Radioactive Tracing, a Short History

Using nuclear medical imaging together with some kind of device suitable for taking anatomical images (most often with CT) makes accurate spatial localisation possible, which can be very important in treatment planning or surgery.

Various examinations can be carried out depending on the type of molecule being used.

Organs that can be examined using nuclear medicine:

 brain: tumours, alterations in blood flow, energy uptake, Alzheimer‟s disease, Parkinson‟s disease (PET)

 thyroid, parathyroid (hyperfunction, tumours)

 heart (perfusion, blood supply)

 lung (embolism, ventilation problems – using radioactive gas)

 kidney, adrenal gland (blood flow)

 bones (tumours, metastases)

 prostate (tumour)

 practically any kind of tumour (PET)

3.1.3 Radioactive Tracing, a Short History

The concept of radioactive tracing was developed by György Hevesy (1885-1966), the Nobel Prize winner Hungarian scientist. According to the basic idea the human body cannot distinguish the radioactive isotope of an element from the non-radioactive one, thus the radioactive isotope of the element can get to all those organs within the body where the element itself can.

In the golden age of nuclear medicine planar imaging was the most widespread and primitive equipment were used. The discovery of the NaI scintillation crystal in 1948 by Hofstadter was a real breakthrough, and it is the scintillator most frequently used in SPECT devices even today. In 1949 the first image with the use of an isotope was taken, the scintillator was made of CaWO4 (Cassen et al.). In 1952 the first focused collimators appeared, and by 1956 the first image created by positron annihilation was reported about (Aronow and Brownell).

157 The isotope most frequently used in the 1950s was I-131, the half-life of which is ~8 days, and it passes out of the body very slowly. Its gamma energy is 364 keV, so especially thick (2”) scintillators are needed to detect sufficient photons.

By the 1960s multidetector systems had appeared. In the device designed by Anger there were 50 detectors below, and 50 above the patient who was moved along these detectors. It is possible to take dynamic images using multidetector systems, while the so-called scanners are suitable for taking static images only.

The first tomographic image produced by a computer-controlled dual-detector system was taken in 1964 by Kuhl and Edwards. They took transverse and longitudinal section images and the device they used served primarily as a means of examining the brain.

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Rectilinear scanner

The entire scanner head could be moved above the target area, this way information could be obtained of the activity of each point.

The next significant breakthrough was the introduction of the Anger camera (Berkley, 1957) designed by Hal Anger. The camera was not prevalently used for a surprisingly long time but by the end of the 1960s it had become more widespread.

Anger’s devices:

The positron emission camera designed and built by Anger had a collimated (focal-plane) detector operated in coincidence with an uncollimated detector (gamma camera). This proved to be useful because this way detecting scattered photons could be avoided. Whole-body images were obtained by assembling overlapping photos (the image below was taken using Fe-52 isotope).

159 Anger with the positron emission camera and an image taken with this device

In the 1970s the „tomoscanner‟ developed by Anger appeared. It was actually a gamma camera onto which a focused collimator was mounted. The collimator could focus at different depths, which was an innovation, and it was named „tomoscanner‟ because it could be used to take tomographic images. Thus always only one plane at a given depth fills the field of view, it is the only section that appears sharp. This method did not become widespread.

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Tomoscanner developed by Anger, focal-plane imaging

Bone scan (scintigraphy) with various methods

The figure below shows bone scans performed using three different methods. The image on the left was taken with a device containing one detector and one collimator (the numbers indicate the distribution). The photo in the middle is a scanned image, in this case the concentration of activity of the isotope in a given region is proportionate to the line density appearing in the image. The image on the right is noticeably the best, it was taken with a gamma camera.

161 Bone scans performed with different techniques

Developments up to 2010:

 in 1974 gated cardiac imaging appeared

 from 1976 multi-headed devices have been used (they render taking a full circle around the patient unnecessary when tomography is performed)

 in 1992 the first SPECT-CT appeared, in which attenuation correction could be applied based on the CT image

 in the early 2000s the first gamma cameras equipped with CZT (cadmium zinc telluride) detectors appeared, the chief advantage of which is portability; however, they did not become widespread due to the high price of the detector

 by 2010 approx. 20 thousand gamma cameras had been installed all over the world

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Development in pictures

Image of a brain taken in 1961 using a scanner

PET-MRI image today

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3.1.4 Collimators

Collimators play a very important role in nuclear medicine. Their presence is necessary for forming an image in every type of equipment bar PET. Their purpose is to ensure that a given pixel of the detector will only be stricken by photons arriving perpendicularly to the detector pixel (in case of parallel hole collimators) or reaching it from a well defined direction (in case of diverging or converging collimators). Of course these constructions cannot be perfect because the probability of a photon arriving at exactly 90° is zero, this way no pixel would be stricken. Furthermore, it would be impossible to create collimators that are „perfect‟. In general we can say that photons can enter the collimator only within a very narrow spatial angle region.

Fundamentally we can say that collimators can belong to one of four types:

 parallel hole collimators

 converging collimators

 diverging collimators (a great surface can be imaged using a small detector)

 pinhole collimators (camera obscura – they can provide a magnified image)

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