1. Nuclear Medicine Introduction
What is nuclear medicine?
The use of radioactive tracers (radiopharmaceuticals) to obtain diagnostic information [and for targeted radiotherapy].
Radiation is emitted from inside the human body cf transmitted radiation in x-ray imaging.
Tracers :-
Trace the paths of various biochemical molecules in our body.
Hence can obtain functional information about the bodies workings (i.e. physiology).

2. Radiopharmaceuticals
Traces physiology / localises in organs of interest
Radioactive
nuclide
Emits radiation for detection or therapy +
Biochemical
Bonding
Q. What is a radiopharmaceutical?

3. The Pharmaceutical
The ideal tracer/pharmaceutical should follow only the specific pathways of interest, e.g. there is uptake of the tracer only in the organ of interest and nowhere else in the body. In reality this is never actually achieved.
Typically want no physiological response from the patient
The mechanism of localisation can be as simple as the physical trapping of particles or as sophisticated as an antigen-antibody reaction
Why would you only want it to follow only one pathway?
- Reduce radiation dose to other organs.
- Uptake in other organs may obscure any abnormality.

4. Radionuclides in Nuclear Medicine
The ideal radionuclide for in-vivo diagnosis :
Optimum half life
of same order as the length of the test (this minimises the radiation dose to the patient)
Pure gamma emitter
No alpha or beta particles, these do not leave the body so merely increase the radiation dose.
Optimum energy for g emissions
High enough to exit the body but low enough to be easily detected. Useful range for gamma cameras is keV (optimum ~ 150 keV).
Suitable for incorporating into a pharmaceutical without altering its biochemical behaviour
Readily and cheaply available on the hospital site.

5. Some Commonly Used Radionuclides in Nuclear Medicine
Radionuclide Production:
Neutron Capture
Nuclear Fission
Charged Particle Bombardment
Radionuclide Generator
Q. Do you know of any other radionuclides that are used in nuclear medicine?
A. Xenon, Gallium, Fluorine, Selenium, Yittrium, Strontium,

6. Producing the Radiopharmaceutical
Radiopharmaceutical kits
Most common radiopharmaceuticals are available as kits. These contain all the necessary freeze-dried ingredients in an air-tight vial, usually the pharmaceutical, a stannous compound and stabilizer. On addition of 99Tcm 04- , the stannous reduces the 99Tcm 04- , makes it charged and "sticky", and Tc forms a bond with the pharmaceutical, labelling it.
For the longer half-life isotopes, the full radiopharmaceutical can be obtained directly from the manufacturer, e.g. SeHCAT labelled with 75Se.

7. Detection of the radiopharmaceutical
Non-imaging
In-vitro (measuring radiation levels in bodily fluids outside the body)
e.g. Blood sample counting for GFR analysis:
Patient
Electronics and
count-rate meter
Inject radioactive
tracer
Extract sample of
bodily fluid
(e.g. blood)
Measure fluid sample
in sample detector
In its simplest form, a non-imaging nuclear medicine test will involve the administration of a radiopharmaceutical, a delay for the tracer to accumulate or be processed in some way, and then the extraction of a sample (or samples).
These are then measured in a sample counter which will indicate the level of activity in the sample.
For example: Glomerular Filtration Rate measurement
3 MBq of 51Cr EDTA injected
10ml blood samples collected at 2,3,4 and 5 hours post-injection.
Estimate of GFR obtained by linear regression fit to these mesurements.

8. Detection of the radiopharmaceutical
Non-imaging
In-vivo (Uptake measurements in organs using a radiation detector probe)
e.g. SeHCAT study for bile salt malabsorption .
Collimator
Scintillation
Electronics and
probe
count-rate meter

9. Detection of the radiopharmaceutical
In Vivo imaging - the gamma camera
Patient
Radioactive
tracer
Gamma
rays
camera
Image
Vast majority of Nuclear Medicine investigations involve the production of an image using a Gamma Camera
Properties of gamma rays
High energy electromagnetic radiation
Can be scattered and absorbed
Cannot be focused

10. The Gamma Camera X Y Z Collimator NaI Crystal Photo Multiplier Tubes
Position
circuitry X Y Z
Collimator
NaI
Crystal
Photo Multiplier
Tubes
Analogue to
Digital Converters
Digital
Output position
& energy signals

11. The Collimator
The purpose of the collimator is to project an image of the radioactive distribution in the patient onto the scintillation crystal.
It is a crude and highly inefficient device, which is required because no gamma-ray lens exists.
In the parallel hole collimator, only incident photons that are normal to the collimator surface will pass through it.
All other photons should be absorbed by the lead septa between the holes
The collimator defines the field of view, and essentially determines the system spatial resolution and sensitivity.
Collimator efficiency: only ~0.1% of photons incident on the collimator pass through it.

12. Spatial Resolution & Sensitivity
Spatial resolution of an imaging device defines its ability to distinguish between two structures close together and is characterised by the blurred image response to a point-source input. For a gamma camera, the overall spatial resolution in the image depends on the collimator (collimator resolution) and the other gamma-camera components (intrinsic resolution).
To improve collimator resolution
Increase the septa depth (d)
Reduce the size of the holes (s)
Resolution ↓ as the source is moved
away from the collimator
- important to image with the camera
as close to the patient as possible
0 cm
5 cm
10 cm
15 cm
20 cm
Output from
Collimator
collimator
Radioactive
pt. source
Q. Why would you want to trade sensitivity against resolution? Why would you not want to always obtain the best resolution? A.
Acquistion may take too long to get reasonsible number of counts
e.g. dynamic imaging (and some SPECT acquisitions) you only have a short time period in which to collect the image information – need to obtain the optimum counts stats (within reason).
To improve collimator sensitivity
Dependent on the number of photons
passing through the collimator
Improved with larger hole sizes and
smaller length septa
Spread of response
to pt. source defines s
collimator resolution d
Spatial distance
resolution and sensitivity are conflicting parameters

13. Scintillation Crystal
The gamma ray causes an electron release in the crystal via the Photoelectric Effect, Compton Scattering or the electron­positron pair production (E > MeV), this excess energy gives rise to subsequent visible light emission within the crystal (scintillation).
Number of light photons produced is roughly  E Hence, this is an energy discriminating detector (important feature as we can use this to reject scattered photons)
Incident
gamma
ray
NaI(Tl) Scintillation
crystal
Light
Photons (~415nm)
Compton Scattering
The energy of a gamma photon is partially absorbed by the atom, ejecting an electron from the atom and scattering the photon.
Photo-electric Effect
The gamma photon is completely absorbed by the atom and the energy is used to eject an electron from the atom.
Why dope the NaI crystal with Thallium?
Activators are impurities that create special sites in the lattice at which the normal energy band structure is modified from that of the pure crystal. Often, thallium is used as an activator in sodium iodide detectors. In small amounts, these impurities enhance the probability of visible photon emission during the de-excitation process.
Once the visible light photons have been produced it is important for them to travel through the crystal unhindered. The crystal therefore needs to be transparent to its own light photons emissions.

14. Image Types
In Nuclear Medicine various forms of data acquisition can be performed:
Static Imaging
The distribution of the radiopharmaceutical is fixed over the imaging period.
Multiple images can be acquired, viewing from different angles (e.g. anterior, oblique).
e.g. kidneys (DMSA), thyroids, bone, lung
99Tcm Thyroid Scan
Whole Body imaging
the camera scans over the whole body to cover more widespread distributions or unknown locations
e.g. bone scan, infection imaging, tumour imaging
99Tcm HDP Bone Scan
Dynamic Imaging
Consecutive images are acquired over a period of time (with the camera in a fixed position) showing the changing distribution of the radiopharmaceutical in the organ of interest.
e.g. renogram, GI bleed, meckel’s diverticulum
99Tcm labelled red blood cells – GI bleed