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.

Transabdominal Ultrasound

Hybrid imaging involving real-time US has been evaluated in different anatomical areas.

The commercially available systems have mainly been tested in the abdominal setting in animal/phantom models and humans, mostly for imaging of the liver (Fig. [ 1 ]) or kidney (Fig. [ 2 ]). The method has a high accuracy in phantoms and bovine livers [ 2 ], [ 3 ] and was regarded useful for detection of small, difficult to visualize lesions in the liver and for evaluation of hepatocellular carcinoma (HCC) ablation, especially in combination with contrastenhanced US (CEUS). The advantage of hybrid imaging in the liver was the possibility of better overview in areas normally considered difficult to visualize sonographically (the liver dome and anterior thoracic wall) (Fig. [ 3 ]); furthermore, it enabled visualization of lesions that could not be seen in conventional B-mode [ 4 ]–[ 6 ].

Fig. 1a Normal image of the right liver lobe. Left: fusion of US and CT. Right: corresponding CT image b. Hybrid US and CT image, oblique section showing the right liver lobe and right kidney. The green box corresponds to the US image. The CT image can be zoomed in or out.

Fig. 2a Longitudinal section of the right kidney. Left: CT image. Right: US image revealing missing color Doppler signals in the upper pole. b A kidney infarction was confirmed in hybrid imaging of contrast-enhanced CT (left) and CEUS (right).

Fig. 3 Difficult visualization of the liver dome due to the presence of air.

Recently, the method has also been evaluated for musculoskeletal and breast applications showing promising results. It enabled US-guided needle insertion in the sacroiliac joint and it aided in the imaging of osteoarthritis and rheumatoid arthritis by correlating the MRI and US images (Fig. [ 4 ]) [ 7 ], [ 8 ]. In breast cancer, guidance for US detection of lesions seen on MRI was possible [ 9 ].

Fig. 4 Hybrid imaging of anterior part of trochlea of the humerus using US (left) and MRI (right).

Fusion of US and MRI images of the prostate has been evaluated in several studies. One study successfully demonstrated a better detection rate for prostate cancer using MRI targeted US-guided biopsy of lesions suspect of cancer [ 10 ]. Further studies are needed to evaluate whether the method will be an advantage for instance in combination with real-time elastography, which has been established as an useful method for depiction of prostate lesions.

Generally, the diagnostic accuracy and image-guided intervention are improved by adding several imaging modalities. By involving real-time US in hybrid imaging it may become possible to visualize areas, which are otherwise difficult to visualize sonographically. Thus, US-guided intervention may become possible for instance in areas hidden behind bone or in air-containing lesions. Furthermore, biopsy or follow-up on PET-positive lesions lying among PET-negative lesions could result in earlier detection of tumors and possibly improved treatment.

Endoscopic Ultrasound

Endoscopic ultrasound (EUS) has also been used in combination with CT/MRI images, based on electromagnetic tracking of the tip of the endoscope by using special magnetic positioning sensors placed nearby the small US transducer on the tip of the echoendoscope [ 11 ]–[ 13 ]. The hybrid EUS-CT procedures were tested in a limited number of patients, showing an accurate positioning of the US transducer (echoendoscope tip), enhanced diagnostic confidence based on multimodality imaging visualization, as well as faster location of the lesions [ 14 ]. This could have significant consequences for the followup of the patients during chemotherapy and/or antiangiogenic treatment based on fusion of power Doppler or low mechanical index harmonic contrast-enhanced EUS with dynamic contrast-enhanced CT or MRI [ 15 ].

Perspectives

Future perspectives of hybrid imaging would include better ways of displaying the information with multiple modes visible on the same screen (e. g. fly through capabilities on the 3D reconstructed images), but also more reliable and faster computer algorithms for the co-registration (e. g. automatic co-registration without external or internal position markers). The possibility of fusing a previously recorded 3D US dataset may allow side-byside evaluation of tumor response to antiangiogenic treatment, for instance with CEUS and time intensity curves. Real-time co-registration of 4D ultrasound images with corresponding multiplanar segmented reconstructions of CT or MRI imaging may be available in the near future, allowing a better follow-up of oncological patients or better preoperative planning in complex surgical situations.

In conclusion, hybrid imaging may be superior to individual imaging techniques both for the accuracy of transducer orientation, but also in terms of image interpretation and real-time guidance of therapeutic procedures. The technique may significantly decrease the learning curve of difficult ultrasound procedures, as well as broaden the use of imaging as a clinical extension through combination of both anatomical and functional characteristics of the displayed images. More importantly, multimodality registration allows the reduction of radiation exposure caused by CT-guided procedures, which can be safely carried out using ultrasound guidance in real-time hybrid imaging systems. Furthermore, multiple modes can be used simultaneously in the same system, e. g. coregistration between PET-CT and realtime US elastography or CEUS. However, clinical impact of these technological advances should be established by further prospective randomized trials.

Contact Jan Tuma [e-mail] for more information or www.efsumb.org

• References

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