PET scan prep will explain all about the pet scan machine and its functioning. What are those points need to keep in the mind before and after scan.
Why prefer PET-CT instead of only pet scans and how to benefit our medical condition.
As compare to MRI, CT scan, X-Ray, and Ultrasound it is different because all the above devices only provide anatomical views/information inside the body.
Unlike the above equipment’s pet scan provides information about the rate of metabolism for a specific region of the body.
Although it’s spatial resolution is less compare to modern CT scan and Magnetic resonance imaging.
But very useful for oncology, functional imaging, radiopharmaceutical, and research.
Table of Contents
PET scan prep for Patients
There are few instructions that need to follow when a PET-CT scan prescribed.
In most cases, instructions are common for pet scan prep to accept any special diagnosis like bone imaging where NaF-18 tracer is used.
For cancer tissues and muscle imaging fluorodeoxyglucose used as a tracer. Due to this reason instruct the patient for fasting before 4 to 6 hours of an appointment.
In case of diabetes, patients need to consult doctors for further precautions.
Also, advise avoiding strenuous activity at least 24 hours earlier than appointment.
When arrive for PET-CT scan provide gown only to wear. This is helpful to avoid any metal or other obstacles during the study.
Around one hour earlier to enter for scanning inject tracer ( 18F-FDG ) to absorbed well inside the tissues before scanning.
Usually, the study finishes in less than an hour during this time advised to steady lie down on the scanning table.
The more body movement can be the cause of repeated study to achieve desired images.
Post scanning precautions for PET scan
After scanning patients can eat and drink normally. The radioactive tracer dose is very low radiation.
Advice to drink plenty of fluids to drain out the remaining radiotracer from the body.
As a precaution, Also advice to avoid close contact with pregnant women, babies, and young children for 6 hours after the scan.
Pregnant women never advised for a Pet-CT scan until not necessary to avoid radiation effects on the fetus.
Breastfeeding women need to consult before scanning that how long will take to radiation effect to nullify so that can feed the baby normally.
Physics for PET scan
Also known as positron emission tomography which clearly indicates the basic working principle.
It is similar to the medical scintigraphy used in nuclear medicine. In which radioactive isotope is attached to medicine as a tracer.
So that it can be traced inside the body with the help of a gamma camera.
As tracer emits gamma radiation captured by a gamma camera and locate in three-dimensional images.
Similarly using cyclotron make tracer like NaF-18, 18F-FDG, etc. for injecting before pet scan.
There are three methods to introduce inside the body are inhale, swallow or directly inject into a blood vein.
Most commonly use 18F-FDG ( Fluorodeoxyglucose ) as a tracer in PET diagnosis because its half-life is just less than two hours or 109.8 minutes precisely.
This is more than other isotopes like Carbon-11 makes it more feasible in use at distant location transportation.
There is a wide application of Fluorodeoxyglucose ( C6H1118FO5 ) in oncology ( branch of medicine for cancer ) study using PET.
As soon as FDG injected into the vein circulate the whole body through blood.
Absorbed by body tissues especially in targeted body organs as glucose.
When 18F9 isotope decays and converts into 18O8 then proton split into one neutron and one positron.
When the speed of positron reaches near zero then it collides with an electron and produces two gamma photons 180 degrees from each other.
The above process is known as annihilation in witch antimatter and matter (positron and electron) collide with each other and produce pure energy as per the Einstein law E = mc2 = hv.
Where E = hv ( Plank’s photon energy equation ) represents the energy of emitted gamma photons ( p = E/c ).
There are only two photons produced in opposite direction follow the conservation of linear momentum which is not possible with one or more than two photons.
PET scan machine and Detectors
Since the source of Positron annihilations ( Tracer ) that are of interest in PET imaging systems is within the patient.
The PET detector starts with a scintillation crystal. This crystal gives off light when it is hit by radiation.
Because the PET radiation is so strong, the crystals are 30mm deep to give the particles a better chance of striking the atoms in the crystal.
The detector subsystem arranged in a circular ring has to surround the patient to obtain a complete pattern of annihilations occurring within the patient.
To provide this capability, a ring of gamma photon detector module sets are placed within the PET scanner.
The purpose of the detector subsystem is to detect gamma photons generated when an annihilation occurs.
The two gamma photons travel in opposite directions and if the direction of flight of the two gamma rays is such that they both penetrate the detector subsystem.
Both the photons should be detected as events occurring from the same Positron annihilation. These events are referred to as coincidence events.
The subsystem must be capable of detecting gamma photons that penetrate the detector and converting.
The detection in a timely manner signals that can be used to find a coincidence between two gamma photons.
scintillation crystal is the material selected to detect these events. BGO or LySO is effective at stopping and converting the high energy 511 KEV gamma photons into visible light photons.
The crystals are bundled into 6×6 or 8×6 or 9×6 packages to be attached to the PM tubes.
When light strikes the tube, it fans out in all directions. This drawing shows what happens when the light from 1 photon on the left-most crystal enters the PM tube.
The crystals are then glued to a quad PM tube. This tube has 4 outputs.
So, in this case, if the signal from the left quadrant of the tube is 6 times as large as the signal from the right quadrant, we can say the light came from the left most crystal.
So, if we have 4 tubes and 36 crystals, how do we know which crystal gave off the light?
By the ratios of the light from the 4 tube outputs, we can calculate the crystal the light came from.
In reality, a simple ratio isn’t close enough, but by using a source, a crystal map can be created.
In some PET machines detector, 8 PM tubes make up 1 detector module.
The module has 24 crystals in the slice direction and 16 around the ring, and 35 modules make a complete detector.
In the 6 by 6 crystal package slice direction, the crystals are always 6 deep. Having more crystals around the ring leads to a better resolution.
The original crystals were made of Bismuth Germanium Oxide (BGO).
The newer crystals are Lutetium Yttrium Silicon Dioxide (LySO).
They give off about 3 times as much light and the light decays about 4 times as fast as BGO.
The crystal depths give a sensitivity that is adequate for present-day PET scanners.
The face of the individual crystals is about 5mm by 5mm. Each individual crystal is surrounded by a white plastic so that most of the light given off will come out on the other end.
The detector generates electrical signals when a gamma photon strikes it. These signals are sent for digitization.
So that mathematical formulas can apply to raw data in the form of software algorithms to obtain images.
Detector Physical Design
Some internal organs in patients are relatively large and to obtain event data from the entire organ, a large slice of the patient’s body is required.
To accomplish this requirement, a maximum axial length of 15 cm. was selected for the detector subsystem.
The design of the system allows for a smaller axial length to be implemented without changing the design of the remaining subsystems.
One of the important requirements of a PET scanner is high resolution.
When a coincidence event occurs, the exact location of the Positron annihilation is required to plot an accurate Line of Response (LOR) of the event.
To obtain this accuracy, the detector subsystem is designed with small crystals. Small enough to pinpoint events to accurate locations.
The present architecture provides 18 direct axial slices in the detector subsystem within the 15 cm. axial length.
There are 672 crystals in the transaxial direction in each of the 18 rings of crystals for a total of 12,096 crystals in the detector ring.
Coincidence timing for PET Scan
If a coincidence occurs and gives off 2 photons that strike 2 different crystals, the signals get back to the signal processing system boards at different times.
It is very close to the exact same time, but there is a few nanoseconds difference due to the tube and amplifier response and the length of the cables from the tube to the Raptor.
Using a pin source, we can create a large number of coincidence events and measure the time difference between every 2 crystals.
The peak must be occurring from the source, so where the peak value is located on the timeline, is the timing difference between the 2 crystals.
Position and Gain detection of coincidence generated photons
Each quadrant of the PM gives a different light output given the same input.
This will affect the ratio that we use to calculate the position. So when calibrating the tube, need to repeat the Position and Gain calibrations until the results are constant.
To calibrate gain and position, the source is used and the outputs of all the tubes are recorded.
The following map shows the light output of a single tube and how many times that occurred.
PET scan for cancer
The pet scan procedure for cancer diagnostic is the same as the pet scan prep procedure for other than a cancer diagnosis.
Only the difference is that it is a very common test in pre and post-cancer treatment diagnosis procedures.
CT Scan and MRI provides anatomical details but pet provides additional information about metabolism and microbiology.
Which helps a lot to identify cancer tissues more precisely. Although it is also useful in other treatments and diagnoses like Alzheimer’s.
also applicable in Cardiac, Osteoporosis, Pancreatic, Prostate, Lung, Cervical, Thyroid, Lymphoma, Head and neck, Colorectal, evaluate certain brain disorders, etc. Also useful in various research studies.
