Pictures of the Future      Fall 2005

The Perfect Patient

Fast computer programs have opened the door to a new category of medical imaging systems—hybrids. These combine different systems to achieve images of unprecedented quality. The result: improved diagnoses and more efficient procedures in hospitals.

High-tech makes it possible. State-of-theart computer tomography (CT) produces images of the heart in a matter of seconds with a resolution of under 0.4 mm, and ultrasound devices generate real-time 3D pictures of a fetus in the mother’s womb. But ultimately it’s software that integrates hightech components into a smoothly functioning whole. This is clearly demonstrated by a new generation of medical devices—hybrid systems. These systems amalgamate gigantic data volumes generated by different imaging methods. The result is improved image quality and more efficient diagnostic procedures. In the spring of 2005, for instance, the University Medical Center in Erlangen, Germany, became the first institution in Europe to start using the new Symbia System from Siemens, a hybrid CT-SPECT (computed tomography and single photon emission computed tomography) system (above).

In CT, an X-ray source rotates around the patient to create sectional images of the body. On the other hand, in SPECT, patients are injected with small doses of slightly radioactive substances that accumulate for a short time in certain organs, such as the heart, or in tumor tissues. The emitted radiation is sensed by a detector to create images that reveal biological activity and metabolism in damaged areas of the heart or in malignant tumors.

Until now, physicians have used two different devices—one for SPECT and one for CT. However, since the patient’s position may differ slightly from one examinations to the other, an exact superimposition of the image information is difficult to achieve. Only when CT and SPECT are combined can the two sets be flawlessly fused. Using Symbia, the two exposures are made in quick succession while the patient remains in the same position. The SPECT signal not only demonstrates the position of the tumor but also provides clues as to its physiology. Very active tumors accumulate high concentrations of radioactive material and therefore emit strong signals. But some of the radiation emitted by this radioactive substance in the body is absorbed or scattered by surrounding tissue. This can degrade the image and make localization and interpretation difficult.

Hybrid CT-SPECT technology can solve this problem. "With the CT data, the software we’ve developed can estimate the amount of attenuating tissue through which the radiation has traveled and, as a result, determine the actual level of radioactivity in the tumor," notes Keith Andress, manager of the Software Department for Molecular Imaging at Siemens Medical Solutions in Hoffmann Estates, Illinois. The CT data can also serve to estimate the depth of the attenuating tissue between the tumor and the detector, thereby improving the precision of tumor localization. The Symbia system benefits patients in yet another way. In the past, they had to make separate appointments in two different departments at different times. As a result, days or even weeks might pass before a diagnosis was finally available. Now, both examinations are performed in quick succession.

Localizing Tumors in 3D. Another hybrid system, the biograph, works much like Symbia. It combines CT with Positron Emission Tomography (PET). In PET, too, the patient is injected with a small amount of a radioactive substance. But, unlike SPECT, the radioactive substance does not emit photons but elementary particles called positrons. The positrons collide with electrons in nearby atoms. Each such interaction creates two photons. These photons can be identified, and their point of origin precisely localized by detectors.

"Nuclear medicine is a service provider for specialists, such as surgeons, which supplies important image information for use in therapy or during surgical intervention," explains Stefan Käpplinger, manager of the Physics Group at Medical Solutions (Med) in Erlangen, where he is also responsible for Molecular Imaging. "Radiologists in nuclear medicine are accustomed to interpreting tomograms. Surgeons, on the other hand, prefer three threedimensional images." That’s why biograph systems have been provided with FusedVision 3D software. This program assembles 2D sectional CT images into a three-dimensional image that can be viewed from any angle. The tumor data generated by PET is incorporated within this image. This combination improves physicians’ ability to check the progress of therapy. Checking a tumor’s response to therapy requires follow-up examinations at regular intervals. The biograph can swiftly and accurately superimpose images from different exams, so that the physician can easily compare the position and size of the tumor, as well as the level of radioactivity in it.

Making therapeutic procedures more precise, efficient and effective was the purpose behind the development of two other combination systems—the Angio CT Miyabi, a CT device combined with an angiographic system (used for imaging blood vessels), and the Primatom, which combines a CT system with a linear accelerator (used to treat tumors). Unlike Symbia and biograph, these devices are not assembled in the same housing. Instead, the CT system is supported on rails, and can be moved into the angio lab or the radiation therapy room. Nevertheless, the patient does not have to be moved, and both procedures can be performed in quick succession.

The Miyabi system was developed especially for treating patients suffering from vascular disease. Here, an X-ray source and an image receptor are juxtaposed on a freely moveable C-arm. The purpose of the procedure is to detect vascular defects, such as narrowing or dilatation. The resulting image information aids physicians in precisely steering catheter during cardiac catheterizations (see "Software-Guided Intervention"). But angiography does not clearly delineate surrounding tissues. That information is provided by CT. Here too, software enables the two data sets to be precisely superimposed, which is particularly useful in evaluating and treating strokes. To minimize brain damage, for instance, it’s essential to decide quickly whether the problem is a cerebral hemorrhage or a clotted artery, because these two conditions require entirely different treatments. In the past, the patient had to be rapidly transferred from one machine to the other. However, with the Miyabi system this will become unnecessary in many cases.

The Primatom has many advantages in imaging-controlled radiation therapy. Tumors in the back, pelvis or abdomen can shift in position from day to day due to organ movements or to volume changes in the digestive tract. The Primatom system generates a highly energetic radiation beam used to destroy tumors, and it also creates a detailed CT image of the same body region. This combination substantially facilitates patient positioning for accurate targeting of the treatment beam.

Ultrasound in 4D. Another area in which software is vital in improving conventional procedures is ultrasound. "Many experts used to think that this technology could not be further improved," says Gerhard Kreitz, marketing manager at Med in Erlangen. "But with the Acuson Antares Premium we’ve brought a device to the market that shows that much more can be done."

The Acuson Antares uses fourSight 4D technology to create images in four dimensions: It depicts the area of interest not only in 3D but also in motion. "That’s especially important in examining fetuses, because their movements provide clues about their neurological development." The system also makes it easier to recognize malformations, such as anomalies of the face or fingers. As a result, the Acuson Antares is used mostly for gynecological studies, and in 90 % of those for prenatal exams.

The Acuson Antares’ innovative transducer doesn’t need to be moved across the patient’s abdomen but remains stationary. Inside it, a small motor causes the sound probe to oscillate in precisely defined trajectories. The advantage? The device always knows the exact position of the sound waves. "We know exactly where each pixel is located," explains Kreitz.

Acuson Antares scans the volume of interest up to 25 times per second and generates tomograms of different tissue layers, which the software combines into a 3D stack—so fast that movements can be displayed in real time. "To do this, the image composition fast be extremely precise, and the software must handle a gigantic data volume—up to 2 Gbyte," notes Kreitz. But the most important issue is what the system can do with the data. That’s why Antares has been equipped with additional functions. The physician can view the volume as a whole, scroll though the sections, select any desired layer, and of course observe the movements of the fetus at normal speed.
                                                                                                                    Tim Schröder