RAD 114 RADIATION PROTECTION RADIATION PROTECTION IN NUCLEAR MEDICINE

Review… Define isotope; isobar; isotone.

Review Question Answers: • Isotope: Same atomic number; different atomic mass (same Z#; different A#) • Isobar: Same atomic mass; different atomic number (Same A#; different Z#) • Isotone: Same number of neutrons (different A and Z#’s)

1

Radioactive Decay Process… • Alpha Emission: Ejection of a subatomic particle that results in a decrease in both A and Z numbers (4 and 2, respectively) in the daughter atom. • Beta Emission: Ejection of a negatively charged particle that results in an increase in the number of nuclear protons in the daughter atom (the atomic mass remains unchanged).

Radioactive Decay Processes… • Positron Emission: Ejection of a positively charged subatomic particle that results in a decrease in the number of nuclear protons in the daughter atom (the atomic mass remains unchanged). • Beta Capture: An electron or beta particle is absorbed by an atomic nucleus resulting in a decrease in the number of protons in the daughter atom.

RADIOACTIVE HALF-LIFE A decay constant that is unique for each radioactive element. It is the amount of time that it takes for a sample to lose half of its radioactive atoms. The values range from fractions of a second to centuries.

2

RADIOACTIVE HALF LIFE

SELECTED RADIOACTIVE ELEMENTS ELEMENT

SYMBOL

T1/2

Polonium-214

214Po

1.64 x 10-4 sec

Decay Mode , 

Radon-222

222Rn

3.8235 days

, 

Radium-226

226Ra

1.6 x 103 years

, 

Carbon-14

14C

5.73 x

Uranium-238

238U

4.47 x 109 years

, 

Thorium-232

232Th

1.41 x 1010 years



103 years



RADIOACTIVE MATERIALS USED IN NUCLEAR MEDICINE The radioactive materials used in nuclear medicine are called RADIOPHARMACEUTICALS. Not just any radioactive substance can be used on patients.

3

RADIOPHARMACEUTICALS MUST POSSESS: • Short half life • Low toxicity • Ability to go to desired tissues (radioactive material is “tagged” with compounds that can be taken up readily by tissues • Be easily eliminated from the body (short biological half-life) • Gamma emitter

RADIOPHARMACEUTICALS Depending on the examination, the radiopharmaceutical must be able to provide information that is related to the physiologic activity of the tissues being imaged.

RADIOPHARMACEUTICALS The images produced are not distinct as in radiography. Instead, the key to interpretation is how dark or light certain areas of the body appear.

4

RADIOPHARMACEUTICALS For example, thallium-201 (T1/2=3.08 days) is used to determine the degree of blood supply to the heart muscle. Areas of decreased density may indicate poor circulation.

RADIOPHARMACEUTICALS Gallium-67 (T1/2=3.25 days) can be used to detect cancerous tumors. Because metabolic activity is increased in areas metastatic involvement, “hot spots” may appear on the scan. As the urinary system eliminates the gallium, the bladder will also look dark (“hot”).

5

SUMMARY OF MOST COMMONLY USED RADIOPHARMACEUTICALS • Technetium 99-m (T1/2=6.02 hrs): The most commonly used radionuclide (“m” stands for “metastable” • Thallium-201: Used in cardiac imaging • Xenon-133: Perfusion imaging (pulmonary) • Positron Emitters: (F-18; C-11; N-13; O-15) used in PET scanning

MOST OFTEN USED RADIONUCLIDES IN NUCLEAR MEDICINE ELEMENT

SYMBOL

DECAY MODE

T1/2

USE

Technetium-99m

99-mTc



6.02 hrs

Multiple diagnostic tests

Thallium-201

201TI

EC, 

73.1 hrs

Myocardial scanning

Xenon-133

133Xe

 -, 

5.3 d

Pulmonary function

Fluorene-18

18F

Iodine-123

123I

EC,   100 min EC, 

13.2 hrs

PET scanning of brain Thyroid scanning

6

CAREER UPDATE CT technologists must become familiar with nuclear medicine concepts in order to take advantage of PET/CT. Although CT training can still be achieved through on-the-job training, nuclear medicine requires an additional year of formal schooling. Additionally, the ARRT CT certification examination now requires knowledge of nuclear medicine specific to PET, and includes a 16hour “structured educational requirement.”

RADIATION PROTECTION Generally speaking, the exposure from radiopharmaceuticals is very low, even for the patient. However, failure to observe simple protection practices can cause unacceptable dose accumulation.

RADIATION PROTECTION Because radiation in nuclear medicine cannot be shut off, time, distance and shielding are very important -especially time.

7

RULE #1 Limit the amount of time that you handle radioactive materials. Patients who have received injections of radiopharmaceuticals are entitled to quality patient care. Use what time is necessary to provide effective patient care. When patient needs are satisfied, step away from the patient.

OTHER RULES • No eating or drinking in the ”HOT LAB” • No storage of food or beverage in refrigerators where radioactive materials are kept • Radiation monitoring is required as in diagnostic radiography

OTHER RULES • Lead aprons are of little practical use in nuclear medicine, so do not bother using them • Use of rubber gloves prevent transmission of infection and radioactive contamination of hands when preparing and handling radiopharmaceuticals

8

Bushberg, et al, The Essential Physics of Medical Radiography, 1994

RADIATION MONITORING and PROTECTION • • • • • • •

Ring Dosimeters (TLDs) Whole Body Dosimeters (TLD or Luxel) Area Monitoring (Geiger-Muller Counters) Lead Barriers (in so-called HOT LAB) Leaded Syringes and Containers Dense Plastic (for Beta) Hooded ventilation

9

POSTED WARNINGS Observe radiation signs indicating “CAUTION: RADIATION AREA.” Exposure rates in those areas are potentially greater than 5 mR, but less than 100 mR per hour. Signs indicating “CAUTION: HIGH RADIATION AREA” warn of potential exposure rates exceeding 100 mR per hour.

10

PET/CT SCANNER

GAMMA CAMERA

11

SPECT (SINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY)

12