Agreement rates among single photon emission computed tomography using gallium-67, computed axial tomography and lymphangiography for Hodgkin disease and correlation of image findings with clinical outcome

Revisiting gallium-67 imaging: investigation of the energy photopeaks

BACKGROUND

Ga-67 has multiple energy emissions of different abundances and the quality of images is affected by these inhomogeneous energy distributions. Thus the purpose of this study was to investigate whether two or three energy peaks of Ga-67 would be the most appropriate setting in terms of γ-camera performance and image quality measurements.

MATERIALS AND METHODS

The physical performance of the γ-camera using Ga-67 has been tested; in particular, the effect of different gallium energy settings (two vs. three energy photopeaks) on the extrinsic and intrinsic uniformity and resolution of the γ-camera. The multicontrast/resolution phantom (OPRAXMEDICAL model 74-345) has been used to compare the two energy settings. The average contrast of the lesions and the contrast-to-noise ratio were measured and the significance of the result was determined using the paired Student's t-test. To test the effects of size and contrast on the detectability of defects, contrast detail curves have been generated for both the acquisition energy settings. For the optimization of the energy window, multiple static liver phantom images were acquired using the dual-energy photopeak acquisition method. However, each image was taken at different symmetric energy window combinations and the window settings were (5, 10, 15, and 20%) for 92 and 184 keV, respectively.

RESULTS

For the two energy window acquisitions, the intrinsic uniformity calculated by the integral and differential equations for the useful field of view was 2.26 and 1.39%, respectively. However, for the three energy windows, the values were 3.04 and 1.81%, respectively. The integral and differential values for the dual-energy flood images were 10.97 and 3.75%, respectively, whereas the integral and differential values for the triple-energy flood images were 13.3 and 4.13%, respectively. A slight improvement in the system's intrinsic spatial resolution was observed in the dual-window setting with a full width at half maximum of 9.24 mm compared with 10.42 mm for the triple-energy acquisition. The full width at half maximum was not drastically increased in the three window acquisitions, but the full width at tenth maximum was found to be larger. The two energy acquisitions yielded a significant improvement by 20.5 and 22% in the mean contrast of the planar images of the phantom filled with 2 and 5 mCi of Ga-67, respectively (P value <0.01). Single-photon emission computed tomography images acquired for the liver phantom showed an improvement in the mean contrast by 29 and 27.6% using two windows (P<0.001) for 2 and 5 mCi activity concentration, respectively. Unlike planar imaging, single-photon emission computed tomography images yielded a 45% increase in the contrast-to-noise ratio using the two window acquisitions with a mean CNR of 5.24±3.47 and 2.9±2.23 for two and three windows respectively; P value less than 0.05. The contrast detail curves showed better detectibality for small-sized lesions and lower contrast when using dual-energy settings. The best contrast, on average, was achieved using 10% for 92 keV and 5, 10, 15, and 20% for 184 keV, respectively, and it was improved by 16, 24, and 35% compared with other settings.

CONCLUSION

A significant improvement in image quality can be achieved by simply applying different window widths over the different photopeaks. Our results indicated that two photopeaks with optimized window widths can significantly outperform three windows in terms of spatial and contrast resolution and lesion detectability with a relatively negligible reduction of count sensitivity.

Hodgkin's lymphoma--patient's assessment and staging

Hodgkin's lymphoma is one of the most curable malignancies today. But treatment is associated with significant toxicity. The objective of high-quality management is to minimize treatment exposure while maximizing cure of disease. Principles of cancer staging and patient's assessment taxonomy are important to improve communication. An orderly patient evaluation and systematic recording of disease extent using the Ann Arbor classification forms the basis for treatment decision, response assessment, and clinical trials. The practice of staging in Hodgkin's lymphoma evolved over the past 40 years from clinical examination and plain imaging to modern anatomic and functional imaging. Although useful in the past, staging laparotomy, lymphangiograms, and Gallium scintigraphy have now been abandoned. Computerized tomography combined with 2-[18F]fluoro-2-deoxyglucose-positron emission tomography form the basis for anatomic disease extent assessment. Although patients' evaluation and staging at diagnosis are important, the management of Hodgkin's lymphoma involves a complex series of algorithms requiring interim and overall response assessment, careful follow-up, repeat assessment, and salvage management of recurrent disease.

Positron emission tomography/computed tomography: diagnostic accuracy in lymphoma

An accurate initial staging of patients with non-Hodgkin lymphoma (NHL) and Hodgkin lymphoma (HL) is critical for the selection of an appropriate treatment. Computed tomography (CT) remains the standard imaging technique, although it is based on anatomic criteria. Positron emission tomography (PET) with 2-deoxy-2-[fluorine-18]fluoro-d-glucose (FDG) provides useful functional information but requires anatomical correlation to localise lesions accurately. We have prospectively compared the accuracy of combined PET/CT with that of CT and PET alone at initial staging in lymphoma patients. Forty-seven newly diagnosed patients were evaluated. PET/CT was superior compared with CT and PET in nodal evaluation and detection of extranodal disease. Using a staging algorithm with PET/CT resulted in the disease stage being increased in 11 of 47 patients (10 NHL and 1 HL) (McNemar test P = 0.012). Therefore, a different treatment strategy based on PET/CT findings was suggested for seven patients (14.8%). PET/CT markedly improves accuracy in the diagnostic work-up of patients with lymphoma.

Gallium-67 scintigraphy: a cornerstone in functional imaging of lymphoma

Until recently, gallium-67 scintigraphy (GS) has been the best available functional imaging modality for evaluating patients with non-Hodgkin's lymphoma (NHL) and Hodgkin's disease (HD). The diagnostic accuracy of GS in detecting lymphoma is based on optimisation of the imaging protocol, knowledge of potential physiological and benign sites of (67)Ga uptake, and the Ga avidity characteristics of the individual lymphoma. As (67)Ga is a tumour viability agent, the role of GS is primarily at follow-up. A residual mass persisting on CT after treatment poses a common clinical dilemma: it may indicate the presence of viable lymphoma, which requires further treatment, or it can be benign, consisting of only fibrotic and necrotic tissues. GS can successfully differentiate between these conditions. Routine follow-up with GS may allow early diagnosis of recurrence and early institution of treatment. Reversion of a positive GS to a negative test, and the rapidity with which this occurs has a high predictive value for the outcome of the individual patient. Lymphoma showing a normal GS early during treatment has a better prognosis than lymphoma with persistence of pathological findings. Other tumour-seeking single-photon emitting agents, such as thallium-201, technetium-99m methoxyisobutylisonitrile and indium-111 octreotide, have been investigated in lymphoma, primarily as an alternative to GS in specific clinical settings, but are of limited value. The role of radioimmunoscintigraphy is gaining importance in conjunction with radioimmunotherapy. Fluorine-18 fluorodeoxyglucose (FDG) imaging of lymphoma using either dedicated or camera-based PET systems is gradually replacing GS for assessment of lymphoma. FDG overcomes some of the limitations of GS while sharing its tumour viability characteristics. The extensive clinical knowledge and experience accumulated over three decades with GS in lymphoma provides a solid background as well as a model for the assessment of new functional imaging techniques.