"The patient is injected with a radiopharmaceutical. Depending on how the isotope is absorbed by the various organs, the body emits gamma radiation. Detectors then measure the gamma emissions to build a three dimensional image."
While clinical nuclear imaging is relatively straightforward, "false positive" and "false negative" errors in diagnosis and treatment are possible. Infarcted heart tissue, for example, will not absorb the isotope. In this instance, the absence of activity, not the presence of activity, indicates heart disease. Physicians, consequently, may overestimate emission activity originating near the body surface, or underestimate radiation from deep within the body.
Attenuation correction normalizes these differences to create a more accurate picture of the patient's true situation. Regrettably, conventional correction methods are also expensive and difficult to maintain. "Profile" attenuation correction, recently introduced by the Nuclear Medicine Group of Siemens Medical Systems for its E.CAM family of nuclear cameras, overcomes these drawbacks in elegant fashion. Based on the progressive shifting of decaying sources, Profile combines the positive aspects of static array (sheet), as well as scanning line, transmission sources--traditional attenuation correction scenarios.
"The simplicity and reliability of a sheet source is desirable," explains Profile's Principle Mechanical Engineer Grant Albert, "but its high cost and heavy weight (due to shielding) make it more applicable to research purposes." Albert adds that the moving line concept, where a single line of transmission sources scans the detector face, is a complex, high-maintenance assembly requiring a linear drive and numerous other electromechanical components.
"For these reasons," Albert says, "we focused our attention on a multiple line array. This strategy spaces a series of stationary individual line sources along an arc, and positions them far enough away so that they appear as a sheet source. Physically, the design eliminates the complexity and moving parts associated with scanning line sources, and reduces the high cost, high weight, and shielding requirements of sheet sources."
Once an isotope reaches its half life, it has lost its effectiveness for nuclear imaging and must be replenished. And while some isotopes have a long half life that would negate replenishment, they are either too expensive for practical use, or their energy level does not match the application. Low-energy radiation, for example, will not sufficiently pass through a patient.
"Periodically replacing each of the line sources in a multiple line array would be cost prohibitive for the customer," Albert notes. "This was a challenge we had to overcome if we were to be successful in bringing the Profile system to market."
Profiling (hence, the name) the majority of transmission strength at the center of the patient where attenuation is greatest, proved to be the solution. Concentrating the activity in the center of the patient not only results in more accurate attenuation maps for better images, but a cost-efficient means of source replenishment.
"As the line sources naturally decay over time," Albert explains, "they can be shifted from the central slots to the periphery. Two new sources are used to fill the vacancies in the center, while the weakest sources replaced at the periphery are returned for recycling." The patented shift-and-replenish scenario, in effect, significantly extends the usable life of each line source, resulting in as much as a 50% reduction of annual operating costs. In addition, shortening the source replenishment cycle to six months vs one-and-a-half years as found in competitive scanning line source designs minimizes the variation in source strength over time.
Mechanical design challenges
Low overall weight, high rigidity, and reasonable cost make for conflicting design goals. Profile weight goals were based on the need for easy manipulation by technologists while simultaneously achieving adequate source shielding. 3D solids modeling using SDRC IDEAS helped cut weight from 23 kg for the prototype to less than 9 kg for the actual product. In addition, a simple low-cost cartridge containing the source, attenuator, shielding, and collimator insert was developed for the line sources themselves. This approach minimized the amount of lead to create a compact shielded package easily handled by the technologists. Source suppliers build the package and ship it directly to customers every six months for array replenishment. Other design features include:
Counter-balanced solenoid. A small-frame solenoid, used for low weight and simple operation, opens and closes the shutter. Because load changes as the system rotates 360° about the patient, engineers had to find a way to ensure uniform solenoid performance, regardless of shutter position. Solution? A counterbalance compensates for weight variation during rotation. As the system orbits, the only load change the solenoid experiences is due to the slight variation in mechanical friction.
Removable boom. To accurately position Profile relative to the patient, regardless of moment load, the system slides forward and back along a linear guide that attaches to a horizontal boom. With Profile in its docked, or stowed position, the boom can be removed for easier patient access. Here, the design problem was the linear guide. Normally, a linear guide will not accommodate a joint because the recirculating ball bearings tend to spew out. The Siemens team, working in conjunction with engineers from bearing supplier THK, solved this problem by slightly undercutting the boom behind the guide near the joint, and securing the guide at a predetermined distance from the joint to cantilever each joint end. The resulting flex proved enough to accommodate ball travel through misalignment.
Profile attenuation correction allows simultaneous acquisition of both emission data sets from the injected radiopharmaceutical and transmission data sets. The system, therefore, adds no additional time to most routine, clinical nuclear imaging procedures.
Also, the additional patient radiation dose is minimal. The replenishment scheme requires only two 20 mCi gadolinium sources to be replaced every six months, while at the same time focusing counts where they matter most. Once a new 20 mCi source is installed in the Profile system, it is not removed for 3.5 years until it has decayed down to less than 1 mCi.
Image quality, however is the most immediate reward from Profile. "It is well known that attenuation artifacts (false readings) increase the complexity of interpretation, and in the worst case, can lead to mismanagement of a patient's condition or to unnecessary tests if a misdiagnosis is made," concludes Simon DeBruin, Profile senior product manager. "Profile attenuation correction is easy to use, reliable, and extremely cost-effective. Most importantly, clinical trials at multiple centers have demonstrated that is improves clinical nuclear imaging accuracy."
"Based on our preliminary results, we believe that attenuation correction provides more precise images." H. Hoffken, MD, Azentrum Radiology, University Klinikum, Marburg, Germany
"We have found that the use of Profile attenuation correction has a positive effect in our hospital." Anthony Fung, MD, Nuclear Cardiologist and Director, Cardiac Care Unit, Vancouver General Hospital, Vancouver, Canada
"The rate of false positives has come down since we started using Profile attenuation correction." James O'Donnell, MD, University Hospitals of Cleveland, Division of Nuclear Medicine, Cleveland, OH, U.S.