Focused Molecular Imaging Using Radiotracers

Hugh F. Stoddart, NeuroPhysics Corp.

Locating, imaging and quantifying concentrations of specified molecules deep within the body are essential in medicine and in drug discovery and development. Together often called molecular imaging, these tasks typically are performed by attaching radioactive atoms, or radiotracers, to the molecules and observing the radiation externally. Another molecular imaging modality uses fluorescent molecule but is mostly limited to distributions close to the animal surface. Molecular imaging should not be confused with “functional” imaging, which usually refers to the use of contrast agents in conjunction with anatomical imagers such as CT, MRI and ultrasound scanners.

Two kinds of atoms are used for radiotracers: One emits positive electrons, or positrons; the other emits x-ray and/or gamma-ray photons. For positron emitters, the particles move about in the tissue until they find an electron, whereupon they are annihilated, producing a pair of 511-keV photons.

Detecting the time coincidence of these “back to back” photons forms the basis of positron emission tomography (PET), which is used widely in clinical medicine to produce three-dimensional images of positron concentrations. It is important to remember that this is not quite the same as the distribution of the tagged molecules — especially if the positrons are energetic with long ranges in tissue. This positron “haze” does not affect the resolution materially for clinical applications but does place a limit on resolution for small-animal imaging.

Unlike positrons, x-ray and gamma-ray photons are detected externally, revealing the positions of the associated tagged molecules directly. The instrument commonly used to do this is called a gamma camera. Invented by Hal O. Anger and used in clinical nuclear medicine, the device consists of a thin, large-diameter crystal scintillation disk. On the front of it — the side facing the patient — is a lead plate, or collimator, of equal diameter, with many small parallel holes that pass through it perpendicular to the disk’s face.

Fastened on the back of the disk is an array of light-sensitive photomultiplier tubes. When absorbing a gamma ray, the crystal emits a flash of visible photons called a scintillation, the location of which is deduced by the relative amounts of light detected by the tubes. By accumulating the locations of the scintillations over time, one ends up with an image of a projection of incoming photons — perpendicular to the face of the gamma camera — of the radioactivity distribution in the body. When made to rotate about the patient, gathering projections at many angles so that a 3-D image can be reconstructed, the device is called a SPECT (single-photon-emission computed tomography) camera.

Gamma cameras have sensitivity and resolution limitations. Sensitivity is limited because very few photons reach the crystal. When a radioactive atom within the patient emits a gamma-ray photon — a process called decaying — that photon usually will stray and be lost. Very occasionally, one will travel in exactly the right direction to one of the holes and strike the crystal. Resolution is limited both by statistical errors in locating the (X-Y) positions of the scintillations using photomultiplier tubes and by the angulations of rays set by the finite diameter and length of the holes in the collimator.

Scanning focal point technology

The limits of gamma camera resolution and sensitivity led to the development of an entirely new method for 3-D imaging of distributions of radioactivity-tagged molecules. The technique is similar to that of the 3-D scanning confocal mi-croscope invented in 1955 and patented in 1957 by Marvin Minsky, which has become widely used in the biosciences (Figure 1).

Figure 1. Minsky’s scanning confocal microscope uses a lamp and lens to illuminate a tiny volume (the focus) within a tissue sample, which is observed by an identical coaxial lens and photocell. By mechanically moving the tissue in a raster scan perpendicular to the optical axis, the optical properties of a plane within the tissue are sampled. Moving the tissue in steps along the axis of the optics produces multiple planes of data that are combined to produce 3-D images.

In the special case of bioluminescence, where light is produced by a chemical reaction that originates within an organism, an external light source is not needed. This also is the case with a distribution of molecules tagged with radioactive atoms that emit gamma- and x-rays. The problem is that practical lenses for imaging these energetic photons are not available. Instead, a focused collimator with a very small focal point and a very large acceptance solid angle replaces the optical lens (Figure 2). To increase the sensitivity, several focused collimators are placed around the object.

Figure 2. A focusing collimator (orange) replaces Minsky’s lens. It is made of radiation-absorbing material and has a 3-D array of conical holes whose axes intersect at the same point in space (focus). The focal point is moved mechanically to perform a raster scan over a transaxial slice of the object. When the data from a slice is complete, the object is moved axially by an amount that is less than that of the focal point resolution, and data for a new scan is recorded. This process is repeated until the entire selected region is scanned. The scintillating crystal (blue) detects photons from activity at the focal point.

In the implementation described here, 12 collimator/detectors were used, each subtending 30° radially around the slice for a total coverage of 360°. The collimators extend ±24° off the slice in the axial direction, providing a solid-angle equivalent to an optical f/1.6 lens. The total scintillating crystal area is 0.31 m².

The diameter of the scanned slice is 200 mm. This field of view accommodates human heads, laboratory primates and other animals. To distinguish the scanner from SPECT devices, all of which are based on medical gamma camera technology, this focused-point scanner is designated the MollyQ-200 (think “molecule”).

As in Minsky’s 3-D microscope, the scanning focal point technology produces very high resolution radiotracer images. One way to demonstrate this is to scan a realistic “phantom” representing the human brain — built up from a stack of thin plastic sheets, each of which is cut in such a way as to provide cavities that represent key brain anatomical features (Figure 3).

Figure 3. These images demonstrate the very high resolution of the scanning focal point MollyQ-200 by using a 3-D plastic “phantom” that simulates the human brain. In the left column are three slices at different axial positions as imaged by MRI, with the phantom filled with a magnetic solution. The middle column shows matching slices using the scanning focal point instrument. In the right column are slices from a state-of-the-art rotating gamma camera SPECT instrument.

To characterize the phantom, it is filled with a magnetic solution and imaged with an MRI scanner to produce a set of “gold standard” slices. The phantom then is filled with a radioactive solution and imaged with radiotracer scanners. The resolution of the focal-point scanner is far superior to that of a state-of-the-art SPECT scanner with multiple rotating heads.

Drug discovery

The Molly Q-200 is being employed as the premier imager using primates to investigate potential drugs for neurodegenerative disease. For example, scientists can give monkeys the compound MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) to induce the symptoms of Parkinson’s disease, then follow the effect of various prospective counteracting drugs over time.

The affected brain structures are the striata (right and left). Each striatum consists of two substructures, the caudate nucleus and the putamen, which use the neurotransmitter dopamine, so they can be made visible with radioactivity-tagged dopamine transporters. In Parkinson’s disease, the concentration of dopamine transporters in the putamen decreases relative to the caudate, allowing the response to Parkinson’s disease drugs to be followed with time. Only the scanned focal point technology can distinguish these very small structures in a small primate (Figure 4).

Figure 4. Focused-scanner slices from an image of a green monkey striatum show clear separation of the caudate (bright spots at top in each slice) from the putamen (dangling below the caudate). The separation of the two is caused by white matter of the internal capsule. The slices are spaced 2 mm apart and demonstrate excellent axial resolution. No other radiotracer imager can provide this essential detail.

A version for mice and rats

A new quarter-size scanner with a field of view of 50 mm has been developed to provide very high resolution for scanning small animals to facilitate drug discovery and development. Resolution scales with size; therefore, the smaller scanner has better resolution than the larger one by a factor of four.

Designated the MollyQ-50, the scanner uses eight very large solid-angle (67¼ × 45¼) collimators, each with 10,042 long-bore converging conical holes that produce a focal point of less than 1 mm. The entrance holes of the collimators are only 343 μm in diameter, and the slice-to-slice spacing is only 400 μm. As with its precursor, the smaller scanner preserves its high sensitivity and resolution uniformly over the entire field of view.

Figure 5. This image is a surface rendering of a 3-D reconstruction from a subset of slices from a mouse injected with inorganic 125I. The rendering was made possible because the radiotracer is distributed in muscle and other tissue. The iodine is particularly taken up by the thyroid and saliva (red). The separated bilateral thyroid glands are at the center of the image, and each are about 1 mm.

The first animal scanned with the 50-mm scanner was a mouse that was injected with inorganic ¹²⁵I. The radiotracer was distributed throughout the muscle and concentrated in the thyroid gland and saliva. The bilateral lobes of the tiny thyroid gland are completely separated (Figure 5). This new radiotracer molecular imaging using scanned focal point technology has the potential for drastically lowering the cost of new drug discovery and for lowering both the cost and time to market of developed drugs.

Meet the author

Hugh F. Stoddart is the chief scientific officer at NeuroPhysics Corp. in Shirley, Mass.; e-mail: hughf@alumni.caltech.edu.