NucMed: Finals

Gas Detectors

Scintillation Detectors (Scintillators)

Semiconductor Detectors

Types Of Radiation Detectors:

Gas Detectors

Contains a gas-filled chamber

air

inert gases

Gas types of gas detectors

argon

xenon

Inert gases:

Gas Detectors

A gas detector is a device used to detect radiation by using a chamber filled with gas.

ionization

When radiation enters the gas chamber, the radiation interacts with the gas atoms. This interaction causes _________

Free electrons

Positive ions

ionization produces:

negative charge

Free electrons charge

positive charge

Positive ions charge

positive electrode

Electrons move toward the ________ electrode

negative electrode

Positive ions move toward the _______ electrode

pulse

The movement of these charges creates a small electric signal called a ________.

Current Mode

Pulse Mode

Modes of Operation of Gas Detecors:

Current Mode

Measures the total or average current

Used when many ionizations occur continuously

Current Mode

Common used in Measuring radiation exposure or dose

Pulse Mode

Measures individual radiation events

Each detected event produces a separate pulse

Pulse Mode

Common use: Counting radioactive emissions

voltage

The behavior of the detector depends on the amount of _______ applied

No Voltage Region

No electric field is present.

The ions and electrons do not move.

They recombine immediately

Recombination Region

Low voltage is applied.

Some ions reach the electrodes, but many recombine before being collected

Recombination Region

Only part of the signal is measured.

Detector is inaccurate.

Ionization Chamber Region

Voltage is high enough to collect all ions and electrons

No recombination occurs

Ionization Chamber Region

The signal is directly proportional to the amount of radiation energy deposited.

Current mode

Ionization Chamber Region Operates in:

Measuring radiation dose

Monitoring x-ray exposure

Measuring patient exposure

Ionization Chamber Region Uses:

Dose calibrators

Ionization survey meters

Ionization Chamber Region Examples:

Accurate dose measurement

Stable and reliable

Ionization Chamber Region Advantages:

Proportional Counter Region

Higher voltage is applied.

Electrons gain enough energy to produce secondary ionization.

cascade effect

(secondary ionization) One ionization produces many more ionizations.

Proportional Counter Region

Larger pulse is produced.

Pulse size is proportional to radiation energy.

Pulse mode

Proportional Counter Region Operates in:

Proportional Counter Region

it is used radiation counting and particle detection

nuclear medicine

Proportional Counter Region is rarely used in ___________.

Geiger-Müller (GM) Region

Very high voltage is applied.

Causes a complete avalanche of ionization.

same large pulse

Every detected event produces the __________, regardless of radiation energy

Geiger-Müller (GM) Region

Can detect radiation presence

Cannot measure exact energy or dose accurately

Geiger-Müller (GM) Region

used in Detecting contamination and Surveying work areas for radiation

Geiger counter

Geiger-Müller (GM) Region Example:

Cannot accurately measure radiation dose

Cannot determine radiation energy

Geiger-Müller (GM) Region Limitation:

Simple design

Reliable

Good for:

Dose measurement

Radiation monitoring

Contamination detection

Advantages of Gas Detectors:

low density

Low sensitivity

Not suitable for imaging

Not ideal for clinical counting in nuclear medicine

Limitations of Gas Detectors:

Scintillation Detectors

work because certain materials can produce light flashes when struck by radiation.

scintillation

This light flash emitted in scintillation detectors is called a

crystal

The performance of a scintillation detector depends on the properties of the ________ used.

more

Dense materials and high atomic number materials absorb _____ radiation.

Density and Effective Atomic Number (Z)

Higher detection efficiency

Better ability to stop gamma rays

Light Output

Amount of light produced after radiation interaction

Better energy resolution

Better spatial resolution

Light Output affects:

better

More light = ______ image quality.

Response Time

How fast the crystal produces and stops emitting light

Response Time

Affects temporal resolution

Faster response allows quicker imaging and counting

Sodium Iodide with Thallium NaI(Tl)

This is the most commonly used scintillation crystal in nuclear medicine.

light

Why thallium is added: Improves _______ production

Light Detection Systems

The light produced by the crystal must be converted into an electrical signal.

Photomultiplier Tube (PMT)

converts light into a strong electrical signal.

1,000,000

About ________ electrons can be produced from one event

Very high gain

Low noise

Cost-effective

Very sensitive

Advantages of PMT:

Avalanche Photodiode (APD)

a solid-state detector that also converts light into electrical signals.

Avalanche Photodiode (APD)

Light enters the APD

Produces electrons

Electrons undergo an avalanche multiplication process

hundrends

Gain of APD:

Higher efficiency (~80%)

Efficiency of APD:

Avalanche Photodiode (APD)

Can work in magnetic fields

This is important for systems combined with MRI.

Silicon Photomultiplier (SiPM)

made of many tiny APD elements.

Each small element acts like a mini Geiger counter.

Silicon Photomultiplier (SiPM)

Radiation produces light

Different mini-elements are triggered

The total number triggered forms the signal

efficiency (~50%)

efficiency of SiPM:

Scintillation detectors

are widely used in nuclear medicine imaging.

Gamma Cameras

SPECT

PET

Applications of Scintillation Detectors

Gamma Cameras

Detect gamma rays from radiopharmaceuticals

Produce planar images

single photon emission computed tomography (SPECT)

SPECT

single photon emission computed tomography (SPECT)

Produces 3D images using gamma rays

Positron Emission Tomography (PET)

PET

Positron Emission Tomography (PET)

Detects annihilation photons

Produces highly detailed functional images

PET/MR

SPECT/MR

Hybrid imaging:

Semiconductor Detectors

detect radiation using a semiconductor material instead of gas or crystals.

Semiconductor Detectors

When radiation enters the detector, it interacts directly with the semiconductor material.

Electron holes pairs

Semiconductor Detectors produces