Todd M. Blodgett

PET/CT imaging in recurrent head and neck cancer

PET/CT offers advantages over PET alone, which is limited by poor anatomic localization and CT alone, which provides morphologic data only. Retrospective fusion of separately acquired PET and CT images allows for potential fusion misregistration in the mobile head and neck between imaging sessions. Indications for PET/CT include recurrent neoplasm, tumor surveillance, and staging. This article will focus on recurrent head and neck neoplasm including, head and neck cancer, thyroid cancer, recurrent skull base tumor. PET/CT may change management in facilitating earlier detection of recurrence than is possible with conventional CT or MR imaging, in guiding biopsy, and in detecting second primary sites and distant metastases. Limitations of PET/CT include physiologic uptake, metabolically active tissue, and muscle contraction during uptake phase. PET/CT, however, is better equipped than is PET alone to mitigate these limitations by precisely localizing FDG uptake to anatomic structures. In addition, small lesions (< 1 cm) may be below scanner resolution and, therefore, a lower SUV (that is < or = 3), may suggest neoplasm. Recent treatment may result in false negative findings, especially when PET is performed within 4 months of radiation therapy. Finally, tumors of low metabolic activity (e.g., salivary gland tumors) may be prone to false negative results. In the future, PET/CT imaging will become more useful in staging head and neck cancer with improved scanner resolution. Development of specific tumor markers may allow for tumor-specific ligands that will increase sensitivity to head and neck neoplasia. Treatment targeting for radiation therapy is an application that is likely to become widely used.

Diffuse bone marrow uptake on whole-body F-18 fluorodeoxyglucose positron emission tomography in a patient taking recombinant erythropoietin.

F-18 fluorodeoxyglucose (FDG)–positron emission tomography (PET) is used extensively in oncology to diagnose, stage, and restage patients with various malignancies. Many patients treated for malignancies develop neutropenia secondary to marrow suppressive chemotherapy and are subsequently treated with synthetic hematopoietic growth factors (HGF), both granulocyte–macrophage colony-stimulating factor (GM-CSF) and granulocyte–colony-stimulating factor (G-CSF). Patients taking HGF can present a diagnostic challenge for those interpreting PET because they can demonstrate diffuse marrow uptake on FDG-PET scans, mimicking diffuse bone marrow metastases. It has not been reported whether bone marrow uptake is affected on PET scans in patients taking erythropoietin, the erythroid-specific cell-line stimulator. We report a case of extensive diffuse bone marrow uptake in a 77-year-old man with a history of colon cancer who began taking erythropoietin 3 weeks before his PET scan. This case demonstrates the need to consider erythropoietin in the differential diagnosis of possible etiologies causing diffuse bone marrow uptake on PET scans.

Does reducing CT artifacts from dental implants influence the PET interpretation in PET/CT studies of oral cancer and head and neck cancer?

In patients with oral head and neck cancer, the presence of metallic dental implants produces streak artifacts in the CT images. These artifacts negate the utility of CT for the spatial localization of PET findings and may propagate through the CT-based attenuation correction into the PET images. In this study, we evaluated the efficacy of an algorithm that reduces metallic artifacts in CT images and the impact of this approach on the quantification of PET images. Methods: Fifty-one patients with and 9 without dental implants underwent a PET/CT study. CT images through the patients dental implants were reconstructed using both standard CT reconstruction and an algorithm that reduces metallic artifacts. Attenuation correction factors were calculated from both sets of CT images and applied to the PET data. The CT images were evaluated for any reduction of the artifacts. The PET images were assessed for any quantitative change introduced by metallic artifact reduction. Results: For each reconstruction, 2 regions of interest were defined in areas where the standard CT reconstruction overestimated the Hounsfield units (HU), 2 were defined in underestimated areas, and 1 was defined in a region unaffected by the artifacts. The 5 regions of interest were transferred to the other 3 reconstructions. Mean HU or mean Bq/cm3 were obtained for all regions. In the CT reconstructions, metallic artifact reduction decreased the overestimated HUs by approximately 60% and increased the underestimated HUs by approximately 90%. There was no change in quantification in the PET images between the 2 algorithms (Spearman coefficient of rank correlation, 0.99). Although the distribution of attenuation (HU) changed considerably in the CT images, the distribution of activity did not change in the PET images. Conclusion: Our study demonstrated that the algorithm can enhance the structural and spatial content of CT images in the presence of metallic artifacts. The CT artifacts do not propagate through the CT-based attenuation correction into the PET images, confirming the robustness of CT-based attenuation correction in the presence of metallic artifacts. The study also demonstrated that considerable changes in CT images do not change the PET images.

18F-FDG Uptake in an Ovary Containing a Hemorrhagic Corpus Luteal Cyst: False-Positive PET/CT in a Patient with Cervical Carcinoma

3Department of Neurology, University of Pittsburgh Medical Center, Pittsburgh, PA 15213. ET with 18F-FDG combined with CT (PET/CT) enables precise localization of FDG uptake to particular structures and is useful for the detection and staging of several malignancies [1–3]. However, benign processes also can result in false-positive 18F-FDG uptake [4, 5]. We report the case of a 31-year-old woman with cervical carcinoma who underwent PET/CT that showed 18F-FDG uptake in an ovary that was misinterpreted as metastatic disease. At surgery 12 days later, the ovary contained a hemorrhagic corpus luteal cyst that was the likely cause for the 18F-FDG accumulation.

Brown fat: atypical locations and appearances encountered in PET/CT.

OBJECTIVE The purpose of this article is to review the physiology and describe the typical and atypical presentations of brown fat on (18)F-FDG PET. CONCLUSION The presence of brown fat on FDG PET has the potential to lead to misinterpretation and unneeded invasive tests, which can be avoided by using measures such as ensuring the patient is warm, reducing FDG uptake in brown fat before the procedure, and correlating PET uptake to a specific anatomic location with PET/CT fusion imaging.

PET/CT artifacts

There are several artifacts encountered in positron emission tomography/computed tomographic (PET/CT) imaging, including attenuation correction (AC) artifacts associated with using CT for AC. Several artifacts can mimic a 2-deoxy-2-[18F] fluoro-d-glucose (FDG) avid malignant lesions and therefore recognition of these artifacts is clinically relevant. Our goal was to identify and characterize these artifacts and also discuss some protocol variables that may affect image quality in PET/CT.

Best Practices: Consensus on Performing Positron Emission Tomography-Computed Tomography for Radiation Therapy Planning and for Therapy Response Assessment

The incorporation of positron emission tomography-computed tomography (PET-CT) into oncological imaging has expanded rapidly since the hybrid scanners were introduced approximately 10 years ago. PET-CT is becoming the standard of practice for the imaging diagnosis and staging of most cancers. Since its introduction, hardware-registered PET and CT images produced by a PET-CT scan were recognized as valuable not only for detection, staging and restaging applications but also for optimizing radiation treatment planning. Even before the introduction of PET-CT, the value of metabolic imaging with the use of FDG PET was recognized as a potentially powerful means of assessing response to various therapies, particularly chemotherapy regimens. To better understand the optimal use of PET-CT in radiation therapy planning and the role of PET-CT in assessing response to therapy, we invited experts from various disciplines to participate in focus group meetings that took place in 2009 and 2010. The Symposia focused on the use of PET-CT imaging in radiation therapy planning (2009) and the use of PET-CT in therapy response assessment (2010). This article will summarize areas of consensus reached by the group regarding many of the discussion topics. The consensus summaries covered in this article are meant to provide direction for future discussions on how to improve the application of this hybrid modality to optimize patient care.

Retroperitoneal fibrosis as a cause of positive FDG PET/CT.

This article educates the reader on idiopathic retroperitoneal fibrosis and emphasizes the importance of considering conditions that mimic this disease on PET/CT through an illustrative case of a 58 year-old man presenting with intermittent abdominal, back, and scrotal pain undergoing successive PET/CT scans, both for diagnosis and following treatment.

Calcified Lymph Nodes Causing Clinically Relevant Attenuation Correction Artifacts on PET/CT Imaging

There are several artifacts unique to PET/CT imaging, with CT-based attenuation correction (AC) artifacts being among the most commonly reported. AC artifacts from calcified lymph nodes represent clinically significant and easily misinterpreted PET/CT artifacts that have received little attention in the literature. In this case series, we report three cases of calcified lymph nodes causing an AC artifact and one case of a highly calcified lymph node without an AC artifact. All three cases of calcified lymph nodes causing an AC artifact would have resulted in a change in patient staging, and likely management, if the nodes had been misinterpreted as malignant nodes. In PET/CT imaging, this artifact needs to be considered as a potential cause of apparent FDG activity when calcified lymph nodes are present on the CT portion of a PET/CT study in order to avoid misinterpretation and potential patient mismanagement.

Scatter Correction Requirements for Likelihood-Based Attenuation Artifact Correction in PET

Artifacts have been observed in CT-based, 511-keV attenuation images for PET attenuation correction. Artifact detection and correction algorithms are being developed based on the Poisson log likelihood function (LLF). The LLF quantifies the level of consistency between an activity and attenuation image pair and the measured emission sinogram. We have shown that the LLF can discriminate more-artifactual from less-artifactual attenuation images at clinical noise levels. Here we use a digital anthropomorphic phantom to examine the effect of scatter on LLF performance. Base sinograms were formed by forward projecting a true activity distribution through a true attenuation distribution. Scatter was simulated by filtering the base sinograms. Noise was considered by producing 128 Poisson-randomized variates of the filtered base sinograms for each of 7 assumed count levels. An artifactual attenuation image was produced by modifying mu-values of regions of the true attenuation image. For each sinogram, two OSEM reconstructions were performed and two LLF values obtained using the artifactual and true attenuation. Accuracy was calculated as the fraction of cases in which the LLF preferred the artifact-free attenuation image. Additional studies were performed to test several scatter correction procedures. Findings show that scatter may render the LLF unreliable even in the limit of high-count scans but that its usefulness is restored by applying an appropriate correction.