PET Neurology Images
Viewing Note: this page contains some true 8-bit (256 color) images. They may not display well if your display/software configuration can't allocate all 256 levels and appropriate color maps.
Here's a jump list of studies available on this page:
Raw tracer Images - images showing raw tracer distribution
(source: SUNY/VAMC) This is an 18-FDG brain metabolism study of a normal volunteer. 18-Flurodeoxyglucose (18-FDG) is a tracer composed of radioactive fluorine (18-F, half-life of 110 minutes) and a sugar (deoxyglucose). This sugar enters cells in the brain in a manner similar to the way normal glucose enters them, and glucose is the primary source of energy for these cells. However, once inside a cell, 18-FDG essentially becomes trapped and is not metabolized further. Since the brain does not store energy but must have glucose supplied essentially "on demand" by the blood, and 18-FDG is taken up at a rate related to normal glucose uptake, we can use the 18-FDG to measure how active various parts of the brain are by measuring how much of it is taken up when tracer amounts are injected. For this study the subject was injected with the 18-FDG tracer as they lay quietly in the PET camera. The tracer was taken up into the brain over a period of 40 minutes, and then a 20 minute scan showing the distribution of the tracer was taken. The resulting image is dark where there is no uptake (metabolism of glucose) and becomes brighter as there is more and more uptake (higher metabolic rates).
Click here (320,671 bytes) to see the whole 31 plane image set. The first image in the upper left hand corner is at the top of the brain and subsequent images work down towards the feet. The total field of view from the 1st to the last image covers a volume of 10.8[cm]. Notice in the 1st few images there is a slight ring of uptake around the brain. This is where the tracer was taken up by the skin & muscle of the scalp.
(source: SUNY/VAMC) This is an image of "15-O water" tracer distribution in a normal volunteer. Trace amounts of 15-O (half-life of 123 seconds) in the form of water are injected into the subject as a bolus and allowed to circulate. 15-O water falls into the category of a "freely diffuseable" tracer which means it can readily cross cell boundaries to obtain a concentration in the cells on par with its concentration in the arterial blood (it also flows out of the cells as easily into the venous blood). This means that when you look at a 15-O water image you're basically going to see activity in areas where there is blood flow (increased flow means you see more tracer). As mentioned in the 18-FDG Normal Brain discussion above, cells in the brain do not store energy - consequently it must be supplied essentially on demand. Since the energy (glucose) is brought to the cells by the blood, increased local energy demand necessitates increased local blood flow and hence increased 15-O water tracer uptake. Combining the data in this image with data indicating the amount of 15-O tracer available in the general arterial blood pool as a function of time (obtained by rapidly acquiring arterial blood samples while the images are being taken) allows us to convert the raw 15-O water tracer images into quantitative images of regional Cerebral Blood Flow (rCBF; see below).
There are several techniques for computing rCBF. One approach, known as the autoradiographic technique, requires you to acquire the image data starting at the moment the tracer arrives in the brain and to continue acquiring the image for a fixed length of time from that point - say 40-60 seconds. Unfortunately, tracer arrival time in the brain varies widely across individuals and may be anywhere from 15 to 40 seconds. To get around this problem, the images are actually acquired as multiple rapid sets (frames) of images. This way the starting image frame - the frame in which the tracer arrives in the brain - can be identified and subsequent image frames added to it until the proper time interval has been built up. The image above represents such a composite image covering the interval from tracer arrival in the brain to 60 seconds post arrival.
The big advantage of the 15-O water tracer comes from the combined properties of short half-life, its ability to measure rCBF, and the fact that rCBF can be related to cellular activity levels. Because of the short half-life of 15-O water, rCBF can be measured essentially once every 10 minutes in a subject. This opens up the possibility of isolating regions of the brain used to perform a particular activity by subtracting an rCBF image when the subject is performing some task (and is in an activated state) from the same subject's rCBF image when they are at rest. By performing six or eight of these studies in a row one can tease out data on which parts of the brain are utilized for subcomponents of a multicomponent task. For example, suppose you want to look at tone discrimination in hearing. You could take a 15-O water image while having the subject press one button when they hear a high pitched tone and to press another when they hear a low pitched tone. However, what constitutes hearing, what constitutes button pressing, and what constitutes the "discrimination" task? To answer this question the investigator might perform several studies. One would be a rest state study, then next study might be simply listening to tones, the next listening to tones and pressing a button whenever a tone is heard, and finally pressing the button to discriminate between tones. By properly subtracting out rCBF values from conditions designed to highlight the tasks you aren't interested, the investigator is left (hopefully) with an image that represents just the discrimination task they were interested in.
Of course each stage of this process is almost a field of study in and of itself - from proper experimental design, to proper image and blood time activity curve acquisition, to proper rCBF image generation, to proper analysis of the resulting rCBF images to identify the activation sites (see Statistical Parametric Mapping (SPM) topic on previous page for one approach to this analysis). However, it is the diversity of skills required to perform such a study correctly that makes PET such an interesting and challenging field.
Click here (408,934 bytes) to see the whole 31 plane image set. These are one minute images (from time of tracer arrival in the brain). The total field of view from the 1st to the last image covers a volume of 10.8[cm].
(source: SUNY/VAMC) What's the difference between the 40 images (available below)? Which is normal, which has a tumor, and which has indications of stroke? Actually they're all the same image of a healthy normal volunteer - just displayed with different color scales. The effects created by various color scales may be visually dramatic, but may also cause one to see distinct boundaries where there are none. With so much image analysis occurring on the computer where dialing up any color scale you like is relatively easy, it is possible to make almost any feature stand out with the right "tweaking" (affectionately referred to as "dialing a defect"). For this reason it is important to include a color scale legend somewhere on these images if they're going to be shared with others so that viewers will have some idea of how the underlying image intensity is being represented (1st and last image are presented with a linear ramp gray scale).
Note: The full series of images below appeared on the December 1996 cover of the Journal of Nuclear Medicine Technology. One of the motivations for creating these images (aside from their artistic merit) was to illustrate that different "interpretations" are possible for the same image under the simple artificial manipulation wrought by adjusting the color scale. An additional potential source of interpretation error was added at the time of publication - image orientation. One must be extremely careful when viewing images in an artificial color scale, especially when they are upside down and left/right reversed ;-).
Click one of these (Large - 493,960 bytes) or (Small - 130,483 bytes) to see the same image in 40 color scale variations. Pay particular attention to the "hot spot" at the base of the image and note how it can appear "hot", "cold" or "disappear" depending on the color scale used.
(source: SUNY/VAMC) Hypermetabolic tumor with 18-FDG uptake.
Click here (346,554 bytes) to see the full 31 plane image set. Note the hypometabolism in the affected hemisphere.