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<title>CT Imaging</title>
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<h1>Computed Tomography</h1>
<span class="image main"><img src="images/pic13.jpg" alt="" /></span>
<h2>1. Introduction to Computed Tomography</h2>
<ul>
<li><strong>Overview:</strong> Computed Tomography (CT) imaging is a non-invasive diagnostic technique that uses X-ray technology to create detailed cross-sectional images of the brain and spinal structures. It is widely used in neurology to detect various conditions, such as hemorrhages, tumors, fractures, and other abnormalities.</li>
<li><strong>Process:</strong> During a CT scan, an X-ray beam rotates around the patient, capturing multiple images from different angles. These images are then combined to produce a comprehensive view of the internal structures.</li>
<li><strong>Advantages:</strong> CT scans are quick, widely available, and highly effective in detecting acute issues such as strokes and traumatic injuries.</li>
</ul>
<h2>2. Types of CT Scans</h2>
<strong>Standard CT Scans</strong>
<ul>
<li>Non-Contrast CT: This is a standard CT scan performed without the use of contrast agents. It provides detailed images based solely on the natural differences in tissue densities. It is commonly used for quickly assessing acute conditions such as hemorrhages, fractures, and large tumors.</li>
<img src="images/ct_fig2.1.jpg" alt="axial" style="width: 24%;"> <img src="images/ct_fig2.2.jpg" alt="coronal" style="width: 25%;"> <img src="images/ct_fig2.3.jpg" alt="saggital" style="width: 24%;">
<p><strong>Figure 1.</strong> Normal brain CT without contrast, with axial <i>(left)</i>, coronal <i>(middle)</i> and sagittal <i>(right)</i> views.</p>
<li>Contrast-Enhanced CT: This scan is performed after administering an iodine-based contrast agent intravenously. The contrast agent circulates through the bloodstream and highlights vascular structures and lesions. It is useful for identifying abnormalities that affect the blood-brain barrier, such as tumors, infections, and inflammatory conditions.</li>
<img src="images/ct_fig3.jpg" alt="contrast" style="width: 22%;">
<p><strong>Figure 2.</strong> Normal axial brain CT with contrast <i>(black arrows)</i>.</p>
</ul>
<strong>Specialized CT Scans</strong>
<ul>
<li>CT Angiography (CTA): TA involves the use of contrast agents to specifically image the blood vessels in the brain and neck. The scan captures rapid sequences to track the flow of the contrast agent through the vascular system. It is used to evaluate aneurysms, arterial stenosis, vascular malformations, and other abnormalities in the blood vessels</li>
<li>Perfusion CT: This technique measures the blood flow to various regions of the brain by tracking the movement of contrast agents over time. It provides information on cerebral perfusion. Perfusion CT is critical in the assessment of stroke, helping to differentiate between salvageable brain tissue and areas of irreversible damage. It is also used to evaluate tumor vascularity and response to treatment.</li>
</ul>
<h2>3. How It Works</h2>
<strong>Data Acquisition:</strong>
<ul>
<li>X-ray Production: X-rays are a form of electromagnetic radiation. In a CT scanner, they are produced by an X-ray tube that emits a controlled beam.</li>
<li>Tissue Penetration: X-rays pass through the body and are absorbed at different rates by different tissues. Dense tissues like bone absorb more X-rays and appear white on the CT images, while less dense tissues like the brain absorb fewer X-rays and appear in varying shades of gray.</li>
<li>Attenuation: The attenuation of X-rays refers to the reduction in their intensity as they pass through tissues. This attenuation depends on the density and composition of the tissues. Differences in attenuation allow CT imaging to distinguish between various types of tissues and identify abnormalities.</li>
<li>Raw Data Collection: During the scan, the X-ray detectors capture raw data from multiple angles as the X-ray tube rotates around the patient. This raw data is used to create individual cross-sectional slices of the body.</li>
</ul>
<strong>Image Reconstruction:</strong>
<ul>
<li>Reconstruction Algorithms:
<ul>
<li>Filtered Back Projection (FBP): A common method where the raw data is mathematically processed to produce images. FBP is fast and effective for most routine CT imaging.</li>
<li>Iterative Reconstruction: More advanced algorithms that refine the image by iteratively comparing the raw data to the reconstructed image, reducing noise and improving image quality.</li>
</ul>
</li>
<li>Windowing Techniques: To enhance the visibility of specific tissues and structures, CT images are often adjusted using different "windows."
<ul>
<li>Bone Window: Optimized for viewing bone structures. This window increases the contrast of bone tissues relative to surrounding soft tissues, making fractures and bony abnormalities more visible. The bone window has a narrow window width and a high window level, which emphasizes the high attenuation of bone.</li>
<li>Soft Tissue Window: Designed for better visualization of soft tissues, such as muscles, organs, and vessels. This window adjusts the image to enhance the contrast between different soft tissues. The soft tissue window has a wider window width and a lower window level compared to the bone window, making it easier to differentiate between tissues with similar densities.</li>
<li>Multiplanar Reconstruction (MPR): After the initial slices are created, they can be reconstructed into different planes (e.g., coronal, sagittal) and even into 3D images to provide a more comprehensive view of complex structures.</li>
<img src="images/ct_fig1.1.jpg" alt="Bone window" style="width: 25%;"> <img src="images/ct_fig1.2.jpg" alt="Soft tissue window" style="width: 24.35%;">
<p><strong>Figure 3.</strong> Non-contrast Axial Head CT Scans with Bone window <i>(left)</i> and Soft tissue window <i>(right)</i>.</p>
</ul>
</li>
</ul>
<p>For a guide to processing and visualizing CT brain images in Python, visit <a href="https://colab.research.google.com/drive/1YQe8rqhup-_ihQJeAmzNhdOqP6ZTHiya?usp=sharing#scrollTo=ug-c57ItBH9c">NeuroNest Tutorial on CT Imaging Classification using Artificial Neural Networks</a>!</p>
<h2>4. Main Components of a CT Scanner</h2>
<ul>
<li>Gantry: The gantry is the large, circular part of the CT scanner that houses the X-ray tube and detectors. The patient lies on a motorized table that moves through the gantry. The gantry rotates around the patient to capture multiple X-ray images from different angles.</li>
<li>X-ray Tube: The X-ray tube generates the X-rays by heating a filament to produce electrons, which are then accelerated and collide with a metal target, producing X-rays. It emits a controlled beam of X-rays that penetrate the body and are captured by detectors.</li>
<li>Detectors: Modern CT scanners use solid-state detectors that convert X-ray photons into electrical signals. They capture the X-rays that pass through the body and convert them into digital signals for image reconstruction.</li>
<li>Computer System:The computer system processes the raw data from the detectors using reconstruction algorithms to create cross-sectional images. It displays the images for interpretation by radiologists and can store the images for future reference.</li>
</ul>
<img src="images/ct_fig4.jpg" alt="scanner" style="width: 30%;">
<p><strong>Figure 4.</strong> Schematic Diagram of a CT scanner machine.</p>
<h2>5. Clinical Applications of CT Neuroimaging</h2>
<h3>Brain Imaging</h3>
<ul>
<li>Stroke: CT scans are crucial for the rapid identification of ischemic and hemorrhagic strokes.
<ul>
<li>Detection: Non-contrast CT is typically used initially to quickly rule out hemorrhage, which is essential in determing treatment. Hemorrhagic stroke will show areas of high attenuation (hyperdense regions) on a non-contrast CT.</li>
<li>Assessment: CT perfusion imaging can assess the extent of ischemic brain tissue. It helps differentiate between the ischemic penumbra (potentially salvageable tissue) and the infarct core (irreversibly damaged tissue), guiding treatment strategies such as thrombolysis or thrombectomy.</li>
<li>Visualization:CT angiography (CTA) can visualize cerebral arteries and identify large vessel occlusions, which is critical for planning endovascular interventions.</li>
</ul></li><li>Traumatic Brain Injury (TBI):
<ul>
<li>Detection: CT is the gold standard for detecting skull fractures, brain contusions, hemorrhages (such as epidural, subdural, and intracerebral hemorrhages), and edema following head trauma. It is preferred due to its speed and ability to quickly identify life-threatening conditions.</li>
<li>Monitoring: CT is used to monitor patients for potential complications such as increasing intracranial pressure or delayed hemorrhage. Repeated CT scans may be necessary to assess changes over time or the evolution of the injury.</li>
</ul></li>
<li>Tumors:</li>
<ul>
<li>Detection: Contrast-enhanced CT helps identify brain tumors by enhancing the visibility of the tumors and delineating their size, location, and relationship to surrounding structures. Tumors typically show up as areas of enhanced contrast due to their vascularity and disruption of the blood-brain barrier.</li>
<li>Guidance: CT is used to guide biopsies and plan surgical interventions by providing detailed information about the tumor's anatomy and its interaction with nearby structures.</li>
</ul>
</ul>
<h3>Spinal Imaging</h3>
<ul>
<li>Spinal Trauma:</li>
<ul>
<li>Detection: CT is essential for diagnosing spinal fractures, dislocations, and other traumatic injuries to the spine. It provides clear images of bony structures and can reveal fractures, subluxations, and damage to spinal ligaments.</li>
<li>Assessment: It helps in assessing the extent of spinal injury and identifying any associated complications, such as hemorrhage or spinal cord compression.</li>
</ul>
<li>Spinal Degenerative Diseases:</li>
<ul>
<li>Detection: CT scans are useful for evaluating degenerative changes in the spine, such as disc herniations, spinal stenosis, and osteophyte formation. They help visualize bony structures and can assist in diagnosing conditions that may not be visible on plain X-rays.</li>
<li>Guidance: CT is used to guide procedures such as spinal injections or minimally invasive surgeries. It provides detailed images of the bony anatomy and helps in precise placement of needles or other instruments.</li>
</ul>
<li>Spinal Tumors:</li>
<ul>
<li>Detection: Contrast-enhanced CT is used to identify spinal tumors and assess their extent. It helps in differentiating between primary spinal tumors and metastatic lesions, and in evaluating their impact on surrounding structures, such as the spinal cord and nerve roots.</li>
<li>Guidance: CT can assist in surgical planning by providing detailed anatomical information about the tumor’s location and its relationship to critical structures, facilitating accurate resection or biopsy.</li>
</ul>
<li>Post-Surgical Monitoring:</li>
<ul>
<li>Detection: After spinal surgery, CT is used to monitor the surgical site for complications such as hardware failure, infections, or residual disease. It helps in assessing the alignment of spinal implants and evaluating the healing process.</li>
</ul>
</ul>
<h2>6. Safety and Precautions in CT Scanning</h2>
<ul>
<li>Radiation Exposure:</li>
<ul>
<li>Risks: Understanding and minimizing radiation exposure is crucial, especially in pediatric and repeat imaging patients.</li>
<li>Techniques: Use of dose-reduction technologies and adherence to the ALARA (As Low As Reasonably Achievable) principle.</li>
</ul>
<li>Contrast Agent Reactions:</li>
<ul>
<li>Allergies: Screening for history of allergies and premedication protocols can help manage potential allergic reactions to iodine-based contrast agents.</li>
<li>Renal Function: Assessing kidney function before administering contrast to prevent contrast-induced nephropathy.</li>
</ul>
<li>Patient Preparation:</li>
<ul>
<li>Instructions: Providing clear instructions to patients regarding fasting, medication adjustments, and removal of metallic objects.</li>
<li>Positioning: Ensuring proper patient positioning to achieve optimal image quality and reduce artifacts.</li>
</ul>
</ul>
<h2>7. Glossary of CT Imaging Terms</h2>
<ul>
<li>Attenuation - Reduction in X-ray beam intensity as it passes through different tissues.</li>
<li>Contrast Agent - Substance (usually iodine-based) used to enhance visibility of internal structures.</li>
<li>Computed Tomography (CT) - Non-invasive imaging technique that uses X-rays to produce detailed cross-sectional images.</li>
<li>Cross-Sectional Image - Image representing a slice of the body obtained through scanning.</li>
<li>Filtered Back Projection (FBP) - Traditional image reconstruction method using mathematical processing to produce images.</li>
<li>Iterative Reconstruction - Advanced image reconstruction technique that reduces noise by iteratively comparing raw data with the reconstructed image.</li>
<li>Multiplanar Reconstruction (MPR) - Process of creating images in different planes (coronal, sagittal) from cross-sectional slices.</li>
<li>Perfusion CT - CT scan that measures blood flow to various brain regions using contrast agents.</li>
<li>Radiograph - X-ray image produced by passing X-rays through the body and capturing the transmitted rays.</li>
<li>Slice Thickness - Width of each cross-sectional image or slice obtained during a CT scan.</li>
<li>Tomography - Imaging technique that produces cross-sectional views of the body by capturing multiple X-ray images from different angles.</li>
<li>X-ray Tube - Component that generates X-rays by accelerating electrons to collide with a metal target.</li>
<li>X-ray Detector - Device that captures X-rays passing through the body and converts them into electrical signals.</li>
<li>ALARA Principle - "As Low As Reasonably Achievable"; principle aimed at minimizing radiation exposure.</li>
<li>Contrast-Enhanced CT - CT scan performed with a contrast agent to highlight blood vessels and abnormalities.</li>
<li>Non-Contrast CT - CT scan performed without contrast agents, relying on natural tissue density differences.</li>
<li>Perfusion Imaging - Technique to evaluate blood flow to tissues, particularly useful in assessing stroke or tumor vascularity.</li>
<li>Reconstructed Image - Image produced from raw CT data using reconstruction algorithms.</li>
<li>Tomogram - Single slice or cross-sectional image obtained from a CT scan.</li>
<li>Computed Tomography Angiography (CTA) - CT scan using contrast agents to visualize and assess blood vessels.</li>
<li>Ischemic Penumbra - Area of the brain at risk but salvageable if treated promptly.</li>
<li>Infarct Core - Brain tissue that is irreversibly damaged by a stroke.</li>
<li>SPECT (Single Photon Emission Computed Tomography) - Imaging technique using gamma rays for functional imaging.</li>
<li>Artifact - Unwanted distortion or error in the CT image caused by various factors.</li>
<li>Hyperdense - Areas appearing brighter (white) on a CT scan due to higher density.</li>
<li>Hypodense - Areas appearing darker (gray) on a CT scan due to lower density.</li>
<li>High-Resolution CT (HRCT) - CT technique providing detailed images of small structures.</li>
<li>Slice Reconstruction - Process of creating images from individual slices of data obtained during a CT scan.</li>
<li>Radial Scanning - Scanning method where the X-ray tube and detectors rotate around the patient.</li>
<li>Spiral CT (Helical CT) - CT scanning technique with continuous imaging as the X-ray tube rotates in a spiral path.</li>
</ul>
<p>For a step-by-step tutorial on neural networks in python, visit <a href="ct_python.html">NeuroNest's tutorial on CT Imaging Classification using Artificial Neural Networks</a>!</p>
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<h2>Funding</h2>
<p> We would like to express our heartfelt gratitude to <strong>Neurohackademy</strong> at the <strong>University of Washington eScience Institute</strong> for providing invaluable training and support. This experience has significantly enriched our understanding of neuroimaging and data science. We also acknowledge the support of the National Institute of Mental Health (NIMH) grant number <strong>5R25MH112480-08</strong>, which made this opportunity possible.</p>
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