Diagnostic SPECT/CT: All in Favor

SPECT/CT of Thyroid Carcinoma: Figure: 47-year-old woman with residual thyroid tissue after total thyroidectomy for papillary carcinoma of the thyroid. A, Planar 131I image at 24 hours shows two foci (arrows) of central neck activity. B and C, Axial CT (B) and fused SPECT/CT (C) images obtained 48 hours after 131I administration show superior focus (arrow, B) localized immediately anterior in relation to tip of hyoid bone, consistent with thyroglossal duct remnant. Normal thyroid tissue is present along track of thyroid embryologic descent. Images provided by American Roentgen Ray Society.

First there was PET imaging, and then emerged the need for anatomic localization. Thus was born the hybrid PET/CT scanner, which currently dominates the market. The acceptance of SPECT/CT is gaining momentum, as more and more clinical uses for the combination imaging are proving advantageous.

By offering the ability to scan pathophysiology and localize it within the anatomy, SPECT/CT not only adds to physician confidence in suggesting functional/molecular abnormalities, but also provides confidence in defining where the disease is located.

“It’s one thing to say, ‘There is a hot spot in neck,’ its’s another thing to say, ‘There is a hot spot in the neck that is 1 cm anterior to the left carotid artery,’” says Ronald D. Petrocelli, MD, chief medical officer, Molecular Imaging, Siemens Healthcare.
Physicians have found SPECT/CT useful in cardiology, to image infection, and for many cancers including head and neck, endocrine, bone, lung and pelvic.

“We did bone scanning for nonmalignant disease for years before the PET or SPECT/CT cameras were available,” says Petrocelli, who is board certified in both nuclear medicine and radiology. “In the old days, we’d see the hot spot and we’d know the disease was sort of in the region of the vertebrae, for example, but we didn’t know exactly where. With a diagnostic CT image, we can see the appearance of the bone trabeculae, for instance, and whether there is expansion or destruction of the bone. Such information enables physicians to make definitive interpretations, not only to determine what the disease is, but also to determine what the disease is not.”

Thanks to the recent debut at the European Association of Nuclear Medicine annual meeting by GE Healthcare of the Discovery NM/CT 670, a 16-slice CT attached to conventional SPECT, all three major vendors now have diagnostic SPECT/CT. Once cleared for sale in the U.S., this will join the Siemens Symbia True Point SPECT•CT and the Philips Precedence that are already available around the world. This validation of sorts is sure to strengthen SPECT/CT imaging applications in a market thus far dominated by stand-alone SPECT cameras. Why? Perhaps due to the increase in clinical applications offered by hybrid SPECT/CT and the increase in the need to use hybrid imagers as multi-purpose cameras, often serving both radiology and nuclear medicine imaging departments. For example, SPECT/CT raises the bar for well-established myocardial perfusion SPECT imaging evaluations, enabling a comprehensive cardiac evaluation including perfusion, attenuation correction and calcium scoring in about five minutes.

Thyroid cancer
Ka Kit Wong, MBBS, and colleagues from the University of Michigan Medical Center, Ann Arbor, sought to determine incremental value of iodine-131 SPECT/CT over traditional planar imaging of patients with differentiated thyroid carcinoma (AJR 2008;191:1785-1794). They found that ¹³¹I SPECT/CT was useful for accurate evaluation of regional and distant activity in characterizing foci as residual thyroid tissue or nodal, pulmonary, or osseous metastasis. They also concluded that suspected physiologic mimics of disease can be confirmed with increased reader confidence using SPECT/CT.

The study involved 53 patients whose planar scans depicted 130 neck foci and 17 distant foci. SPECT/CT further characterized these as thyroglossal duct and thyroid bed remnant (98), cervical nodal metastasis or local residual disease (26), physiologic activity (11) and distant metastasis (12). Investigators concluded that SPECT/CT’s superior lesion localization and additional anatomic information added incremental diagnostic value over planar imaging in nearly half of foci.

“We routinely perform diagnostic ¹³¹I studies to complete postoperative staging of disease and to guide selection of radioiodine therapy,” Wong says. “Physicians at many centers omit the diagnostic ¹³¹I study and proceed directly to fixed-dose radioiodine therapy. Inherent in this protocol, however, is the inability to complete staging until after the patient has received the therapeutic dose of radioiodine.”

Wong and colleagues also found SPECT/CT useful to distinguish lesion activity from brown adipose tissue uptake (Clin Nucl Med 2008;33:346-348). The case involved a woman with suspected primary hyperparathyroidism, who underwent dual phase technetium-99 sestamibi imaging. Focal uptake in the anterior thorax on SPECT images, which potentially may have been misinterpreted as ectopic parathyroid tissue, was demonstrated on SPECT/CT as uptake in brown adipose tissue. Researchers said the study provides further evidence that other radiopharmaceuticals, such as F-18 FDG, ¹²³I MIBG and Tc-⁹⁹m tetrofosmin, also may accumulate in brown adipose tissue.

SPECT/CT market
Frost & Sullivan reported that sales of SPECT/CT scanners could help “rebuild the North American nuclear medicine equipment market,” which was hit hard by the Deficit Reduction Act of 2005. The combination of physiologic and anatomic imaging will open up “numerous lucrative opportunities in niche markets such as neuroendocrine tumor imaging,” according to the report.

The North American nuclear medicine equipment market earned revenues of $298.4 million in 2007. Frost & Sullivan’s analysis estimates the revenues will reach $325.3 million in 2014.

“With the adoption rate of SPECT/CT progressively increasing, it opens up new market potential for both imaging system vendors and radiopharmaceutical agent companies,” says Frost & Sullivan Research Analyst Travis Chong. “The rising numbers of new niche markets are expected to drive growth in this segment, create opportunities for revenue expansion and establish SPECT/CT as a mainstream tool in nuclear medicine.”

SPECT/CT scanners have been available in one form or another since 2004 and sales of the hybrid scanner are about equal to stand-alone SPECT, which translates into about 10 percent growth per year, says Petrocelli. A change in reimbursement in the United States could help propel sales of the hybrid scanner. Currently, physicians will get reimbursed for a SPECT/CT scan only if the referring physician orders both a SPECT study and a CT exam. If nuclear medicine physicians decide on their own to perform a CT scan, even if it adds significant clinical information, they will not get reimbursed for the CT portion of the study.

Breast & brain cancer
Iris M. C. van der Ploeg and colleagues from The Netherlands Cancer Institute in Amsterdam found SPECT/CT to be useful as an additional lymphatic mapping tool in patients with breast cancer (World J Surg. 2008;32(9):1930–1934). According to the researchers, conventional lymphoscintigraphy does not always define the exact anatomic location of a sentinel node.

In their own experience, the investigators found SPECT/CT provided a “meaningful surgical roadmap,” but there was little literature on the topic. They therefore reviewed the literature and found that the majority of investigators did not formulate indications for additional SPECT/CT after conventional imaging but scanned all patients eligible for sentinel node biopsy. Van der Ploeg and colleagues reported that sentinel nodes were visualized in 89 to 100 percent of studies by combined conventional imaging and SPECT/CT, with sentinel nodes depicted only by SPECT/CT in up to 14 percent.

Katia Rozovsky and colleagues from Hadassah-Hebrew University Medical Center in Jerusalem, found that 123I MIBG SPECT/CT increased the diagnostic certainty in 89 percent of discordant studies of stand-alone CT or SPECT in patients with pheochromocytoma or neuroblastoma. Overall, SPECT/CT provided additional information in eight of the 15 cases. They concluded that in cases of equivocal diagnostic CT, “SPECT/CT bridges the gap between 123I MIBG scintigraphy and diagnostic CT, with guidance of the diagnostic CT and characterization of its findings.”

SPECT/CT offers nuclear medicine physicians reduced scanning time, attenuation correction, increased diagnostic confidence and improved outcomes. The hybrid modality is here to stay and should command more of the market in the coming years.