Hybrid Imaging by SPECT/CT and PET/CT: Proven Outcomes in Cancer Imaging

https://doi.org/10.1053/j.semnuclmed.2009.03.003Get rights and content

The last decade has seen the development of hybrid imaging technologies combining positron emission tomography (PET) or single-photon emission computed tomography (SPECT) with x-ray computed tomography (CT). Numerous studies demonstrate the superiority of PET/CT and SPECT/CT over stand-alone PET and SPECT in terms of diagnostic accuracy. For PET with 18F-fluorodeoxyglucose (FDG), this has been demonstrated for bronchial carcinomas, high-grade lymphomas, melanomas, and head and neck tumors, to name a few. Combined imaging of structure and biochemistry is expected to be even more important for tracers such as 124I that are more specific for tumor tissue. Similarly, SPECT/CT has revolutionized the field of conventional nuclear medicine. Available evidence indicates that this hybrid imaging technology will become the gold standard for conventional scintigraphy, including bone imaging performed for staging malignancy, and also for the so-called tumor scintigraphies that visualize neoplastic foci via tumor-specific agents such as octreotide labeled with 111In or 131I. Another important indication for SPECT/CT is sentinel lymph node scintigraphy, where SPECT/CT fusion helps considerably in localizing the first lymph node draining a tumor. Technological progress never stands still; hybrid cameras combining PET and MRI have already been introduced. These systems will lead medical imaging to new horizons, and they will offer the virtually unlimited potential of simultaneously acquiring morphologic, functional, and molecular information about the living human body.

Section snippets

Positron Emission Tomography/X-ray Computed Tomography

The essential prerequisite for PET/CT investigation is a PET indication. (The CT part of the investigation may be adapted, the indication of a stand-alone CT investigation.) Therefore, we refrain from repeating all possible PET indications, which are essentially the same in PET/CT.

Usually, oncologic positron emission tomography (PET) uses the radioactively labeled glucose analog 18F-fluorodeoxyglucose (FDG). The investigation is based on the principles discovered by Warburg et al2 that tumors

Methods

PET and CT are well-established diagnostic procedures. PET focuses on biochemical characteristics of diseases and may afford poor special resolution, whereas CT mostly relies on imaging morphology and structure and needs to have a very high spatial resolution. Thus, the combination of the 2 is very promising, especially with regard to synergistic effects. The advantage is exclusively on the side of PET. The high quality CT image is quickly acquired and can be used for attenuation correction of

Methodological Problems

Using CT data for attenuation correction for PET as performed in PET/CT will automatically lead to an impairment of PET images by artifacts already present in CT. However, the probability and the extent of artifacts caused by spatial and temporal misalignment of the PET and the CT image depend on the applied protocols and the thoroughness of their performance.

Local misregistration between the CT and the PET in integrated PET/CT or the use of CT contrast media may lead to artifacts in

Impact of PET/CT in Oncologic Staging and Restaging

In a review reflecting the improvements in cancer staging with FDG-PET/CT, Czernin et al26 concluded that improvements in the staging and restaging accuracies of FDG-PET/CT over FDG-PET or CT alone for different cancers have now been established. These improvements are frequently statistically significant and average about 10%-15%. Patients with cancers of the head and neck, thyroid, breast, lung, and gastrointestinal tract; cancer of unknown primary; lymphoma; and melanoma especially benefit

Application of Tracers Beyond FDG

Parallel to the development of, and probably stimulated by, PET/CT, there has been a diversification of the application of positron emitter labeled radiopharmaceuticals. Past and ongoing radiopharmaceutical developments led to numerous PET tracers beyond FDG. Regardless of tracer, PET/CT proves to be of special value—compared to PET alone—both for tracers that show highly specific accumulation and for tracers that, by contrast, show a low specific accumulation with an inhomogeneous background

Single-Photon Emission Computed Tomography/X-ray Computed Tomography

The advantages of PET/CT hybrid imaging described in the previous section of this article should also hold true for cameras integrating SPECT and CT. Therefore, in 2000, SPECT/CT hybrid cameras were introduced in the market, integrating a two-headed state-of-the-art SPECT camera with a nonspiral low-dose CT scanner.44, 45 The CT images from these cameras are of nondiagnostic quality; nevertheless, they allow a fairly accurate localization of foci of pathologic tracer uptake and attenuation

Miscellaneous Tumor Imaging Procedures

123I-meta-iodobenzylguanidine accumulates in cells via the action of the reuptake transporter for norepinephrine. This protein is expressed in a minority of neuroendocrine carcinomas, benign and malignant pheochromocytomas, and neuroblastoma, a malignant tumor affecting children.48, 79 Studies have highlighted the potential of SPECT/CT for this tracer also, similar to what has been published for 131I- and somatostatin receptor-SPECT (Fig. 8).80, 81, 82, 83 Here again, hybrid imaging combining

Sentinel Lymph Node Scintigraphy

Sequential scintigraphy performed after intra- or peritumoral injection of 99mTc-labeled colloids allows the identification of the first lymph node draining a tumor, ie, the sentinel lymph node (SLN). After its removal by radio-guided surgery, it is examined histopathologically. If it does not harbor tumor cells, lymphatic tumor spread is highly unlikely, and the patient can be spared the lymphatic basin dissection frequently associated with severe lifelong side effects such as lymphedema and

Use of SPECT/CT for Dosimetry of Molecular Radiotherapy

SPECT suffers from attenuation artifacts.114 These lead to considerable inaccuracies in the quantification of the concentration of therapeutic radiopharmaceuticals in neoplastic foci. Therefore, SPECT is as yet rarely used for dosimetry of molecular radiotherapy. CT-based attenuation correction of SPECT has now become possible. First data show that it is quantitatively exact for tracers labeled with 99mTc or 111In115, 116, 117; data for its performance for 131I are still missing.

Studies using

Conclusion

The last decade has seen the introduction of PET/CT and SPECT/CT as the first techniques capable of simultaneously acquiring information on the structural and molecular alterations caused by disease. Soon after development of these techniques, a wealth of data on the superior diagnostic value of these hybrid imaging technologies was collected and published, particularly in oncology.

PET/CT has already made stand-alone PET obsolete. The authors expect a similar development to occur in the field

Acknowledgments

Professor Kuwert lectures on behalf of Siemens Medical Solutions. From 2005 to 2007, his institution was lent a SPECT/spiral-CT system by this company. Dr Schmidt and Professor Kuwert acknowledge helpful discussions with PD Dr Rainer Linke from the Clinic of Nuclear Medicine in Erlangen. They are deeply indebted to the late Professor Werner Bautz, without whose continuous and generous support correlative imaging in Erlangen would not have been possible.

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