Applied Radiology

Imaging specialists can now more accurately localize lesions detected by abnormal radionuclide uptake with a system that simultaneously acquires an anatomic map. With new technology promoted as a powerful merger of form and function a fusion of anatomy and physiology a gamma camera equipped with digital x-ray tomography provides tomographic slices that show the structural landmarks of radionuclide activity.

X-ray anatomic landmarks acquired by Millennium VG gamma camera

Unveiled at The Society of Nuclear Medicine's (SNM) 46th Annual Meeting (held June 1999 in Los Angeles), the functional anatomic mapping technology developed for the Millennium VG evolved from the 1998 acquisition of Elscint's nuclear medicine business (Haifa, Israel) by GE Medical Systems (Milwaukee). The system is now undergoing clinical evaluation at Vanderbilt University Medical Center (Nashville), Duke University Medical Center (Durham, NC), and Hadassa and Rambam Hospitals (Israel). With marketing clearance from the Food and Drug Administration (FDA) pending, GE expects the system to become available commercially by the end of the year 2000.

"I believe that this technology is a defining moment in nuclear medicine, and will have a tremendous impact on how we practice nuclear medicine in the future," said Martin Sandler, MD, vice chairman of radiology and radiological sciences at Vanderbilt. "Knowing the precise location of the disease, monitoring the response to therapy, and noninvasively differentiating benign from malignant lesions is critical to good patient care," Dr. Sandler added.

"Anatomic mapping is a major breakthrough in nuclear medicine and it's going to improve patient care over the next several years," said R. Edward Coleman, MD, director of nuclear medicine and vice president of the Department of Radiology at Duke.

Ever since tomographic anatomy became clinically routine with x-ray computed tomography (CT) and magnetic resonance (MR), nuclear medicine pioneers have pursued techniques for correlating or fusing those images with radionuclide studies. Years ago at the low-tech end of the spectrum, transparent plastic templates with anatomic maps were produced to overlay on single-photon emission computed tomography (SPECT) films to help localize lesions. Today, there are sophisticated techniques for fusing the digital data from SPECT, positron emission tomography (PET), CT, and MR, but the technology is not ready for everyday clinical practice.

Localization and attenuation correction

The functional anatomic mapping available from GE is not a fusion of a diagnostic CT scan with a SPECT scan (or with a dualhead, coincidence-detection, positron-emission scan). Instead, it is a method of providing tomographic x-ray anatomic landmarks for the radionuclide image. The x-ray and radionuclide data are acquired simultaneously, without moving the patient, which should eliminate registration problems inherent in fusing separately acquired studies.

Beth Klein, GE's general manager of global nuclear medicine and PET, pointed out that functional anatomic mapping can rise to meet two key challenges in today's nuclear medicine practice:

(1) Localization-Although nuclear medicine has progressed to the point where radionuclide studies quite accurately detect the presence and extent of disease, precise and accurate localization has remained elusive.

(2) Attenuation correction-In addition to providing anatomic map images, the x-ray transmission data can also serve to correct for attenuation in the radionuclide emission data, which will be particularly helpful in nuclear cardiology.

Advantages of a slip-ring gantry

The x-ray tomography in functional anatomic mapping comes from a combination of a low-dose x-ray tube (about 300 millirads of exposure to the patient) and a solid-state digital x-ray detector mounted directly on the slip-ring gantry. The low-dose x-ray tube in the GE gamma camera exposes the patient to about one-tenth the radiation of a diagnostic CT scan, according to Nathan Hermony, manager of GE's Nuclear Medicine R&D, Engineering, and Manufacturing Center of Excellence (Haifa, Israel).

The slip-ring gantry design allows for continuous revolution of the gamma detectors, x-ray tube, and x-ray detectors around the patient, "making it possible to cover the entire volume without breaks, pauses, or the need to 'rewind' the detectors after one or two revolutions," explained Mr. Hermony. Although most x-ray computed tomography (CT) systems have slip-ring technology, he added, very few gamma camera systems have it.

Economic impact

In response to a question about the economic impact of the system, GE did not provide a list price or an estimated cost per procedure. However, "when you improve diagnostic accuracy, you decrease healthcare costs," said Dr. Coleman. With better evaluation of a tumor, for example, unnecessary surgery can be avoided and more appropriate chemotherapy can be planned, he explained.

Unlike other tranmission data sources used for attentuation correction, such as gadolinium or cesium, which are usually replaced every year, the x-ray tube in the Millennium VG functional anatomic mapping system should last for the lifetime of the camera. From this point of view, noted Mr. Harmony, this new technique for attenuation correction could make more sense economically.

GE illustrated case studies of patients with lung cancer (figure 1), melanoma (figure 2), colon cancer, and breast cancer.

FIGURE 1. An FDG image fused with x-ray functional anatomic mapping in a patient with lung cancer.

FIGURE 2. An FDG image fused with x-ray functional anatomic mapping in a patient with lymphoma.

Evaluation of lung cancer and lymphoma

Dr. Sandler presented brief case reports of two patients to illustrate how functional anatomic mapping can help in the evaluation of lung cancer and lymphoma. Both patients underwent dual-head, coincidence-detection studies with fluorine-18 fluo-rodeoxyglucose (FDG).

In a patient with a history of lung cancer that led to removal of the left lung, Dr. Sandler showed a diagnostic CT study with a small nodule in the right lung and an enlarged lymph node in the mediastinal region. CT, of course, does not indicate whether nodules or enlarged lymph nodes are benign or malignant. The FDG image, however, showed increased uptake in both areas. When the functional anatomic map was superimposed on the FDG image, the uptake correlated perfectly to the nodule and lymph node, said Dr. Sandler, providing a noninvasive test that identified metastatic cancer in the right lung with involvement of the mediastinum.

In a patient with lymphoma, the diagnostic CT study showed significant shrinkage of an abdominal mass 3 months after chemotherapy. The dilemma, said Dr. Sandler, was determining whether the patient was in remission or had active disease. The FDG scan alone did not show whether the FDG activity was part of the mass or unrelated to the mass. With functional anatomic mapping, however, it was clear that the hot spot represented residual tumor activity.

Useful for more than FDG

Although the clinical examples shown at the SNM were FDG oncology studies, functional anatomic mapping can apply to other positron emitters used in dual-head, coincidence-detection, gamma camera systems, or with any radiotracer used for SPECT. For example, said Dr. Coleman, functional anatomic mapping could become useful for infection imaging with gallium-67, for monoclonal antibody studies in patients with cancer, and for nuclear cardiology studies

Improved reconstruction algorithms and radiation therapy planning

Besides improving the diagnostic accuracy and interpretation of radionuclide images, said Dr. Coleman, the anatomic map data can provide two additional benefits: improved radiation therapy planning and more accurate reconstruction of radionuclide data.

With current technology, a nuclear medicine image is often overlaid on a CT image from a radiation planning system, "which is not ideal," said Dr. Coleman. Correlating a SPECT/PET/ anatomic map image with a radiation planning CT could improve the accuracy of radiation therapy targeting. In addition, smart algorithms will be created to use the x-ray transmission data for better reconstruction of SPECT images, said Dr. Coleman.

X-ray anatomic map: not a diagnostic CT image

When asked whether nuclear medicine technologists will need to learn CT imaging skills, Dr. Sandler said that the x-ray image should not be confused with a CT study. "I hope that we identify clearly what we are doing from the start so we don't create a tangled web that we have to untangle over the next few months or years," said Dr. Sandler.

Although the anatomic map image from the GE system is generated by x-ray tomography, it is not the same as a diag-nostic CT study. In fact, at the SNM presentation, Drs. Sandler and Coleman always called the x-ray image an "anatomic map," not a "CT image". The Millennium VG will not be used as a diagnostic CT. The x-ray transmission data are used only for attenuation correction and anatomic mapping.

Every state has different regulations on the use of x-ray sources, noted Dr. Sandler. In Tennessee, for example, nurses can perform simple x-ray procedures after completing a relatively short training program. "We should be able to establish similar training programs for nuclear medicine technologists," he added. Having an x-ray tube in a gamma camera will probably be comparable, in relation to using x-ray equipment in a nuclear medicine department, to x-ray bone densitometry, he added.

Nuclear medicine: "bigger player"

It is possible that functional anatomic mapping may reduce the need for some anatomic imaging studies in a diagnostic workup, said Dr. Sandler, particularly when the main purpose of such studies is to provide anatomic localization of radionuclide-detected lesions. Rather than viewing this new technology as a way for nuclear medicine to take referrals away from other modalities, however, Dr. Sandler sees it more as a technique for improving diagnostic accuracy. "I think it will improve the ability of nuclear medicine to become a 'bigger player," he predicted.