PET Scan: Particle Physics And Electronics in Medical Imaging

Numerous imaging modalities are there to view anatomical structures in our body. They include X rays, Ultrasound imaging, MRI and many other procedures where we can see normal or diseased tissues in our body. They tell us ‘the location’ (position) in the body the image corresponds to. If we wanted to see what were happening in these locations, we would then need to perform functional imaging techniques like PET scan or fMRI.

In Positron Emission Tomography or PET, a radioactive isotope that decays by positron emission is introduced into the body. Positron emitting radioisotopes are prepared by bombarding stable atomic nuclei by protons. Protons are speeded up in a particle accelerator called cyclotron which then impinge upon the stable nuclei, and knocks out one neutron from its nucleus. The proton now occupies the position where the ousted neutron once stayed. But this atomic configuration is unstable, so the proton now decays. It decays by emitting positron, a particle resembling an electron in all aspects except that the charge is positive and not negative. In other words, a positron is an antimatter: an anti-electron.

Many different radioisotopes are there, such as Fluorine18, Oxygen15, and Carbon11. 18F is the most commonly used isotope. It replaces hydroxyl (OH) group in molecules of interest. We can use 18F Fluorodeoxyglucose (18FDG) an analog of glucose for probing the activity of brain. Our brain uses glucose for its metabolism, so when it encounters 18FDG, it stores them. The FDG in the brain begins emitting positrons. These particles travel only a short distance before they meet nearby electrons and annihilate. Two gamma ray photons, each having 511 keV of energy are produced--- Photons because they are electromagnetic waves, and gamma ray because the frequencies correspond to the gamma ray spectrum of electromagnetic waves.

So by detecting these photons, we can find out where they came from, since we know that these photons are emitted back to back, 180 degrees apart. (They aren’t exactly collinear as their initial velocity is not zero, and some computational error always creeps in). For the detection part, we need a detection array which will convert these photons into electrons. This is done by scintillators. Bismuth germanate, Luterium oxyorthosilicate (LSO) are some of them. Photons which are incident on them produce electrons by photoelectric effect. These electrons are then guided through a vacuum tube, which has many positive electrodes (dynodes) held at successively higher voltages. These dynodes of this photomultiplier tube accelerate these electrons, which in turn knock-off more electrons from the dynode plates. Thus we get more electrons than what we started with. The signal has now been amplified and we now have a measurable current.

With advancement in detection technology, silicon avalanche photodiodes (silicon APD) has now shown promise to replace the vacuum technology (photomultiplier tube). As the name suggests, APDs work in a similar way an avalanche gains its momentum as it descends from the mountain-avalanche effect. Detecting photons aren’t sufficient. We need to detect only co-linear (coincident) photons. Each collinear photon pair (i.e. 180 degrees apart) will constitute an event. All other photons (noise) must be rejected. About 10^7 to 10^8 or more ‘events’ must be registered in order to have a good signal to noise ratio. Image faithfulness varies proportionally with the square root of the number of events.

By acquiring a large number of events, the computer software is able to determine exactly where these radioactive tracers are located. This, in our case, means the locations where the neurons are accumulating (accumulation is a function of utilization of glucose) 18FDG. Thus we get a functional map. In order to know ‘what’ these structure were, we need to combine anatomical imaging like MRI or CT with PET. This combined PET-CT or PET-MRI let us know what structures are doing how much.

PET scan is very useful in neuroscience researches, clinical diagnoses like cancer detection, receptor analyses and even watching gene expression in molecular biology.

References: A good site with animation: PET animation

Physicsworld

Last Mod: 10 Mar, 2014

Out of Body Experience: Want to Have One?

The poor old man complained of intractable tinnitus, a clinical condition where the patients hear 'ringing' noises in the ears. The great artist Vincent Van Gogh also suffered from this condition. He was so disturbed by the auditory hallucination that he cut off part of his ear. It is said that he used to poke ice picks inside his ear in order to get relief from tinnitus.

Our man in question, too had this symptom. He was given conservative treatment for this {which include betahistine (a histamine analog), sodium fluoride, antihistamines such as meclizine, diuretics (drugs that increase urine output thus lowering the accumulated fluid in the ear) and many others}. But all these failed. The doctors did the next best thing. They put in a tiny electrode in his right superior temporal gyrus and stimulated it electrically. Lo and behold! The patient said that he felt as if he was about 50 centimeters behind his body and to the left.

The scientists led by Dirk De Ridder of Antwerp University, Belgium, immediately straightened up and arranged for PET scanners, machines that detect metabolic (glucose) activity in real time. They noticed that in addition to the area being stimulated, other areas were also showing increased glucose metabolism. For example, supra marginal gyrus, an area that processes informations from the inner ear was also lighting up. Needless to mention, internal ear also senses our (head) position and acceleration/deceleration. Thus activity in supramarginal area may have given the old man (63 year old) the goose pimples: an out of body experience.

It was long known that drugs used in dissociative anesthesia (ketamine, for example) imparted a sensation akin to this, a sensation felt by persons who have been on the verge of certain death, a 'near death experience'. In a condition known as Alice in Wonderland syndrome (Todd's Syndrome) which may occur in patients suffering from migraine or epilepsy, such intense feeling of distortion of ones own self, coupled with fast forwarding of time sense occur.

But scientists didn't stop at that. So they went off with an experiment. They made a guy sit on a chair. Then they fitted two cameras at (behind) his back. The cameras scanned his back and the image was transferred to his eyes by special goggles. So, in essence, he was seeing his own back. The naughty scientists then scratched the poor chap's back. The guy saw one thing (his projection) and felt one thing (real sensation on his real body). Disoriented, he mixed it up and had an out of body experience. How about one yourself.

Right is Might

You have forgotten your car keys and done hours of thinking; but to no avail. You give up in despair and start sipping your evening cup of coffee. Suddenly, out of nowhere you remember where you kept your keys. What you could not remember thinking, came so abruptly on its own. This is intuition and it is said that its the right brain (right half of the brain, obviously!) that is responsible for this. The famous scientist, Albert Einstein said “The only real valuable thing is intuition.”

When we copy a 'normal' picture, the 'logical' brain breaks down the picture into graphical pixels. These pixels are then plotted in their graphical co-ordinates. This is rather like mathematics. The picture on the left have been intentionally placed upside down. When you try to draw this picture, you just can not take anything for granted, because the orientation is just not right. The mathematical plotting can not be activated. Where reasoning ends, intuition begins. It is much easier to draw (copy a picture) a picture placed upside down, since our logic takes the back seat then; and the intuitive role of the right hemisphere ('intuitive brain') starts to play. We are no longer primed to 'assign' any coordinates preemptively. The right hemisphere of the brain is not only associated with intuition and drawing (art), but also emotion, music and other artistic abilities. The left brain is rather logical and deals with mathematics and other tasks that require calculations. When a subject is told to recall words, the activity in left frontal cortex increases (left parahippocampal area is also stimulated); while activities in the right frontal cortex (along with para hippocampal areas of both sides) are increased when a subject recalls sceneries or pictures. These activation patterns of the brain have been measured by fMRI scans (functional Magnetic Resonance Imaging), PET scans (Positron Emission Tomography), ERP studies (event related potential or evoked potential) and other medical investigations.

Have you noticed that when we hold babies in our arms, we put them on our left? This left cradling is puzzling, because its the right arm that is stronger. So, why do mothers hold their babies on their left laps? It has been suggested that our expressions show better on the left side of the face and left cradling fosters stronger mother-baby relationship. This arises from the fact that the hemispheres of the brain is cross-wired. The (some of the) neural pathways from the brain cross to the opposite sides, as they enter the spinal cord, to supply peripheral circuits. The right brain (the one responsible for emotion etc) is wired to the left side of the body and the left brain is wired to the right. Thus the 'emotion generator' right brain, will have its manifestation on the left side of the face. Mothers possibly don't bother about all this, but evolutionary anthropology has left its indelible footprints on bipeds.

To become a creative genius, you need both intelligence (a faculty usually attributed to the left brain) and intuition (typically a right brain's domain). Albert Einstein, it is said, had strong intuitive powers, which helped him to formulate his famous theory of relativity, ahead of the contemporary knowledge. So, its time we wake up to this fact and start exercising our right brains as well.