Cross Sectional Imaging PDF

# Lecture 5: Cross Sectional Imaging ## Subject: Cross-Sectional Imaging ## Date: - - **Plain Radiography (2D)**: - Periapical - Panorama - Cephalometry - Occlusal - **Cross-Sectional Imaging (3D):** - Computed Tomography (CT) - Cone Beam Computed Tomography (CBCT) -...

# Lecture 5: Cross Sectional Imaging ## Subject: Cross-Sectional Imaging ## Date: - - **Plain Radiography (2D)**: - Periapical - Panorama - Cephalometry - Occlusal - **Cross-Sectional Imaging (3D):** - Computed Tomography (CT) - Cone Beam Computed Tomography (CBCT) - Magnetic Resonance Imaging (MRI) ### Main Difference between the Imaging Techniques: Superimposition - **Plain Radiography**: Any object that passes through the ray will appear on the image. - **Panorama**: Anything within the focal trough will appear as a superimposition. - **Upper Molars**: Appear with superimposition (buccal root superimposes on palatal root). ### Limitations of Plain Radiography **1. First Limitation:** - **Small Structures**: Obscured by large overlapping structures. So small periapical pathosis may not appear. - **Roots**: May obscure anything in furcation area. **2. Second Limitation:** - **Cannot Identify Superficial Structures from Deep Structures.** ### Solution: - **Different Angulation:** Use different angulation to take cuts to define superficial and deeper structures in different cuts. ### Cross-Sectional Imaging - Allows to see the same object in different planes. ### Cross-Sectional Imaging Modalities 1. Conventional Tomography 2. Medical CT 3. MRI 4. Cone Beam Computed Tomography (CBCT) ### Regarding Implantology - **CBCT**: - Used now. - **Medical CT**: - Used now. - **Conventional Tomography**: - Not used nowadays. - **MRI**: - Practically speaking, not used in implantology. ### Why is Conventional Tomography Not Used Nowadays? - **Conventional Tomography**: - Can only give lateral and frontal views for skull and mandible. - No transverse view. ### Advantages of Conventional Tomography - **Buccal-Lingual Dimension** ### Disadvantages of Conventional Tomography - Very high radiation dose - Poor sharpness and resolution (hazy image) - Complex technique - Magnification - Metal artifact ## Medical Computed Tomography (CT) ### Terminology - **Helical CT**: Older name. - **Spiral CT**: Older name. - **Multislice CT (MSCT)**: Used now. - **Multidetector CT (MDCT)**: Used now. ### Advantages of CT over Plain Radiography 1. Eliminated superimpositions 2. High contrast resolution. - Can show soft tissues. - CT is only digital; data is acquired digitally, so there is no need for film-based acquisition (less errors). ### CT Machine - The patient lies on a bed inside the Gantry (Scanning unit). - The Gantry rotates 360 degrees around the patient. - While it rotates, the x-ray source emits rays and on the other side, a set of detectors captures these rays. - When it completes a rotation, it captures a single slice of the patient. - Instead of using one row of detectors, multiple rows are arranged on top of each other, so in one rotation, you can capture more than one slice. This is why it's called Multidetector CT. ### Hounsfield Numbers (Units) - Hounsfield numbers are used to assign a number to each structure, according to its density. - **Water**: 0 - **Air**: -1,000 - **Cortical Bone**: +1,000 - **Density increases**: More radiopaque (white). - **Density decreases**: More radiolucent (black). - **CT**: Has high contrast resolution. It can differentiate soft tissues, allowing it to assign a specific number for muscle, fat, and glands. - **Objects denser than cortical bone (more than +1,000):** - Amalgam - Base metal - Titanium implant - Enamel ### Benefits of Hounsfield Units - The computer assigns a number to each object based on its density and this number is converted into a color to make it easier to interpret. - The main purpose of the Hounsfield unit is to measure density. ### Terminology (Expressions) - **Pixel:** Picture element. - **Voxel:** Volume Element (3D). ### Windowing - Hounsfield numbers: -1,000 to +1,000 - Gray scale of screens: 256 shades - Human eye: About 40 shades. - **CT**: Can output 2001 shades, but a screen can only display 256 shades. - **Human eye**: Can only differentiate 40 shades in the best conditions. - **Windowing**: Allows you to see the range that is important, and the rest is not displayed, as the human eye cannot differentiate between the range of colours from the CT scan. ### Windowing Width - The range of CT numbers. ### Windowing Level - The central value of the range. ### Setting Window Levels - **Soft tissue:** Set Hounsfield unit near 70. - **Bone:** Set Hounsfield unit near 400. - **You cannot see soft tissues in the bone window.** ### Window Level and Window Width in CBCT - **CBCT**: Replaced by contrast and brightness. - **Adjust Contrast and Brightness**: To adjust window width and window levels. - **Contrast:** The difference in brightness between two areas. - **Brightness**: The overall brightness of the image. ## Reconstruction (Reformatting) - **Reformatting** is the ability to reconstruct or rearrange the image to any direction (plane) you want. - The software allows you to do this. ## Resolution ### Types of Resolution - **Spatial Resolution:** Ability to distinguish between small adjacent objects. - Smaller pixel = higher resolution. - **Contrast Resolution:** Ability to differentiate between structures according to their colors (density). - **Temporal Resolution** ### Example of Contrast Resolution - **Periapical X-RAY**: You can distinguish between enamel and dentin as they have different minerals, resulting in different densities. But you cannot differentiate between dentin and cementum, because their desnities are very similar. - **CT**: Has high contrast resolution, allowing it to show soft tissues. ## Advantages of CT over Plain Radiography 1. **Eliminate Superimpositions** 2. **High Contrast Resolution**: - Data is acquired digitally. - Data from a single image can be reformatted into different planes. - A 3D reformatted image can be obtained. ## Limitations of CT 1. **High Radiation Dose.** 2. **Spatial Resolution is Lower than Plain Radiography**: - Lamina dura and Periodontal ligaments look better in plain radiography. 3. **Contrast Resolution of Soft Tissue is Lower than MRI:** - For soft tissue, MRI is better, especially salivary glands. 4. **Metal Artifacts** (more in CT than CBCT) 5. **High Cost** ## Cone Beam Computed Tomography (CBCT) ### Terminology 1. **CBCT** 2. **Digital Volumetric Tomography (DVT)** 3. **Dental CT** ### Machine - Same size as a panorama machine. - This increases the chances of having a CBCT in a clinic. ### CT vs. CBCT - **CT**: Multiple detectors. - **CBCT**: Area detector, allows it to capture all structures in a single rotation. This reduces the time. ### Voxel - **Anisotropic Voxel**: - Dimensions are not equal, so it is unable to easily reformat. - **Isotropic Voxel**: - Produced by CBCT. - All dimensions of this cube are equal. ### CBCT Needs Computers and Special Requirements - You can only get a few cuts in the direction you want with a regular CT. - CBCT sends the entire volume and you can get cuts in any direction you want. ## Advantages of Cone Beam Computed Tomography over CT 1. **Interactive Software:** Allows you to take cuts in any direction you want, and manipulate the scan with more options. You would not get as many options with a CT scan and relying only on the viewer. 2. **Less Radiation Dose**: Dramatic reduction in radiation dose. 3. **Smaller Machine**: So you need less space. 4. **Less Expensive** 5. **High Spatial Resolution**: The major factor that controls the spatial resolution is the voxel. - The smallest voxel in CT is 0.5mm - The largest voxel in CBCT is 400 microns to 75 microns. - This means the resolution of CBCT is better than CT. - Smaller voxel = More radiation dose. 6. **Ability to Control Field of View (FOV):** - You can control the volume of the x-ray beam. - You can use CBCT to image the jaws, a quadrant, or individual teeth. - You cannot control the volume you want to image with CT. - You can only image either the maxilla or mandible. ## Limitations of Cone Beam Computed Tomography 1. **More Scattered Radiation**: Due to the presence of an area detector, so there is less contrast resolution in soft tissues (scattered radiation makes tissues look the same. 2. **Poor Contrast Resolution of Soft Tissues**. 3. **Cannot Measure the Density**: The Hounsfield unit is not accurate in CBCT. - The number is unreliable. ### Side Notes - **Field of View**: The larger the field of view, the more radiation the patient receives. - **Voxel Size**: The smaller the voxel size, the better the image resolution, but this means the patient will be exposed to more radiation if *keeping the field of view fixed.* - **Dental Problems**: You need high resolution and small coverage, with as least amount of radiation as possible. - You can use a small voxel with a small field of view to reduce radiation dose. - On the other hand, for fractures you need a large field of view with less coverage. If you are using a large voxel size, then you do not need high resolution, so the patient won't be exposed to a high radiation dose. - **Implantology**: Adjust the voxel size to 0.2-0.3 mm. ## Display Modes The CBCT allows you to view the image in more than one display mode. ### 5 Main Display Modes in CT 1. **Orthogonal**: Axial, coronal, saggital. 2. **Oblique:** Ability to take oblique cuts in any direction from the orthogonal planes (axial, coronal, saggital). - **Curved planar (reformatted panorama)**: Gives occluso-cervical and mesiodistal dimensions. - **Serial cross section 3D**: Takes the cuts in the third dimension (bucco-lingual dimension). - **Volume rendering (3D):** Not used in planning.