological effectiveness of the radiation
- DT.R is the dose averaged over the organ of interest, T, due to the radiation R
The unit is the joule per kg with the special name the Sievert (Sv) Effective Dose (E); Sv
Not all tissues are equally sensitive to the effects of radiation. Tissue weighting factors (WT) have been assigned to reflect this
The effective dose is given by:
The unit is the Sv Problem:
Determine the effective dose to a patient who has been exposed to a 100 kVp x-ray beam, and has received doses of 3mGy, 4 mGy and 5 mGy respectively to the gonads, the bladder and the colon
Ans: 1.4 mSv (old weighting factors) 1.0 mSv (recent weighting factors)
Gonads Bladder Colon Total
Absorbed Dose 3 mGy 4 mGy 5 mGy N/A
Equivalent Dose 3 x 1 = 3 mSv 4 x 1 = 4 mSv 5 x 1 = 5 mSv N/A Effective Dose 3 x 0.08 = 0.24
mSv
4 x 0.05 = 0.2 mSv 5 x 0.12 = 0.6 mSv 1.04 mSv
The risk to the patient is determined by Total Effective Dos Biological Effects of Ionising Radiation
These can be classified as:
- Somatic - Genetic
5. INTRODUCTION TO DIAGNOSTIC RADIOGRAPHY
a. What is Diagnostic RadiographyDiagnostic Radiography produces images of the structure and function of the body to assist medical diagnosis and medical decision making. Patients who have injuries or illness are often referred for diagnostic imaging.
b. Role of the Diagnostic Radiographer6yj
Receive imaging referrals for patients who present with a wide range of symptoms from life threatening trauma to simple procedures to complex 3D & 4D sectional imaging needs.
• They are responsible for performing the most appropriate medical imaging procedure for the presenting condition.
• Use digital imaging software to improve the image quality and they evaluate the resultant images ensuring that the images are clinically appropriate.
• Ensure the comfort and safety of patients while they are in their care.
• To work with health professionals such as the Radiologist, Nurses, Speech Pathologists, Doctors and Surgeons etc. to obtain the best possible images and diagnosis for the patient.
c. History and Development of Diagnostic Radiography
• As early as 1785 a Welsh mathematician William Morgan conducted experiments which were seen as the first steps.
• The first major discovery and presentation of x-rays occurred in 1895 by Wilhelm Conrad Roentgen, a Professor at Wuerzburg University in Germany.
• In 1901 he received the Nobel Prize in physics for his discovery.
• One of Roentgen's first experiments late in 1895 was a film of the hand of his wife, Bertha.
• Only 6 months after Roentgen announced his discovery, X-rays were being used by battlefield physicians to locate bullets in wounded soldiers.
• X-rays are created by bombarding a tungsten target with electrons inside a device known as the x- ray tube.
• A very high voltage potential is applied to the cathode which heats it.
• Electrons are fired out of the cathode and strike the anode which is rotating.
• X-ray energy is released from the anode.
d. Plain Film Imaging
•Moveable x-ray tube
• Table
• Erect Bucky
• Mobile – wards, theatre
Advantages Disadvantages
• Inexpensive
• Commonly available
• Relatively “easy’ to interpret
• CR and DR provide flexibility in image distribution
• Limited Contrast Resolution – soft tissue visualisation
• Uses radiation
e. Fluoroscopy
• Allows for continuous imaging and function assessment.
• Shows a continuous X-ray image on a monitor, much like an X-ray movie.
• The movement of a body part or of an instrument or contrast agent through the body can be seen.
• The most common fluoroscopy examinations are Barium swallows, meals and enemas - to view the gastrointestinal tract.
Advantages Disadvantages
• Commonly available
• Shows anatomy
• Shows function
• Patient preparation not pleasant
• Barium tastes like chalk
• Radiation dose can be high f. Mammography
What is mammography?
A mammogram is an x-ray picture of the breast.
Screening mammograms are used to check for breast cancer in women who have no signs or symptoms of the disease.
Diagnostic mammograms are used to check for breast cancer after a lump or other sign or symptom of the disease has been found.
The history
In the late 1950s Robert Egan at the University of Texas M.D. Anderson Cancer Center combined a technique of low kVp with high mA and single emulsion films to devise a method of screening mammography for the first time.
He published these results in 1959 in a paper, and subsequently in a book in 1964 called Mammography.
The "Egan technique", as it became known, enabled physicians to detect calcification in breast tissue
The role of mammography
Mammography has a specific role in imaging, as it targets soft tissue exclusively.
It is used in conjunction with ultrasound, and interventional techniques, to image and treat the breast.
