Focal FDG Uptake in Lung Parenchyma Without Structural Alterations on CT

Serial Migration of Iatrogenic Microembolus on 18 F-FDG PET/CT Images

ABSTRACT

A 51-year-old woman who had a history of partial nephrectomy underwent an 18 F-FDG PET/CT image for a routine health checkup. Focal intense FDG avidity without any anatomical correlation on CT was detected in the lung. On the delayed image after 20 minutes, the focal activity migrated to a more peripheral portion. An iatrogenic microembolus is a rare but crucial false-positive finding that nuclear physicians should be aware of. Our case emphasizes the importance of meticulous FDG injection and cautious interpretation. In addition, delayed PET/CT imaging through wet reading can aid in diagnosing and help prevent unnecessary investigations.

Hot Embolus Artifact Mimicking Disease Progression in Post-therapy 177Lu-DOTATATE Scan: Incremental Value of SPECT/CT

Peptide receptor radionuclide therapy (PRRT) has become an established treatment for patients with inoperable and/or metastatic, well-differentiated neuroendocrine tumors with overexpression of somatostatin receptor type 2 (SSTR-2). The post-therapy 177Lu-DOTATATE whole-body scan not only assesses the biodistribution of the lesions seen on pre-therapy 68 Ga-SSTR PET/CT scan but also provides a quick assessment of disease status and dosimetry during treatment. Like any other radionuclide scan, the whole-body 177Lu-DOTATATE scan may also show abnormal radiotracer uptake, which may require further imaging to establish its exact etiology. Though radiotracer emboli mimicking focal pulmonary lesions have been described with 18F-FDG and 68 Ga-DOTANOC PET/CT scans, similar artifacts with post-therapy 177Lu-DOTATATE scans have not been described. Herein, we report two cases of hot emboli in the post-therapy 177Lu-DOTATATE scans.

The hot embolus of 18F-fluorodeoxyglucose

Scanning oncological patients with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) for their disease staging, evaluation of treatment response, and monitoring/management has become a standard of care. The use of the radioactive fluorine in the FDG molecule helps establish cell/tissue lines high on glucose consumption and hence metabolically active. Abnormalities are detected on the scan as areas of increased uptake. However, these areas of increased (hot) uptakes do not necessarily translate into a pathological finding. A comprehensive knowledge of the uptakes of the tracer and the potential "pitfalls" that may be associated with them should be known and kept in mind during scan reading. One such pitfall is the "hot clot" or "pulmonary emboli," and we report two such cases encountered at our setup and discuss their causes and how they should be identified and avoided.

Less frequent cardiac and extracardiac findings during 99mTc-methoxyisobutylisonitrile myocardial perfusion single-photon emission computed tomography with radiological correlates

Myocardial perfusion single-photon emission computed tomography (SPECT) as a common imaging procedure in nuclear medicine laboratories may encompass findings in the heart or beyond it, in the thorax or abdomen, which may be related or unrelated to the symptoms of the patients. Sometimes, these findings may be discovered incidentally. In the present article, it is aimed to present some less frequent cardiac and extracardiac findings including radiolabeled emboli, transposition of great arteries, breast prosthesis, breast tissue uptake, pericardial effusion, hiatal hernia, hepatocellular carcinoma, ascites, aortic aneurysm, splenomegaly, and polycystic kidney disease during ^99mTc-methoxyisobutylisonitrile myocardial perfusion SPECT with their radiological correlates.

Sub-clinical abnormalities detected by PET/MRI in household tuberculosis contacts

Background

The understanding of early events following TB exposure is limited by traditional tests that rely on detection of an immune response to infection, which is delayed, or on imaging tests with low sensitivity for early disease. We investigated for evidence of lung abnormalities in heavily exposed TB contacts using PET/MRI.

Methods

30 household contacts of 20 index patients underwent clinical assessment, IGRA testing, chest x-ray and PET/MRI scan using 18-F-FDG. MRI images were examined by a radiology/nuclear medicine dual-qualified physician using a standardised report form, while PET/MRI images were examined independently by another radiology/nuclear medicine dual-qualified physician using a similar form. Standardised uptake value (SUV) was quantified for each abnormal lesion.

Results

IGRA was positive in 40%. PET/MRI scan was abnormal in 30%, predominantly FDG uptake in hilar or mediastinal lymph nodes and lung apices. We did not identify any relationship between PET/MRI findings and degree of exposure or IGRA status.

Conclusion

PET-based imaging may provide important insights into the natural history following exposure to TB that may not be available from traditional tests of TB immune response or imaging. The clinical significance of the abnormalities is uncertain and merits further investigation in longitudinal studies.

Beyond Pulmonary Embolism; Nonthrombotic Pulmonary Embolism as Diagnostic Challenges

Nonthrombotic pulmonary embolism (NTPE) is less well understood and is encountered less frequently than pulmonary embolism from venous thrombosis. NTPE results from embolization of nonthrombotic material to the pulmonary vasculature originating from many different cell types as well as nonbiologic or foreign materials. For many radiologists NTPE is a challenging diagnosis, presenting nonspecific or unusual imaging findings in the setting of few or unusual clinical signs. The aim of this paper is to review the pathophysiology of diverse causes of NTPE, which should aid radiologists to better understand and, more importantly, diagnose these infrequent events.

Abnormal Fluorodeoxyglucose Uptake in Lung without Structural Abnormality on Computed Tomography

Fluorodeoxyglucose (FDG) positron emission tomography-computed tomography (PET-CT) is a useful proven imaging modality in the management of many types of cancers. It is being used at various stages of treatment of cancer. Knowledge regarding the physiological biodistribution and false-positive findings should be kept in mind for correct interpretation. Pulmonary FDG uptake can be due to different causes such as infection, inflammation, and metastases which are invariably associated with structural abnormality on CT. In rare circumstances, there can be a focus of FDG uptake in the lung with no corresponding structural abnormality which might be due to an inflammatory vascular microthrombus or due to iatrogenic microembolism caused during the injection of radiotracer. It is important to be aware of this as it can cause difficulty in interpreting the scan and can lead to false-positive findings. It also highlights the importance of hybrid imaging in the form of PET-CT as there is a definite possibility of misinterpreting this as a site of metastasis in a known carcinoma patient if there was no corresponding CT image.

Challenges in Interpretation of Staging PET/CT in Thoracic Malignancies

¹⁸F-fluorodeoxryglucose (¹⁸F-FDG) PET/CT imaging is routinely performed in the evaluation of patients with known or suspected thoracic malignancy. Indications for its use include staging of malignancy, assessment of response to therapy, evaluation of suspected disease recurrence, and evaluation of a solitary pulmonary nodule. In this article, we will discuss specific technical artifacts and also review potential pitfalls in the interpretation of PET/CT in thoracic malignancies including normal variations in physiologic uptake of FDG, benign conditions (such as infection, inflammation, posttreatment changes, and iatrogenic factors) that can result in increased FDG uptake, and malignancies that demonstrate scarce to no FDG uptake.

18F-FDG PET/CT lung 'focalities' without coregistered CT findings: an interpretative clinical dilemma

AIM

The aim of the study was to assess the prevalence of focal fluorine-18 fluorodeoxyglucose (18F-FDG) activity in the lungs using 18F-FDG PET/computed tomography (CT) without matching CT findings.

MATERIALS AND METHODS

A total of 10,500 consecutive 18F-FDG PET/CT records over 4 years were reviewed for focal incongruence between PET and CT, potentially indicating an artifact 18F-FDG (2.2 MBq/kg) was injected through a butterfly needle, followed by a 10 ml saline flush. Non-contrast-enhanced low-dose CT (140 kV and 40-80 mA) was coregistrated with PET.

RESULTS

Sixteen patients (12 men and four women; mean age 63 years, range 44-83) had focal pulmonary areas of high 18F-FDG activity [mean maximum standardized uptake value (SUV max) 15.8; range 3.5-81.0], typically peripheral, with a mean maximum diameter of 1.3 cm (range 0.5-2.2 cm) on PET. 18F-FDG focality was singular in 14 patients, whereas two patients had two foci each. None had corresponding CT abnormalities. Identification of these 'focalities' during the acquisition phase led to late respiratory gated thoracic PET acquisitions in eight patients at 2 h with no significant changes in the location or size of the 'focalities'. Five PET/CTs repeated at 48 h did not confirm the 'focalities'. Five had negative follow-up contrast-enhanced CT. 18F-FDG-positive 'focalities' at PET/CT without anatomical correlation findings were considered as 'artefactual accumulation' of the tracer.

CONCLUSION

In the absence of morphological abnormality, focal pulmonary 18F-FDG activity is very rare (1.5 cases/1000 PET scans) but potentially very 'dangerous'. Artefact identification during acquisition can lead to late respiratory gated images for more confident interpretation, avoiding erroneous reports or further imaging procedures.