Transcript Document

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).
Radiopharmaceuticals
+
Pharmaceutical
Traces physiology /
localises in organs of
interest
Biochemical
Bonding
Radioactive
nuclide
Emits radiation for
detection or therapy
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
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 50 - 500 keV (optimum ~ 150 keV).
• Suitable for incorporating into a pharmaceutical without
altering its biochemical behaviour
• Readily and cheaply available on the hospital site.
Some Commonly Used Radionuclides in
Nuclear Medicine
Radionuclide
Half-life
Tcm (Technecium)
111
In (Indium)
123
I (Iodine)
131
I
201
Tl (Thallium)
6h
2.8 days
13 h
8 days
73.5 h
99
Pure g
emissions ?
y
y
y
n
y
Energy of main
g’s (KeV)
Source of
production
140
173, 247
160
280, 360, 640
68-80
On site generator
Radionuclide Production:
•
•
•
•
Neutron Capture
Nuclear Fission
Charged Particle Bombardment
Radionuclide Generator
Cyclotron
Cyclotron
Reactor
Cyclotron
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 0 - , makes it charged and "sticky", and Tc forms a bond
4
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.
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:
Inject radioactive
tracer
Measure fluid sample
in sample detector
Extract sample of
bodily fluid
(e.g. blood)
Patient
Electronics and
count-rate meter
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
probe
Electronics and
count-rate meter
Detection of the radiopharmaceutical
• In Vivo imaging - the gamma camera
Gamma
rays
Radioactive
tracer
Image
Gamma
camera
Patient
Properties of gamma rays
• High energy electromagnetic radiation
• Can be scattered and absorbed
• Cannot be focused
The Gamma Camera
Collimator
NaI
Crystal
Photo Multiplier
Tubes
Analogue to
Digital Converters
Position
circuitry
X Y Z
Digital
circuitry
Output position
& energy signals
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 gammaray lens exists.
PARALLEL
COLLIMATOR
LENS
Object
Image
Object
Image
•
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.
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).
0
Collimator
Radioactive
pt. source
cm
Output from
collimator
5 cm
- important to image with the camera
as close to the patient as possible
10 cm
15 cm
s
d
Spatial distance
20 cm
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
Spread of response
to pt. source defines
collimator resolution
To improve collimator sensitivity
• Dependent on the number of photons
passing through the collimator
• Improved with larger hole sizes and
smaller length septa
resolution and sensitivity are conflicting parameters
Scintillation Crystal
NaI(Tl) Scintillation
crystal
Incident
gamma
ray
Light
Photons (~415nm)
•
The gamma ray causes an electron release in the
crystal via the Photoelectric Effect, Compton Scattering
or the electronpositron pair production (Eg > 1.022 MeV),
this excess energy gives rise to subsequent visible light
emission within the crystal (scintillation).
•
Number of light photons produced is roughly  Eg
Hence, this is an energy discriminating detector
(important feature as we can use this to reject scattered photons)
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
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
•
Thyroid Scan
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