Nuclear Imaging Enters the Digital Age
Remember the vacuum tube?
It made electronics possible, but its size and fragility made televisions, radios and even computers bulky, temperamental and prone to breakdowns. The advent of solid-state electronics--semiconductors and transistors--largely condemned the vacuum tube to the backwaters of history.
Sure, you can find an occasional hi-fi enthusiast who thinks the “warm” sound produced by tubes is irreplaceable, but vacuum tubes have otherwise disappeared--with one notable exception: the nuclear imaging industry.
Gamma cameras used in nuclear medicine and industrial imaging remain bulky, immobile instruments, weighing as much as 5,000 pounds, because physicists have encountered tremendous difficulties in producing solid-state detectors for the high-energy particles called gamma rays.
But this final toehold of vacuum tubes is about to give way. In May, the Food and Drug Administration approved a new solid-state gamma camera about the size of a laptop computer. The camera, manufactured by DigiRad Corp. of San Diego, promises to revolutionize nuclear imaging in the same manner that transistors forever altered consumer electronics, making smaller, lighter cameras available.
And the new invention’s introduction comes at a time when the use of nuclear imaging is growing widely. Cancer specialists, for example, have found that nuclear imaging can detect many breast tumors that do not show up on conventional mammograms, especially when breast tissue is unusually dense.
Cardiologists have shown that the technique can quickly determine which cardiac patients in the emergency room are actually having a heart attack. About 40% of the 5 million Americans who go to emergency rooms each year with cardiac symptoms are not having a heart attack and are often unnecessarily hospitalized. And as many as 50,000 of those who are having a heart attack are mistakenly sent home.
Nuclear imaging, which measures how well the heart is functioning, can quickly distinguish between the two possibilities.
Outside the world of medicine, nuclear safety experts are searching for new ways to control the spread of radioactive materials and the development of a nuclear weapons capability by Third World countries. Gamma detectors, particularly portable ones, represent the most useful way to address those concerns.
Gamma detectors not only reveal the presence of radioactive materials, but also identify the ratio of various isotopes, which is useful to distinguish between harmless material destined for civilian use and bomb-grade matter. Portable detectors would make it much easier to inspect nuclear facilities in countries where access time is limited.
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“This is a fabulous development,” said physicist Tony Lavietes of the Lawrence Livermore National Laboratory. “There are a tremendous number of applications for these [detectors]. . . . I’ve been biting my tongue for a long time waiting [for permission] to talk about them.”
Dr. Michael Kipper of UC San Diego has used one of the new cameras for nearly six months and is quite pleased. “What we have seen is, at a minimum, equal in quality” to images obtained with conventional cameras based on vacuum tubes, he said. “It is somewhat better with respect to resolution--the details of the image--allowing us to be certain of what we are looking at.”
Nuclear imaging is quite different from use of conventional X-rays. With the latter, X-rays are directed through the body onto film, and differing absorption of the radiation by tissues shows the anatomy of body parts.
In nuclear imaging, specially prepared, harmless radioactive isotopes are injected into the body, and their motion through the heart, for example, or their accumulation in a tumor, is monitored by detecting their faint gamma-ray emissions. Instead of showing what the heart looks like, as an X-ray does, nuclear imaging shows how well it is functioning.
An estimated 12 million nuclear medicine procedures are conducted each year in the United States at a cost of $5 billion, and the number is increasing rapidly.
Researchers have been trying for more than 20 years to develop solid-state gamma-ray detectors, but the problem has been finding an appropriate crystalline material that would function at room temperature. Unfortunately, there is really no rational, or logical, way to design such crystals, said Lavietes. Instead, he said, it is more of “a black art.”
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Traditional gamma cameras are based on two-part devices called scintillation counters. The first part of the device contains a material that, when struck by a gamma ray, emits light. The second part--the vacuum tube--converts that light into an electrical signal. The process itself is not very efficient but can become so by making the apparatus very large.
In a solid-state device, the gamma ray produces an electrical current directly. High-purity germanium crystals, for example, can achieve this, but only if they are cooled to 77 degrees above absolute zero, liquid nitrogen temperature. That makes them too expensive for medical applications.
Physicist Jack Butler and his colleagues at DigiRad discovered that crystals of cadmium zinc telluride would work as a detector at room temperature and developed a technique to grow large crystals. They also developed new ways to collect the electrical current from the crystals and to combine the crystals in large arrays for high sensitivity. The result is the new camera, which has a detection “head” the size of a laptop computer connected to a personal computer on a rolling cart.
“We really appreciate the portability,” Kipper said.
Now, if nuclear imaging is used to evaluate cardiac patients, they must be taken from the emergency room to the nuclear medicine department, which is usually in the basement. With the new camera, he said, they could be evaluated in the emergency department within sight of emergency personnel.
Similarly, most mammography is performed in outpatient clinics that do not have nuclear imaging capabilities. The new camera--although it is no cheaper than current ones--is small enough to be used in such clinics.
The smallness of the camera head also allows radiologists to examine portions of the breast that are difficult to image with current cameras but that often contain tumors, said Dr. Iraj Khalkhali of Harbor-UCLA Medical Center.
And finally, said Dr. Frank Papatheofanis of UC San Diego, the output of the camera is completely digital, so its pictures can be fed directly into a hospital’s medical information system and viewed by doctors throughout the facility.
“We are in the infancy of a technology explosion,” said DigiRad Chief Executive Officer Karen A. Klause. “The same thing that happened to computers will now happen in medical imaging applications.”
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Miniaturizing Medicine
DigiRad Corp. has received approval from the Food and Drug Administration to market a new solid-state camera for nuclear medicine applications. The camera, which weighs only 50 pounds, replaces vacuum tube-based cameras that weigh as much as 5,000 pounds.
* Conventional gamma ray detection system
The cameras now used in nuclear medicine are based on scintillation counters. When a gamma ray from a radioisotope inside a patient strikes the scintillation crystal, the crystal emits a photon of light. The light is detected by a photomultiplier (vacuum) tube. Large arrays of such tubes are needed for efficiency and high resolution.
* DigiRad solid-state detection system
When a gamma ray strikes a solid-state detector, it causes an electron to be separated from the “hole” in which it normally resides. The electrical charge can then be detected by electrodes on the surface of the crystal.
