Radiology Lecture 15.docx

Lecture 15 Chapter 27 – Three-Dimensional Imaging Dental imaging is no longer limited to two dimensions; three-dimensional imaging is now available. When only two-dimensional radiography is utilized, the dental practitioner may not have the ability to evaluate pathology (e.g., bony and soft tissu...

Lecture 15 Chapter 27 – Three-Dimensional Imaging Dental imaging is no longer limited to two dimensions; three-dimensional imaging is now available. When only two-dimensional radiography is utilized, the dental practitioner may not have the ability to evaluate pathology (e.g., bony and soft tissue), distances to critical anatomic landmarks (e.g., maxillary sinus, mandibular canal), locations of impacted teeth, eruption patterns, or other concerns of the oral and maxillofacial complex. Three-dimensional imaging provides more detailed information, allowing for a more accurate interpretation. Terminology Cone-beam computed tomography (CBCT): Term used to describe computer-assisted digital imaging in dentistry; this imaging technique uses a cone-shaped x-ray beam to acquire information and present it in three dimensions. Cone-beam volume tomography (CBVT): Term used to describe computer-assisted digital imaging in dentistry; used interchangeably with cone-beam volume imaging (CBVI); these terms are used to differentiate this procedure from medical computed tomography (CT). DICOM data: The universal format for handling, storing, and transmitting three-dimensional images; the acronym refers to Digital Imaging and Communications in Medicine. Field of view (FOV): The area that can be captured when performing imaging procedures. Multiplanar reconstruction (MPR): The reconstruction of raw data into images when imported into viewing software to create three anatomic planes of the body. Plane, axial: A horizontal plane that divides the body into superior and inferior parts; runs parallel to the ground. Plane, coronal: A vertical plane that divides the body into anterior and posterior sides; runs perpendicular to the ground. Plane, sagittal: A vertical plane that divides the body into right and left sides; runs perpendicular to the ground. A midsagittal plane describes a plane that runs through the midline of the body. Resolution, contrast: The number of gray-scale colors available for each pixel in the image. Resolution, spatial: A measurement of pixel size in multiplanar reconstruction. Three-dimensional digital imaging: An image that demonstrates the anatomy in three dimensions. Three-dimensional volume rendering: A three-dimensional shape that is created from two- dimensional images. Voxel: The smallest element of a three-dimensional image; also referred to as volume element or three-dimensional pixel. For years, three-dimensional imaging was primarily used in medicine. Today, manufacturers of CBCT units have developed three-dimensional imaging specifically to evaluate the oral and maxillofacial (head and neck) complex. CBCT uses a cone-shaped x-ray beam to acquire three-dimensional information. The source of radiation in CBCT machines rotates around the head of the patient, as in panoramic imaging. As the divergent rays exit the machine, some radiation is attenuated (DILUTED) by the patient, and some of the radiation passes through the patient and is received by a digital receptor. The information that the receptor receives is termed raw data. In contrast to producing a single intraoral image, as in two-dimensional imaging, raw data is three-dimensional in volume and undergoes reconstruction, forming a “stack” of axial images termed DICOM images. DICOM images are imported into the viewing software that allows the dental practitioner to see the FOV in three dimensions. Once the images are imported, the data is viewed in three planes: axial (X), coronal (Y), and sagittal (Z). The axial plane is a horizontal plane that divides the anatomical features within the FOV into superior and inferior slices. The coronal plane is a vertical plane that divides the anatomical features within the FOV into anterior and posterior slices. The sagittal plane is also a vertical plane that divides the anatomical features within the FOV into right and left slices. When viewed together, axial, coronal, and sagittal images are referred to as multiplanar reconstructed images (MPR images). Anatomic features within the field of view (FOV) provide accurate dimensional measurements of the patient with a 1 : 1 ratio relationship. If the field of view is small, items that need to be included in the radiograph may be accidentally excluded. Therefore, field of view size is an important aspect of 3D imaging. One of the advantages of using DICOM data is that images can be shared among dental professionals, imaging centers, and referring physicians. The volume of data produced is similar to medical CT, but CBCT uses much less radiation to acquire the images. Most dentists who use the CBCT imaging techniques do not have the formal training that is required to interpret data on anatomic areas beyond the maxilla and the mandible. The American Academy of Oral and Maxillofacial Radiology (AAOMR) recommends that CBCT images be interpreted only by a board-certified oral and maxillofacial radiologist, or by a dentist with adequate training and/or experience. Factors that can be altered when scanning the patient are FOV size and resolution. Contrast resolution refers to the number of gray scales available, and spatial resolution is the measurement, in millimeters, of the size of pixels in the MPR images. Contrast and spatial resolution, along with the size of the field of view can be adjusted. Specialized equipment (a computer) is connected to the CBCT machine accepts raw data and reconstructs the data into a stack of axial images (DICOM images). The viewing software allows the dental practitioner to view axial, coronal, and sagittal images, select the region of interest such as a presurgical dental implant site or the location of an impacted canine, and scroll through these images on a computer monitor to create three-dimensional information. CBCT imaging applications greatly improve interpretation, diagnosis, and treatment planning in many aspects of dental care. Some of the common uses of three-dimensional imaging include the following: Implant placement Extraction or exposure of impacted teeth Definition of anatomic structures, such as inferior alveolar nerve and mental foramen location Endodontic assessment Airway and sinus analysis Evaluation of temporomandibular joint (TMJ) disorders Orthodontic evaluation Evaluation of lesions and abnormalities Trauma evaluation The patient should be advised how long they will be need to remain still and what position they are expected to be in prior to the CBCT scan. Patients also need to be informed that all jewelry and removable dental appliances will have to be removed prior to exposure. Interpretation of CBCT images requires training so that the dental practitioner can recognize findings outside the region of interest, specifically outside the maxilla and the mandible. Such regions include the cerebral hemispheres or areas lateral to the oropharynx (for hard tissue calcifications) and the paranasal sinuses. These regions are not typically evaluated on a routine basis, although clearly visible with three-dimensional imaging. Each CBCT image requires interpretation and must be completed and documented in the patient record. Failure to interpret CBCT images may result in poor treatment outcomes for the dental provider and, more importantly, negative consequences for patients. Advantages Lower radiation dose. Compared with traditional medical CT scans, CBCT imaging involves a lower radiation dose to the patient. Studies have found the effective dose estimates for a dental CBCT scan to be comparable with three or four full-mouth series of intraoral images. Brief scanning time. With some machines, cone-beam data can be acquired with a quick 8- to 10-second scan. This short exposure time decreases the chance for motion artifacts to occur and encourages a high level of patient cooperation. Anatomically accurate images. CBCT eliminates the superimposition of structures, and the magnification of measurements does not occur. Cone-beam data provides an accurate measurement of anatomic structures with a 1 : 1 ratio relationship; this is more accurate that two-dimensional imaging. Ability to save and easily transport images. Three-dimensional images can be saved and shared digitally in a .jpg (Joint Photographic Experts Group) or .bmp (bitmap) format and then viewed online, placed on a compact disc, or printed on paper or film. The images can also be shared electronically. Disadvantages Patient movement and artifacts. Motion artifact occurs when a patient moves during the imaging procedure and is one of the most common reasons for image degradation. This type of artifact may occur with both two-dimensional and three-dimensional imaging. CBCT machines are equipped with devices to stabilize the patient's head and neck, and the patient should also be instructed prior to the procedure to remain still during exposure. Radiation is stopped and may not reach the receptor when it interacts with an area of high attenuation, such as a metal crown, bridge, or large amalgam restoration. Streak artifacts from these types of metallic restorations may eliminate or obscure the surrounding anatomy Size of the FOV. If the FOV is small, findings or pathology in other regions of the oral and maxillofacial area may be missed. The FOV should include not only the region of interest but also anatomic features related to the region of interest. For example, a patient presents with pain in a maxillary molar tooth. To appropriately diagnose this condition, the FOV should include the maxillary posterior teeth, the temporomandibular joint complex, the auditory complex, and the paranasal sinuses. Cost of equipment; training needed for imaging software. The cost of the setup of CBCT equipment may be prohibitive for many dental offices. CBCT machines range in cost from $80,000 to $175,000. Because this technology has been accessible for more than 10 years now, previously owned machines are available for purchase. In addition, becoming fully acquainted with the imaging software and correctly using DICOM data require time and dedication to learn the skills needed to create accurate three-dimensional volume images. Lack of training in interpretation of image data on areas outside the maxilla and the mandible. Many dental professionals who incorporate this technology into their practices have not had the training required to interpret data on anatomic areas beyond the maxilla and the mandible. The AAOMR recommends that CBCT and implant imaging be interpreted only by a board-certified oral and maxillofacial radiologist or a dentist with adequate training and/or experience. The ADA suggests that the CBCT image be evaluated by a dentist with appropriate training and education in CBCT interpretation.