ישומי פיסיקה ברפואה

רפואה גרעינית

ישומי פיסיקה ברפואה רפואה גרעינית

* Introduction Atomic and Nuclear Physics Radioactivity Interaction of photons and matter Photon Detectors Radiopharmaceuticals Radiation Safety Study Types Gamma Cameras

Introduction

Nuclear Medicine combines Physics and Medicine in a very strong way.

Nuclear Medicine uses non-invasive methods to image the physiology of human body by detecting the radiation emitted by radiopharmaceuticals inside the body.

Understanding how radiation is detected is important in order to use optimally Nuclear Medicine detectors.

In the following hour we hope to describe in sufficient detail the basics of the phenomena of the emission of radiation and its detection.

Electromagnetic Waves

Characterized by wavelength.

Wavelength related to frequency and energy: 1 ev = 1.6 x 10e-19 joules

1 kev = 1000ev ; 1 Mev = 1000000 ev wavelength frequency Energy Comments

[m] [Hz] [eV]

3.0e+03 1.0e+05 6.6e-11 LF, MF

3.0e+00 1.0e+08 6.6e-08 VHF,UHF,FM

3.0e-03 1.0e+11 6.6e-05 m-wave,radar

3.0e-06 1.0e+14 6.6e-02 IR,Light, UV

3.0e-09 1.0e+17 6.6e+01 UV, X-ray

3.0e-12 1.0e+20 6.6e+04 X-ray,gamma

3.0e-15 1.0e+23 6.6e+07 gamma

Atomic Physics

The Atom can be divided into the nucleus and the electron envelope.

The electrons generate all chemistry (and biology) and are in large responsible for interaction between radiation and matter.

All radiation detectors are mainly based on the interaction between radiation and the electron envelope.

NucleusNucleus

ElectronsElectrons

Atomic Physics The electrons are arranged in “layers” (or “shells”) each

with its binding energy. (For each layer n there 2n-1 sublayers , and 2

electrons sit in each sublayer) . The innermost layer has an index of 1 and is called the

K layer. The next layer is called L layer and so on.

LL

KK

“Free” Electrons

4-5 keV

33.2 keVBinding Energy

Atomic Emissions The electrons tend to fill the atomic layers from the bottom up, i.e.

if a K shell electron is kicked out , an outer shell electron will move to take its place, releasing energy. This energy can be released in two ways:

– Characteristic X-ray photons E=BK-BL

– Auger electrons E=BK-2BL

LL

Nucleus

K-shellVacancy

AugerElectron

KKCharacteristic X-ray

Ionization Exitation

Ground state

kev

100

200

300

400

400 kev gamma

800

The Nucleus The nucleus is comprised of protons and neutrons:

– Z = number of protons

– N = number of neutrons

– A = Z + N , the total mass of the Nucleus

Z defines the number of electrons in an atom, therefore defining which element it is.

The atom is noted by AZxN.

AX is sufficient to define a nucleus.

For example Iodine 131:

13153I78

131I

The Nucleus The nucleus is kept together by the strong nuclear force, which is

active at short distances. The protons and the neutrons in the nucleus can be arranged in

energy shells, not much different from the arrangement of electrons in atoms.

The nucleus can be on:

# Excited state: Unstable, decays promptly to ground state

# Ground state: Stable

# Metastable state: Unstable decays slowly (lifetime > 10-12 sec) to ground state

Metastable nuclei are very important in nuclear medicine: 99mTc is a metastable nucleus.

Isotopes

Isotopes are nuclei with the same number of protons , but different number of neutrons.

Many elements exist in nature with different number of neutrons in the nucleus. Examples: 238U and 235U, 36Cl and 35Cl.

All isotopes of an element have the same chemical characteristics.

Isotopes have different nuclear properties, i.e.. some are ground state, some are in excited states etc..

Radioactivity Unstable isotopes will try to reach the ground state by emitting

radiation There are 4 main types of radiation:

– alpha rays: nuclei of helium, 2 protons and 2 neutrons

– beta rays: electrons

– gamma rays: electromagnetic radiation of short wavelength

– neutrons

alpha rays: change the number of protons and neutrons by 2:

AZXN

A-4Z-2YN-2

beta- rays: turn a neutron into a proton: AZXN

AZ+1YN-1

gamma rays: keep the same nuclear numbers, just the state change. neutron emission is usually associated with fission, when a heavy

nucleus break into lighter parts.

Radionuclides

Natural Exist in an unstable state in

nature ( Z > 82 )

Artificial Produced by bombarding

stable nuclides with high-energy particles.

Interaction of Radiation and Matter Gamma and Beta rays interact with the electrons in the Atom Alpha rays interact both with electrons and nuclei Neutrons interact only with the nucleus

Atoms are either Ionized or excited by radiation

LL

Nucleus

KK

Excitation

Ionization

EMRadiation

Interaction of Photons and Matter Photons interact with matter through 3 processes:

– Photoelectric effect

– Compton Scattering

– Pair production

LL

KK

Photon

Electron

Nucleus

Photoelectric

PhotonElectron

Compton

Photon Anti-Electron

Electron

Pair Production

> 1.02 Mev !

Photon

Radiation Detectors Detectors are devices that translate radiation into recognizable

signals The signals can be electric, light or even visual Ideal detector is:

– Fast

– Precise

– Linear in Energy

– Efficient All radiation detectors work by the principle that radiation deposits

energy in matter. Atoms are ionized and free negative charges (electrons) and

positive charges (cations or holes) are created. 3 types of detectors:

– Gas detectors : Geiger - Muller counter

– Solid State detectors

– Scintillation detectors

Scintillation Detectors

Scintillators produce light in presence of radiation Scintillators have to be:

– Efficient

– Generate light proportional to Energy

– Transparent (Low Absorption)

– Fast Among the Scintillation detectors, NaI(Tl) is the most popular in NM. NaI(Tl) - Thallium-activated sodium iodide crystal. The purpose of

thallium impurities to create activator centers to trap electrons “kicked out” by gamma rays.

Scintillators NaI(Tl) scintillation:

holes

radiation

electrons

Drift to impurity center and Ionize it

Free

Holes and electrons recombineproducing excited atoms

Light radiation

Gamma Ray CollimatorCollimator

NaI(Tl) CrystalLight

Electrons

Dyn

od

es

Dyn

od

es

Anode Hig

h V

oltag

e Su

pp

ly

Photocathode

SignalPre - Amp

Amp

념넮녈넮녁

Summary Emission of radiation Absorption and Detection of Radiation Scintillation Detectors Transformation of Scintillator Light into electric pulses We have all the ingredients needed to start with Nuclear Medicine

devices

Gamma Camera

Radiopharmaceuticals

Radiopharmaceuticals are radioactive agents or drugs used for diagnostic or therapeutic procedures.

Consist of two parts:

1. A radioactive substance to provide the signal.

2. A ligand that determines the molecule’s distribution in the body.

Purpose:

To follow their absorption, distribution, metabolism, and excretion through the use of detection device.

Physical Properties Gamma or x-ray emission with an energy between 60 and

400 kev

Physical half - life between 1 hour and 1 year.

Almost ideal agent:

Tc-99m: 140 kev & 6.02 hour half life.

Radiopharmaceuticals Ideal properties: Readily available. Easy to prepare. Short half - life. Pure gamma emitter. Localization in only the tissue or organ desired. No significant radiation exposure to critical organs.

Common Radionuclides Nuclide Half-Life Application 67Ga 78 hr Tumor/infection imaging 201Tl 73 hr Myocardial imaging 131I 8 days Thyroid imaging & therapy 99mTc 6 hr Nuclide for majority of

radionuclide imaging 123I 13 hr Thyroid imaging 133Xe 5.2 days Ventilation imaging 111In 68 hr Labeling white blood

cells, antibodies

Radiation Safety Radiation Exposure: Intensity of ionizing radiation, The number of ions produced when

radiation passes through a specific volume of air at a standard temperature and pressure.

Exposure is measured in roentgens (R)

Radiation Absorbed Dose: Measured the amount of energy that is

deposited per gram of substance. 1 Rad=1 erg/gram tissue 1Gy=100 Rads

Radiation Safety Radiation Dose Equivalent accounts for the quality of

radiation.

Quality Factor is a measurement of relative

biologic damage caused by a specific type and energy of radiation.

1 Rem (Roentgen equivalent man) = 1 Rad * Q.F (quality factor).

x-rays, gamma rays and beta particles are assigned a quality factor of 1.

Radiation Protection Natural Exposure ( 300mRem/yr)

150 - 250 mRem/yr (W. Body)

Radiation worker - 5000 mRem/yr 1 chest X rays - 80 - 150 mRem

C.T - 1500 - 2000 mRem

W.B Bone Scan - 750 mRem, ALARA - As Low As Reasonably

Achievable. To reduce radiation exposure: Time Distance Shielding

Study Types

Static Imaging Dynamic Imaging Whole Body Scan Gated Imaging SPECT Imaging PET Imaging TET Imaging

APEX 409

03-5643367