Combined PET/MRI: from Status Quo to Status Go. Summary Report of the Fifth International Workshop on PET/MR Imaging; February 15-19, 2016; Tübingen, Germany

This article provides a collaborative perspective of the discussions and conclusions from the fifth international workshop of combined positron emission tomorgraphy (PET)/magnetic resonance imaging (MRI) that was held in Tübingen, Germany, from February 15 to 19, 2016. Specifically, we summarise the second part of the workshop made up of invited presentations from active researchers in the field of PET/MRI and associated fields augmented by round table discussions and dialogue boards with specific topics. This year, this included practical advice as to possible approaches to moving PET/MRI into clinical routine, the use of PET/MRI in brain receptor imaging, in assessing cardiovascular diseases, cancer, infection, and inflammatory diseases. To address perceived challenges still remaining to innovatively integrate PET and MRI system technologies, a dedicated round table session brought together key representatives from industry and academia who were engaged with either the conceptualisation or early adoption of hybrid PET/MRI systems. Discussions during the workshop highlighted that emerging unique applications of PET/MRI such as the ability to provide multi-parametric quantitative and visual information which will enable not only overall disease detection but also disease characterisation would eventually be regarded as compelling arguments for the adoption of PET/MR. However, as indicated by previous workshops, evidence in favour of this observation is only growing slowly, mainly due to the ongoing inability to pool data cohorts from independent trials as well as different systems and sites. The participants emphasised that moving from status quo to status go entails the need to adopt standardised imaging procedures and the readiness to act together prospectively across multiple PET/MRI sites and vendors.

Contrast-enhanced magnetic resonance angiography of peripheral vessels. Different contrast agent applications and sequence strategies: a review

In this article the relation between contrast medium (CM) application and sequence parameters will be discussed with respect to clinical use of the contrast-enhanced magnetic resonance angiography (CE-MRA) in the peripheral vessel region. The adjustment of the sequence parameters, the CM application timing and the bolus geometry is necessary for an effective use of CE-MRA. Investigation protocols for several vascular regions differ mainly corresponding to varying fields of view and slab thickness. Restrictions of increasing the measurement time are expected in peripherally localized vessels if fast arteriovenous transit time occurs. The vessel contrast depends from (1) optimal CM bolus timing and (2) bolus geometry defined by the parameters of the intravenous bolus injection (flow rate, dose and NaCl flush volume). Our study results have shown that the bolus remains compact but also shorter if a higher flow rate is being applied at equal dose. The enlargement of the NaCl flush volume has evidently caused an increased intraarterial CM concentration and a slightly bolus lengthening. The exact timing regimen requires an automated mechanical CM injection pump. In most countries, a total dose of 0.3 mmol/kg Gd is allowed for application during one investigation. Therefore, obtaining an angiogram of the entire iliac and leg region this total dose must be separated. 0.1 mmol/kg for each of the three measurements can be recommended. Otherwise, using this lower CM dose results in less spatial resolution. At least a dosage of 0.2 mmol/kg Gd is necessary to achieve a higher spatial resolution. The calculation of CM dosage should be also related to the dedicated vessel region of interest than to the body weight only.

SPIO-enhanced 2D-TOF MR angiography of the portal venous system: results of an intraindividual comparison

The purpose of this study was to investigate whether MR angiography (MRA) of the portal venous system may be improved by means of superparamagnetic iron oxides (SPIOs) during accumulation phase imaging and to study the underlying contrast mechanisms. MRA of the portal venous system was performed on 48 patients before and after intravenous injection of a new SPIO (Resovist, Schering AG, Berlin, Germany). Resovist, as a predominantly liver parenchymal darkening agent on T2-weighted MR images with uptake into the reticuloendothelial cell system, was administered intravenously by bolus injection of 8 to 12 micromol Fe/kg body weight. Patients were scanned with breath-hold coronal and axial two-dimensional (2D) time of flight (TOF) MRA (TR = 31.0 msec, TE = 9.8 msec, flip angle (FA) = 50 degrees, and 6.9-second acquisition time per section) sequences. Signal intensity values of liver parenchyma, the portal venous system, and background were obtained for quantitative analysis. The clinical relevance of additional plain and contrast-enhanced MRA studies for surgical planning was assessed by independent reading of three readers. Liver signal-to-noise ratio (SNR) significantly decreased following iv injection of Resovist; however, SNR values of the portal veins or hepatic veins did not change significantly. Visibility of the portal venous system improved significantly (tertiary branches visible: pre in 15.2% versus post in 87.0% of patients). Resovist-enhanced 2D-TOF MRA may improve planning of liver resections by better demonstrating the relationship of central liver lesions and vessels on source images. The decrease in liver SNR at a constant vessel SNR after iv injection of Resovist improves MRA of the liver. SPIO-enhanced 2D-TOF MRA scans are superior to plain 2D-TOF MRA studies and may be added for the workup of preoperative patients.

[Combined use of ECK-triggered 2D-phase contrast MR angiography and 2D-time-of-flight MR angiography for planning and follow up before and after vascular intervention of pelvic and leg arteries]

PURPOSE

To develop a strategy for the complete work-up of vessel lumen and vessel wall for planning and follow-up of radiological interventions of lower extremity arteries.

METHODS

A total of 36 patients (21 pre-, 8 post- and 7 pre- and postinterventional) were studied. MRA studies were performed using an ECG-triggered phase contrast technique for the demonstration of intraluminal flow and an axial high resolution time-of-flight technique to assess the vascular wall. All MRA studies were analysed by intraindividual DSA comparison for the assessment of flow and wall structures.

RESULTS

Combined MRA techniques provided a good correlation with DSA for the assessment of vascular flow. The kappa test revealed a value of greater than 0.61 for most on the vessel segments proving a good correlation of both methods. Orthogonal high-resolution TOF-MRA provided additional information for the assessment postinterventional wall haematomas and hard plaques.

CONCLUSION

Combination of PCA to study flow and axial TOF to study wall pathology improves the usefulness of peripheral MRA.

[Phenomenology of pulmonary HRCT. Definitions, differential diagnosis and clinical relevance]

High-resolution computed tomography (HRCT) has proved to be the imaging technique of choice for the lung in numerous studies, allowing precise depiction of normal pulmonary anatomy-including the lobular architecture-as well as relevant pathologic changes. Its sensitivity, specificity and accuracy in this respect are markedly superior, not only to chest radiography, but also to conventional CT. Guidelines for standardized assessment and interpretation of HRCT findings have not been established in the German literature. The purpose of this paper was to develop a systematic classification of pulmonary HRCT findings based on the appearance of morphologic abnormalities (descriptive classification) and their relationship to anatomic structures of pulmonary parenchyma (anatomic classification). Reticular, nodular, and non-reticular/non-nodular densities, as well as lucencies, can be differentiated. Abnormalities of the pleural space are discussed separately. Following the presentation of typical CT findings, morphologic correlation and differential diagnosis are discussed. In addition, we point out indications and specific features of the clinical value of the technique.

[Phase-contrast MR angiography of the lower extremity. Comparison of methods and clinical application]

PURPOSE

To investigate whether phase-contrast MRA is a clinically suited approach to examine arteries of the pelvis and lower extremities.

METHODS

The study was divided into two parts, a volunteer study and patient study. Three MRA techniques-2D TOF with venous saturation, 3D magnitude contrast and 2D phase contrast with ECG triggering-were intraindividually compared in 15 volunteers and evaluated by three blinded readers. Subsequently, a total of 230 vessel segments of 45 MRA studies using ECG-triggered phase contrast were compared with intraarterial DSA. All vessel segments were scored by three blinded readers using a five-point scale with DSA serving as the gold standard.

RESULTS

ECG-triggered phase contrast provided better image quality than the other MRA techniques as assessed by the Friedman test. Clinical studies demonstrated a significant correlation of DSA and MRA as assessed by the Spearman correlation and kappa statistics for individual readers.

CONCLUSION

MRA of the pelvis and lower extremities may be performed with 2D ECG-triggered phase-contrast MRA within a reasonable time frame (< 30 min). MRA slabs provide orientation similar to that with DSA projections and good to very good correlation of vessel pathology as shown by kappa statistics.

[Atypical hypertrophic cardiomyopathy with calcified, left ventricular apical thrombosis]

A case of atypical hypertrophic cardiomyopathy (HCM) with a calcified apical thrombus is presented. A 42 year old asymptomatic patient was admitted for evaluation of an abnormal electrocardiogram (ECG) which was recorded when the patient suffered from a bronchitis. The ECG showed giant negative T-waves in leads II, III, aVF, V3 to V6 associated with high QRS voltages in the precordial leads. The chest X-ray and fluoroscopy demonstrated a calcification in projection to the apical region of the heart. Echocardiography and the left ventricular (LV) cineangiography showed hypertrophy of the apical LV myocardium and an obliteration of the apical LV cavity. Magnetic resonance imaging identified a calcified thrombus in the apical cavity of the LV in the setting of an atypical HCM.

Chondroid tumors arising from the meninges--report of 2 cases and review of the literature

Chondroid tumors are rare intracranial tumors usually arising from the base of the skull. We present 2 cases of intracranial cartilaginous tumors with unusual location. In case 1, a 19-year-old woman, a chondroma of the falx cerebri with extensive secondary ossification was diagnosed. In case 2, a 30-year-old woman, a low-grade chondrosarcoma was resected from the right frontal lobe. Both patients showed an uneventful clinical course without evidence of disease 4.5 and 6 years after total extirpation. Our cases show that chondromas and low-grade chondrosarcomas of the dura and meninges usually occur in young adults with a good prognosis after complete extirpation.