A look at cone beam CT, the X-ray-based imaging technique that, like a conventional medical CT scan, provides fast and accurate visualisation of bony anatomical structures in three dimensions.
Cone beam computed tomography (CBCT) is a variant of traditional computed tomography (CT) and was first described in the late 1970s. The main difference between the two approaches is the volume of the object that is imaged at one time. In traditional CT, a narrow slice of the patient is imaged with a “fan beam” of X-rays. For an extended volume of the anatomy via CT, the patient must be imaged multiple times through the fan of X-rays as it rotates. In contrast, in CBCT, a large-area detector images an extended volume of the patient in a single rotation. (See Figure 1).
In traditional CT reconstruction, the z axis spatial resolution (the spatial resolution in the direction of motion of the patient) is determined by the speed of translation of the patient through the imaging X-ray fan, coupled with the speed of rotation of the X-ray source around the patient. In this z-axis direction (i.e. the sagittal and coronal planes), the resolution from traditional CT is typically lower than in the perpendicular x-y plane (i.e. the axial plane).
Compare this imaging complexity with CBCT, which offers a more simplified system design. With CBCT, there’s no need for high-speed “slip ring” technology. It also delivers a volumetric reconstruction that has isotropic spatial resolution in all three directions.
The Carestream OnSight 3D Extremity System is a cone beam CT system for upper and lower extremities, including the capability of performing weight-bearing exams of the lower extremities.
What are the advantages of the OnSight System?
The ability to more accurately determine the relative placement and orientation of the bones in the foot, ankle and knee under realistic load conditions is an advantage of weight-bearing volumetric exams. See Figure 3 for an example of images from the OnSight system comparing non-weight-bearing vs. weight-bearing scans of a patient’s ankle to show narrowing of the sub-talar joint.
One underappreciated aspect of many CBCT systems is patient workflow. The Carestream system is designed with patient entry to the imaging volume as one of its most important features. The proprietary “patient entry door” (Figure 4) allows easy patient setup for both standing and sitting configurations. In addition, this design allows for imaging of a single knee, foot or ankle in a natural, weight-bearing configuration.
Imaging only one extremity at a time also reduces patient exposure below the typical dosages associated with traditional CT systems.
The OnSight system is easy to install as it works with standard electrical power and minimal shielding requirements.
How does OnSight handle volume reconstruction?
To create a high-quality, reconstructed 3D volume from a cone beam system requires several corrections. Due to the increased volume that’s imaged at one time, X-ray scatter plays a more significant role in CBCT than in traditional CT. The OnSight system incorporates a scatter-correction approach designed to remove much of the scatter in the reconstructed volumes.
As with traditional CT, the presence of highly attenuating objects, such as metal implants, can seriously compromise the reconstructed volume’s clinical utility. OnSight uses a proprietary method for metal-artifact reduction (MAR) designed to improve the visibility of the patient’s anatomy in the vicinity of metal components. See Figure 5.
What type of facilities will benefit from Onsight?
The OnSight system can extend the use of high-quality 3D imaging to a much wider audience than previously served by traditional CT. The small physical size, low-cost installation, optimized patient workflow, weight-bearing imaging and state-of-the-art reconstruction with advanced scatter and metal correction features make OnSight a well suited system for point-of-care imaging. Hospital-based radiology and emergency departments along with orthopaedic offices, trauma centers and imaging centers can benefit from an Onsight system.
For more information about Carestream, visit www.carestream.com