Nuclear IMAGING

 

Nuclear medicine:

The branch of medicine that deals with the use of radioactive substances in research, diagnosis, and treatment.

Nuclear imaging is based on the functionality and metabolism of the body and or the specific organ that is being diagnosed.

Medical nuclear imaging radioactive tracers that are injected will be traced in the desired location by the mean of a gamma camera. The radioactive material used are with varying life span.

Nuclear imaging procedures, which are noninvasive, safe and painless, are used to diagnose and manage the treatment of:

  • Cancer
  • Heart disease
  • Brain disorders, such as alzheimers disease
  • Gastrointestinal disorders
  • Lung disorders
  • Bone disorders
  • Kidney and thyroid disorders

Nuclear medicine

Why is it called nuclear Medicine ?

It is referred to a medicine that is attached to a small quantity of radioactive material to be used for testing a specific organ.

Choosing a radionuclide

What properties should a radionuclide have in order to be used inside the body and not to be dangerous?

Why gamma?

It must be a gamma emitter:

  • Can pass through body tissue
  • Not very ionising 

It must have a short half life :

  • Gives out radiation quickly
  • Decays quickly 

Alfa :

  • Lowest penetration of objects
  • The highest ionizing power
 

gamma

  • A useful gamma emitter is technetium-99m. It is a product of the decay of molybdenum-99.

The following decay chain shows how Tc-99m is produced: 

technetium-99m

Radiopharmaceutical tracers

Radiopharmaceutical = Radionuclide + Pharmaceutical 

Radioisotope is chemically bonded to other molecules that are taken up by tissue type that the medic wants to image. 

For Example:

  • For bone scan , Tc-99m is bonded to a phosphor containing chemical.

 

  • When 99mTc is chemically bound to exametazime (HMPAO), the drug is able to cross the blood–brain barrier and flow through the vessels in the brain for cerebral blood-flow imaging.

Nuclear Medical Scan

Nuclear imaging systems 

The instrument used in nuclear medicine for the detection of gamma rays is known as gamma camera.

 

Nuclear imaging systems

Gamma camera

  • The gamma camera is a device that detects the gamma rays emerging from the body of the patient.

Developed by Hal anger at Berkeley in 1957 therefore also called anger camera

It’s main parts are :

  • Gamma camera heads
    • The collimator
    • The scintillator
    • An array of photomultiplier tubes
    • A computer
  • Gantry (encased in metal and plastic and most often shaped like a box)

Light guide

The collimator

  • Collimator is made from lead
  • Maintains the quality of image
  • Spaces between holes known as septa
  • Collimator consisting of a series of holes in a lead plate can be used to select the direction of the rays falling on the crystal. 
  • It should be close to the patient for more resolution 
  • The collimator determines the resolution and sensitivity of gamma camera.

There are 4 types of collimator:

  • Parallel-hole collimator (most collimators in use)
  • Diverging
  • Converging 
  • Pin-hole collimator (Used for studies focusing on a small are e.g. in ophthalmic imaging ,thyroid) 

The collimator

Scintillator (Crystal)

  • The heart of the camera is a scintillation detector 
  • Sodium iodide with thallium Nal (TI) or CsI[Tl]
  • Size : 50cm diameter and 1cm thick 
  • The main function of crystal in convert gamma ray to photons of visible light process called scintillation
  • Thicker crystal are better for imaging radiopharmaceuticals with higher energies , but have decreased resolution
  • Thinner crystals provide resolution but not efficient with higher KV

Scintillator (Crystal)

Photomultiplier Tube (PMT)

  • Following the light guide. 
  • The photomultiplier (PMT) is an instrument that converts light to electrical signals.
  • Gamma camera contains 37-91 PMT. 
  • It detects and amplifies the electrons that are produced by the photocathode. The photocathode, when stimulated by light photons, ejects electrons. The PMT is attached to the back of the crystal.
  • Only a very small amount of light is given off from the scintillation detector. Only one electron is generated for every 7 to 10 photons incident on the photocathode. This electron is focused on a dynode that absorbs it and re-emits many more electrons (usually 6 to 10). These new electrons are focused on the next dynode and the process is repeated over and over in an array of dynodes. 
  • At the base of the PMT is an anode that attracts the final large cluster of electrons and converts them into an electrical pulse.

Photomultiplier Tube (PMT)

Data Analysis Computer 

  • The amount of charge given by PMT is very small. So a very sensitive amplifier is therefore needed to amplify this signal. This type of amplifier is generally called a pre-amplifier. After that use amplifier to amplify the signal as need 
  • Position circuitry receive the electrical impulses. This allows the position circuits to determine where each scintillation event occurred in the detector crystal. The amplitude of each electrical pulse from the amplifiers is measure in the electrical circuits of the pulse-height analyzer. Peak height analyzer and computer convert the light into a useful anatomical image 
  • Finally, a processing computer is used to deal with the incoming projection data and processes it into a readable image of the 2D spatial distribution of activity within the patient. The computer may use various methods to reconstruct an image, such as filtered back projection or iterative reconstruction.

Gantry 

  • A gamma camera system attached with gantry. 
  • All circuits and motors related to movement (longitudinal , rotational ,up & down) of gamma camera placed in gantry. 

Gantry

SPECT (Single Photon Emission Computed Tomography )

  • The same as gamma camera but creates 3-D images.
  • Typically use Tc-99m. 
  • Standard camera(single detector) that is moved in various positions around the patient.
  • Dual-head camera allows simultaneous anterior and posterior imaging and may be used for whole body-body bone or tumor imaging. 
  • Triple-head systems may be used for brain and heart studies.
  • Application:
    • Heart imaging 
    • Brain imaging
    • Tumor detection 
    • Bone scans 

SPECT

PET (Positron Emission Tomography)

  • It is a nuclear medical imaging technique which produces a 3D image of functional processes in the body.
  • The first pet camera was built in 1973

How it works?

  • A short-lived radioactive tracer isotope is injected.
  • As the radioisotope undergoes positron emission decay ,it emits a positron , an antiparticle of the electron with opposite charge.
  • After traveling up to a few millimeters the positron encounters an electron.
  • The encounter annihilates them both, producing a pair of (gamma) photons moving in opposite directions.
  • These are detected when the reach a scintillator in the scanner device, creating a burst of light which is detected by photomultiplier tubes. 
  • The technicians can then create an image of the parts of your brain, for examples which are overactive.

PET

PET Radionuclides 

  • Positron-emitting radionuclides (emit positron (e+) from Nu (p excess).
  • Relatively short half-lives and high radiation energies (compared to general NM imaging).
  • Produced by cyclotron or generators.
  • These are the only radioactive forms of natural elements that will pass safely through your body and be detected by the scanner.
  • For example, for looking at a tumor, we use glucose (FDG) and watch how it is metabolized by tumor. 

PET Radionuclides
Benefits

SPECT vs. PET, Which is Best?

  • The main positives of SPECT are that its much more available and widely used and much cheaper than PET.   
  • While PET is more expensive in terms of purchasing equipment, SPECT radio tracers also have half-lives of up to six hours, allowing a lot of imaging time, while PET tracers have a shorter half-life. SPECT radio tracers are also much cheaper and more abundant than PET tracers. 
  • SPECT has issues, including long scan times and low-resolution images prone to artifacts and attenuation. 

 

Advance technologies 

Multi-modality imaging (MMI) 

(Hybrid Imaging Modalities) 

  • Comparing the functional/molecular images of Nuclear Medicine with the more anatomical/ morphological modalities like CT or MRI has been done in the past with side-by-side comparison techniques or by the use of software based fusion, overlaying the two sets of data information. 

 

Why hybrid?

  • Nuclear medicine procedures can be time consuming. It can take several hours to days for the radiotracer to accumulate in the body part of interest and imaging may take up to several hours to perform, though in some cases, newer equipment is available that can substantially shorten the procedure time.
  • The resolution of structures of the body with nuclear medicine may not be as high as with other imaging techniques, such as CT or MRI. (functional information gained from nuclear medicine exams)

The PET/CT scanner

PET/MRI

  • Presently, the main clinical fields of PET-MRI are oncology, cardiology and neurology. 
  • The technology combines the exquisite structural and functional characterization of tissue provided by MRI with the extreme sensitivity of PET imaging of metabolism and tracking of uniquely labeled cell types or cell receptors. 

PET/MRI

SPECT/CT

  • SPECT-CT is where two different types of scans are taken and the images or pictures from each are fused or merged together. The fused scan can provide more precise information about how different parts of the body function and more clearly identify problems such as tumours (lumps) or Alzheimers disease, etc.

SPECT/MRI

  • MRI has distinct advantages over CT, such as better soft tissue contrast and lack of ionizing radiation. This has led to the recent introduction of combined PET/MRI systems that permit simultaneous acquisition of both modalities.