Advanced Biomedical Imaging

1. Lecture 8 Isotope scans, Radiation unites & Radiation hazards Advanced Biomedical Imaging Dr. Azza Helal A. Prof. of Medical PhysicsFaculty of MedicineAlexandria University

2. Isotope scans

3. Isotopes: nuclei with same no of protons but different no of neutrons. 6C12, 6C13, may be stable or unstable • Unstable nuclei (radioactive) have some unbalance in number of protons & neutrons. It has excess energy redistributed among nucleons. • To achieve a state of low energy, one of particle may gain enough energy to escape from nucleus. • Ejection of a particle changes the nucleus from one form to another. This is referred as decay or radioactivity • Isomers: nuclei with same no of protons and neutrons but different energy states. Tc99m, Tc99.

4. Modes of decay α β γ β- decay • Excess of neutrons • Electron (beta particle) ejected from the nucleus β+ decay. • Excess of protons • Positron ejected from the nucleus α decay • Excess of nucleons (heavy nuclei) • Nucleus ejects alpha particle (helium) γ decay • γ rays are emitted during any radioactive decay

5. T1/2 of different atoms. Half life time (T1/2 ) is the time required for a given nuclei to decay to ½ of its original value.

6. Radiopharmaceuticals: • Combination of Radionuclides with chemical compounds (pharmaceuticals) • Once administered to the patient, it can localize to specific organs allowing diagnosis of a disease based on the cellular function rather than the anatomy.

7. Radiopharmaceuticals (desirable properties): • Short t1/2(few hours) • Decay to stable daughter, • Emits γ rays (isomeric transition) • Emits mono-energy γ rays (50-300) (150Kev). So 99mTc is the best one. • Radiation exposure is low: • It has a short t1/2 (6hrs) • Emits γ ray allowing small amounts to be detected. • Its quick decay into the less radioactive 99Tc.

8. Technetium-99m generator (technetium cow) • it is used to extract 99mTc from decaying of 99Mo. • 99Mo is used as: • has a t1/2 of 66 hr • easily transported to hospitals where it decays to 99mTc (t1/2 6hr, inconvenient for transport). • Mo99 decays to pertechnetate. Pulling normal saline solution through the column of 99Mo elutes the soluble 99mTc, resulting in Na pertechnetate which is then injected to the patient.

9. After a wk, generator is replaced.

10. Uses ofTc99m: 99Mo → 99mTc + β− (in generator) 99mTc → 99Tc + γ (in patient) Na pertechnetate is used for imaging of tissues as thyroid, gastric mucosa & salivary glands. it can be labeled to a wide variety of compounds e.g. • HMPAO for cerebral imaging. • HIDA for billiary studies. • MAG3 for renal studies. • DTPA for lung ventilation studies • Diphosphonate for bone imaging.

11. Precautions FOR handling radionuclides: Segregation: separated area for preparation, storage, injection, patient to wait & image, temporary storage of waste. patient is considered as a source of external radiation. Follow inverse square low to decrease the exposure. The room should be with continuous floor, glass painted wall and formica topped benches. Simple mixing with the compound to be labeled at room temperature, use shielded syringes to transfer.

12. Personal protection: Use the three important factors for protection, time, distance & barriers. Time should be as short as possible (no eat, drink….) Distance from the source should be as long as possible so use long handle forceps Barriers used as: Lead shielded generators, bottle & syringes Lead glass barriers. Special containers for transport water proof surgical gloves, cover cuts and abrasion,

13. Patient protection: • Check activity, avoid contamination, deal with the spills, vomiting, incontinence. • if there is contamination: • Mop the floor with absorbent pads and seal the swabs in plastic bags & continue cleaning and monitor to check that the dose is low. • If necessary cordon off the area or cover it with impervious sheet till sufficient decay has occurred.

14. Disposal of the waste: Contaminated materials are treated as waste. Gaseous waste vent to atmosphere, aqueous waste diluted with water via designated sink solid waste are placed in designated sack for disposal by incineration. Contaminated cloths and bedding is bagged & stored in a secure protected area until sufficiently decayed.

15. Dose to the patient:increases in proportion to: • Activity and the fraction taken up by the organ. • Effective t1/2 in the organ. • not affected by scan time / number of images Decrease the dose to the patient: • Drinking a lot of water • Empty the bladder to reduce dose to gonads • Avoid contamination

16. Gamma imaging While X ray, CT and MRI isolate anatomic changes in the body, gamma imaging is capable of detecting areas of molecular biology detail even prior to anatomic change. This is by counting the radioactivity and its distribution in the organ. Isotope scans

17. Gamma camera components (scintillation counter): Gantry (collimator,detector crystal & photomultiplier tube in evacuated glass envelope) & data analysis computer. Gantry is connected to computer & both control operation of the camera, acquisition and storage of images. Collimator: lead disk with many holes separated by septa to absorb most gamma rays not parallel to collimator.. NAI crystal is connected to 100 photomultiplier photomultiplier components: photocathode, 10-12 dynodes and anode.

18. When a gamma ray interacts in the crystal, it produces light which strikes photocathode producing low energy electrons. These electrons are focused and accelerated by the dynodes until about one million reach the anode. Size of pulse which emerges from anode is α to number of electrons which are ejected from photocathode which is α to amount of light in turn α to energy which was absorbed from gamma-ray photon by the crystal. A computer is used with gamma camera to electronically process these signals

21. Gamma Camera Planar Imaging single projection view no image reconstruction is required

22. Tomography with radionuclide

23. It includes SPECT (Single Photon Emission CT) & PET (Positron Emission Tomography). Both rely on similar principles to produce images: use of radioactive tracer & detection of gamma rays. tracer is chemically combined with biologically active molecule. waiting period for active molecule to be concentrated in tissues of interest then the patient is placed in the imaging scanner. During scan a record of tissue concentration is made as tracer decays

24. Differences between SPECT & PET

26. Single head SPECT Double head SPECT Triple head SPECT

27. 30.000 (PET) F18 undergoes positron emission decay. After traveling up to a few mm positron encounters an electron producing a pair of annihilation photons moving in opposite directions and detected.

28. PET CT images show metastases to left supraclavicular lymph node a and to liver b

29. Differences between X-ray Imaging &Isotope scans

31. Radiation unites

32. Roentgen: an exposure dose It is amount of x or gamma rays which produce ionisation of 1cc of dry air producing ion pair carrying + & -ve charge each carries one esu. RAD: Radiation Absorbed Dose: It is the energy deposited/unit mass (100ergs/ gm) by any type of ionizing radiation in any medium due to excitation & ionization Gy = 100 RAD.

33. Biological effect of a given absorbed dose of any type of radiation will not be of same magnitude as effect of same absorbed dose of anther radiation. Also it differs in different tissues For the same dose to the organ, alpha or neutron radiation will cause greater harm compared with gamma rays, x rays, or electrons because the ionization events produced by alpha or neutron radiation will be much more closely spaced.

34. The absorbed dose is multiplied by a radiation weighting factor wR (equivalent dose)& tissue weighting factor wT (difference in tissue sensitivities) (Effectivedose) Rem (Roentgen equivalent man): Effective dose Amount of any type of radiation which produces same biological effect as one RAD of x or gamma ray. Sievert (Sv)= 100Rem Accounts for different tissues sensitivities. It measures radiation & organ damage.

35. Equivalent dose= absorbed dose X wR Effective dose = absorbed dose X wR X wT Dr Azza Helal

36. Unites of radioactivity Curie Radioactivity of a sample which decays at a rate of 3.7X1010 dis/sec Becquerel Radioactivity of a sample which decays at a rate of one dis/sec Ci=37GBq

37. Radiation hazards

42. Irradiation of the foetus results from: • Placental transfer. • External irradiation from radioactivity in the mother's organs and tissues. • Since radionuclides in maternal tissues contribute to foetal dose so foetal dose can be reduced by: • Maternal hydration and frequent voiding • Using smaller administered activities and longer imaging times

43. 1 mSv

46. Nursing mothers must be counseled about the need to interrupt / discontinue breast feeding • Pumped milk may be refrigerated and used after the radioactivity has decayed

47. Effect of radiation on the body • Early effects • Whole body • Partial body • gonadal • Late effects • Acute radiation syndrome • Teratogenic effects

48. Radiation protection

49. Radiation protection • To be sure that the dose received by person is small as possible so that the consequent damage never constitute a significant hazards to the heath of irradiated person. Maximum permissible dose (MPD) • The dose which is not expected to cause any detectable body injury for the person during his life time and till his 3rd generation.

50. Radiation exposures divided into 3 categories: Occupational exposure. (2Rem/yr, 20 mSv). Medical exposure, (special group), persons work in controlled area (1.5Rem/yr) Public exposure 1/10 occupational exposure. Pregnant worker 1msv Dose to embryo 5 mSv = 0.5 rem in 9mth