Hybrid Imaging Techniques

Introduction

Hybrid imaging is most commonly known as the combination of functional images from Positron Emission Tomography (PET) or Single Photon Emission Computed Tomography (SPECT) with anatomical images from Computed Tomography (CT) or Magnetic Resonance Imaging (MRI). These techniques have gained widespread use within the last decade, while systems obtaining both radionuclide images and anatomical images are commercially available [ 1 ].

Fusion of images obtained from different anatomical imaging modalities and combinations of cross-sectional images and endoscopical or surgical images has also become available within recent years. Thus for some years, ultrasound (US) images and endoscopical ultrasound (EUS) images have been included in hybrid imaging systems with CT, MRI, PET/CT, endoscopical and intraoperative images resulting in new perspectives for diagnosis and therapy benefitting from the strengths of each method. US or EUS may add specific benefit to hybrid imaging because of the live images, the lack of radiation exposure and the ease of image-guided interventions, but in some of the newer techniques also for intraoperative navigation.

Within recent years, several commercially available US/EUS systems with incorporated software for hybrid imaging have become available. To our knowledge, all are based on a magnetic positioning system including software in the US system, a magnetic transmitter placed close to the patient and magnetic sensor(s) attached to the transducer. The magnetic positioning system spatially tracks the transducer, and the previously recorded CT/MRI data set can be reformatted to fit the actual US scan plane. The commercially available systems do not yet provide fusion with endoscopical or surgical images.

Thus, pre-procedure CT/MRI images are uploaded into the US scanner and 3D reconstructions are performed from the 3D dataset, followed by co-registration with US/EUS images in real-time based on the magnetic positioning tracking of the US or EUS probe. Different co-registration methods using external markers or internal anatomical landmarks, or even automatic co-registration, have been proposed and are still under development.

Due to the magnetic positioning system, it is possible to mark target lesions on one image and the marker will be shown on the corresponding image from the other modality. While moving away from the target lesion, colored boxes indicate the distance from the target lesion. This is an advantage when more than one lesion is present, for e.g. in the liver, and differentiation between them is necessary. It may also be helpful in combination with realtime elastography to distinguish between multiple lymph nodes for instance in esophageal, gastric or lung cancer.

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