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Hybrid SPECT/CT Imaging in Clinical Practice

2006 Edition, May 22, 2006

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Active, Most Current

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ISBN: 978-0-8247-2854-0
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Product Details:

  • Revision: 2006 Edition, May 22, 2006
  • Published Date: May 22, 2006
  • Status: Active, Most Current
  • Document Language: English
  • Published By: CRC Press (CRC)
  • Page Count: 261
  • ANSI Approved: No
  • DoD Adopted: No

Description / Abstract:


Nuclear medicine imaging in general, and single-photon emission computed tomography (SPECT) in particular, enable the diagnosis of a physiologic or pathophysiologic process by providing high target to background images that have high specificity and hence only sparse anatomic detail.

To improve the anatomic localization of the functionally active process identified by the nuclear medicine procedures, investigators have used a variety of methods to coregister scintigraphy with other imaging techniques. The history of fusion evolved from visual comparison of function and anatomy. Physicians with a primary interest in image interpretation, as well as those involved in patient care, have relied for decades on their mental capabilities to integrate information obtained from different diagnostic modalities, performed separately, exhibited side-by-side, or recorded in their memory. Further improvements in this crude type of fusion have been sought by drawing regions of interest on operator-visually-aligned slices.

First real image fusion was obtained by coregistering results of separately performed studies, usually nuclear medicine and computed tomography (CT), using dedicated software and workstations. Coregistration of functional and anatomic images is technically feasible and its clinical utility has been demonstrated in ‘‘fixed'' organs such as the brain, but is more difficult in other regions of the body. The separate acquisition of SPECT and CT, at times days apart, makes superimposition of the two images difficult. Even with the use of fiducial markers, it is impossible to reproduce exact positioning during the gamma camera and CT acquisition. The need to manipulate the data using sophisticated software is also problematic. Discrepancies between functional and structural data that were detected following coregistration could therefore be attributed to either technical problems of coregistration or to clinically significant findings.

Because coregistration techniques were considered cumbersome, they did not gain wide application and were excluded from routine clinical use. The technique has been limited to a few centers with dedicated medical physics groups, and a few highly skilled clinical teams performing dedicated research involving image fusion. In order to be widely accepted, coregistration has to use simple acquisition and processing technology, it has to stand up to proven and accepted quality control criteria, and it needs to be readily accessible to review. Based on, and inspired by the pioneering work of Bruce Hasegawa and his team, hybrid imaging has become a welcomed reality.

In 1999, the first hybrid SPECT/CT imaging system (Millenium VG & HawkeyeTM, General Electric Healthcare Technologies) was introduced. It consisted of a dual-detector gamma camera combined with a single-slice, low-power X-ray CT system. Each modality is imaged separately and sequentially during a single patient examination. The transmission data obtained with the CT component are useful for attenuation correction of the emission SPECT data and also as an anatomic template on which the scintigraphic images are superimposed in order to provide a fused image of the two modalities.

The concept of using X-ray–based attenuation correction of SPECT images was initiated by the need to improve imaging of the heart. CT-attenuation corrected myocardial perfusion scintigraphy is considered at present the state-of-the-art modality in this field. Within a short time, however, hybrid imaging has opened a whole new world of opportunities. It permits a better understanding of the role of imaging in the assessment of disease, as well as the impact it generates on patient care.

SPECT/CT, as well as positron emission tomography/computed tomography (PET/CT), have been rapidly proven their value in tumor imaging. Simultaneous assessment of SPECT and CT images provides previously unavailable information on the nature, size, and localization of tumors, and improves the diagnostic ability and accuracy of in vivo imaging of neoplasms. Hybrid imaging provides an explanation for several phenomena previously observed in cancer patients but not clearly understood, for example, the fact that a tumor mass is not necessarily the only finding seen in tumor imaging. Also, with the help of hybrid imaging, early detection of cancer relapse can be obtained before an abnormal mass is detected by conventional, anatomic imaging modalities. Hybrid imaging enhances the awareness to a possible dissociation between mass and disease in cancer, which may further alter management of the individual cancer patient. It is obvious today that hybrid imaging, in only a short period of six years, has led to a change in the routine concept of noninvasive assessment of cancer.

With hybrid imaging, unique physiologic information benefits from a precise topographic localization. Over many decades, nuclear medicine and anatomic imaging (such as plain X-rays, computed tomography, and ultrasound) have been viewed as competitive modalities and a tremendous effort has been invested to prove the advantages of one technique over the other. The medical community has now come to the understanding that none of these modalities provide the only answer to complex clinical questions. The need for simultaneous evaluation of anatomic and metabolic information about normal and disease processes, using registration of complementary data, has evolved from these important clinical requirements.

While this book was in the final editing stages, new, second-generation SPECT/ CT devices have become available for clinical use and research, characterized mainly by improvements at the CT end of the system (Symbia, Siemens Medical Systems; Precedence, Philips Medical Systems, Hawkeye-4 & Infinia LS, GE Healthcare).

This volume includes chapters that illustrate the current state-of-the-art clinical SPECT/CT imaging. In many instances, the technique is essential for accurate anatomic localization of radiotracer accumulation. This is particularly relevant to tumor imaging using radiopharmaceuticals such as 111In-octreotide, 111In-capromab pendetide (ProstaScint®), 131I-MIBG,131I as NaI,67Ga, and 99mTc-colloid agents for lymphoscintigraphy, as well as for non-oncologic examinations for diagnosis of parathyroid adenoma and infectious processes, or bone scintigraphy, all representing procedures with great pathophysiologic specificity. Paradoxically, this apparent advantage is also the limiting factor as it consequently results in very little nonspecific tracer distribution that may enable anatomically localizing a suspicious focus of uptake.

It is remarkable that even in cases where there is reasonable identification of anatomic structures on nuclear medicine procedures, the availability of hybrid fusion images improves diagnostic accuracy and raises the level of confidence of the interpreting physician and referring clinician. An additional advantage of hybrid imaging, although difficult to quantitate, is the fact that combining anatomic mapping and radiotracer distribution reduces the gap between experienced nuclear medicine readers and trainees, as well as radiologists who do not specialize in nuclear imaging, and furthermore makes the doubts of the clinicians confronted with ‘‘scintigraphic dots'' disappear.

This volume is also privileged to include contributions regarding future applications of our specialty. The use of SPECT/CT for dosimetry calculations is of utmost importance in an era when radioimmunotherapy is becoming an important therapeutic tool in cancer patients. Also, small animal metabolic and hybrid imaging is a new and valuable tool in reaching an understanding of basic biological processes and subsequently in new drug developments.

Why did it take longer for SPECT/CT than for PET/CT to conquer the place it deserves in the world of modern imaging? The answer is partially related to technical limitations that are or will be, hopefully, solved in the near future. Also, nuclear medicine physicians have acquired a high level of expertise in reading planar and SPECT studies, and some of us harbored the conservative insight that there was not much more to add.

The editors of this book, as well as the authors of the various chapters, have had the fortunate opportunity to be among the first users of hybrid imaging with SPECT/ CT. This initial work has helped make the difference. As always, success has come from teamwork, from joint efforts between scientists involved in basic research and engineering teams, between academic groups and industry, and between numerous clinical research teams in different corners of the world. None of the above could have been accomplished single-handedly.

We are on the right path, but there is still a great deal of work to do. We need to establish indications for performing SPECT/CT, and the optimal workflow. Risk-tobenefit ratios related to additional radiation also need to be considered.

The way of SPECT/CT from ‘‘toy to tool'' has been a challenging adventure, met with doubts, high expectations, but also with a great amount of appreciation by our colleagues in the imaging and clinical communities. The availability of new SPECT/CT devices, further developments in technology and radiopharmacy, as well as the growing interest in the clinical potential of hybrid SPECT/CT, confirm our belief that there are exciting times ahead in nuclear medicine.

This project could not have been envisioned without the help of good friends in our clinical departments in particular and in the nuclear medicine community, in general. Working together and stirring the enthusiasm of each other has made this effort possible, and to all we extend our deepest gratitude.