Ultrasound Fusion Imaging System in Neurology Practice

The ultrasound fusion imaging (UFI) system is a new promising imaging modality that combines live ultrasound investigations with preregistered CT, MRI, or PET images. In this study, we want to present our initial experience with the new method that combines the transcranial color-coded sonography (TCCS) in different insonation planes and the 3T-weighted MRI cerebral images. The study vali-dates the diagnostic capabilities of the system to detect different normal cerebral structures in healthy volunteers. In the present paper, we also discuss the advantages of US fusion imaging technology and its clinical applications in Neurology.


INTRODUCTION
The ultrasound fusion imaging (UFI) system is a new promising imaging modality that combines live ultrasound investigations with preregistered CT, MRI or PET images. [1] The system was first used for analysis of lesions located in the liver [2] , breasts [3] , and prostate [4] . It has a special diagnostic approach for ultrasound-guided biopsies of suspicious lesions. [5] Recent publications reported its new application in neurosonology for diagnostics and teaching. [6,7]

AIM
The aim of the present study was to confirm the method of ultrasound fusion imaging with transcranial colour sonography using different planes of insonation, thus detect different normal cerebral structures, and confirm the diagnostic capabilities of the new method in healthy volunteers.

MATERIALS AND METHODS
For this small study, we examined 20 healthy volunteers at the mean age of 35 years with no pathology on MR images. All patients signed an informed consent prior the investigations and all of them had no contraindications for electromagnetic field studies. For the current investigations, a premium Philips EPIQ Elite ultrasound system with PercuNav fusion software package and PercuNav accessories was used. Prior to examination, cerebral images from 3T-weighted MR were loaded into the ultrasound machine in DICOM format. The system is compatible with all types of CT/MR machines. The patient lied down in a supine position. The researcher sat on the side of the patient's head and nearby an adjustable electromagnetic field generator that continuously emits a local electromagnetic field directed to the studied area was fixed. A small electromagnetic patient tracker (motion control sensor) was fixed to the patient's forehead and remained there until the end of examination. A second electromagnetic tracker was attached to a low frequency ultrasound probe used for transcranial insonation. The patient lied still but after fixing the first tracker, he could move his head slightly and breathe calmly. At least three intracranial or extracranial markers between the ultrasound and MR images were used to perform the fusion. The whole process of preparation, positioning, and matching described above took about 10 minutes. During investigation, the images from the ultrasound appeared on the US machine screen and the combined US/MR images were visualized accordingly.

RESULTS
The presented images are a combination between previously performed 3T weighted MR cerebral images of healthy patients and live provided transcranial color-coded sonography. Ultrasound transcranial colour-coded sonography is a safe method and patients do not need any special preparation but still there are certain limitations. As ultrasound waves are absorbed by the skull bones there are only three available bone windows through which intracranial parenchymal and vascular structures can be studied -transtemporal, suboccipital, and transorbital. Structures like midbrain, cerebral parenchyma, cerebrospinal cisterns, and different segments of middle cerebral artery, anterior and posterior cerebral arteries, as well as anterior and posterior communicating arteries can be observed via transtemporal access in axial insonation plane (Fig. 1).
By changing to transtemporal coronal insonation plane (Fig. 2), distal segments of internal carotid artery and some variations of middle cerebral artery as early bifurcation can be easily seen. The access also allows a coronal view of normal and pathological brain structures.
In suboccipital insonation plane (Fig. 3), foramen magnum, medulla oblongata, intracranial parts of vertebral arteries and their branches, and basilar artery can be observed.
Despite the limitations of bone windows, the achieved images represent brain structures live and in 3D reconstructed MR images.

DISCUSSION
Ultrasound transcranial colour-coded sonography provides real-time imaging, uses no ionizing radiation, and is easily accessible. However, there are certain limitations as ultrasound waves are absorbed by the skull and brain structures are not presented in details. On the other hand, MRI or CT cerebral images are of high quality, but rather expensive, time consuming, and impossible to repeat frequently. The ultrasound fusion imaging system gives new promising possibilities combining the benefits of both modalities. The system allows live and detailed examination of cerebral structures and safety follow-up examinations.
The fusion technique was first used for abdominal investigations and ultrasound guided biopsy of suspicious lesions. [1,2] US fusion imaging can also be associated with   advanced US imaging techniques such as color Doppler US, elastography, and contrast-enhanced US (CEUS). [8] The only limitation is that patients with pacemakers should not be exposed to the electromagnetic field of the systems and therefore cannot be examined.
Schreiber et al. first described the applications of the system in neurosonology [6] giving some important instructions about verification of different structures on the differ-ent insonation planes. They also pointed the advantages of the method for repeated measurements in cases of subdural hematoma, hydrocephalus, or arachnoidal cyst. Walter et al. described the benefits of the use of magnetic resonance-transcranial ultrasound fusion imaging for postoperative control of deep brain stimulation (DBS) electrode location. [9] They concluded that this technique may decrease the need for repeated CT or MRI scans in DBS patients.
Other researchers made further investigations and found other applications of the method in neurology. [10] These authors used 7T MR neurography-ultrasound fusion for peripheral nerve imaging. They demonstrated the utility of UFI system for more accurate identification of Guillain-Barré syndrome and Charcot-Marie-Tooth disease type 1A.
In our study, in order to validate the diagnostic opportunities of the method for normal anatomical structures, we chose only healthy volunteers, with no abnormalities on cerebral MR images. The mean age of the patients was 35 years, expecting good bone window and excellent quality of the images. We did not include patients with any contraindications for magnetic field studies. In all patients, we achieved very good rate of successful images in the two main insonation planes (transtemporal and suboccipital). In this way, we managed to detect in detail different anatomical structures that could be difficult to observe if using only ultrasound transcranial colour-coded sonography. Another advantage of the method is that we can repeat the investigations if we want to follow up our patients. The patients also felt very comfortable during the study. The examination also provided live information, for example, about the current flow direction and velocity of the vessels, which could not be diagnosed during MR study. We expect certain limitations if we include older patients with poor bone window and patients with contraindications for magnetic field studies. Our further investigations will be focused on patients between 40 and 65 years of age with cerebrovascular disease proving the diagnostic opportunities of the method in this pathology.

CONCLUSIONS
The ultrasound fusion imaging system is a new promising imaging modality in neurology with multiple reproducibility providing real-time monitoring. In our study, we managed to validate the method for detecting different transcranial structures in healthy volunteers. We also proved that the combination of the two imaging methods (magnetic resonance and ultrasound imaging) increased the volume of diagnostic opportunities for detecting different cerebral structures.