UPSC Exam > UPSC Notes > Medical Science Optional Notes for UPSC > Computed Tomography (CT)
Computed Tomography (CT) | Medical Science Optional Notes for UPSC PDF Download
| Table of contents |
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| Introduction |
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| CT-HU Scale |
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| CT Variants |
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| Cardiac CT and limitations |
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Introduction
CT utilizes ionizing radiation, specifically x-rays, in conjunction with an electronic detector array to capture a density pattern and generate an image of a tissue "slice" or "cut." The x-ray beam revolves around the object within the scanner, allowing multiple x-ray projections to pass through it. The X-ray tube and detector bank complete a 360-degree rotation around the patient, acquiring, for instance, 720 projections at 0.5-degree intervals.
X-rays that are not impeded by the body are detected by sensitive x-ray detectors positioned 180 degrees opposite the X-ray tube.
- Areas with higher X-ray attenuation (e.g., bone) exhibit heightened density on the resulting image.
- Conversely, areas with lower X-ray attenuation (e.g., soft tissue) manifest as regions of lower density.
CT-HU Scale
The linear attenuation is expressed in terms of HU (Hounsfield Unit) values, spanning from -1000 to +1000:
- Air (-1000 HU)
- Fat (-120 to -200 HU)
- Water (0 HU)
- Soft tissues (20-40 HU)
- Blood (50-60 HU)
- Bone (800-1000 HU)
Question for Computed Tomography (CT)Try yourself: What is the purpose of using an electronic detector array in conjunction with ionizing radiation in CT imaging?View Solution
CT Variants
- Axial CT vs. Helical (Spiral) CT
In axial CT, the couch motion is characterized by stepping, whereas helical CT involves the continuous progression of the patient through the scanner beam. - Multidetector Computed Tomography (MDCT) - Multislice CT (Helical MDCT is the current standard)
Instead of the linear array of detector elements employed in conventional and helical CT scanners, multidetector computed tomography utilizes a two-dimensional array of detector elements. This configuration enables CT scanners to capture multiple slices or sections simultaneously, significantly enhancing the speed of CT image acquisition.
Cardiac CT and limitations
Principle: Electron beam CT relies on a fixed detector array and radiation source. X-rays are generated by an electron beam continuously sweeping across the target anode ring. Employing electrocardiographic triggering, often accompanied by a brief breath-hold of 10-15 seconds, allows the acquisition of both static and cine images.
Clinical Applications: Preferred Investigation - Provides 100% negative predictive value for excluding coronary artery disease (CAD) in low to intermediate-risk patients.
- Assessment of cardiac masses and thrombus
- Evaluation of the status of bypass grafts and stents
- Calcium scoring and CT angiography
Limitations:
- Involves ionizing radiation
- Requires iodinated contrast (poses challenges for patients with renal insufficiency or contrast allergies)
Image courtesy-Author Jto410 (GNU Free documentation license)
Question for Computed Tomography (CT)Try yourself: What is the main difference between axial CT and helical (spiral) CT?View Solution
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FAQs on Computed Tomography (CT) - Medical Science Optional Notes for UPSC
| 1. What is the CT-HU scale? |  |
The CT-HU scale, also known as the Hounsfield scale, is a measurement used in computed tomography (CT) scans to quantify the radiodensity of different tissues and structures in the body. It assigns a numerical value to each tissue based on its density, with water having a value of 0 Hounsfield units (HU) and dense materials like bone having higher positive values. This scale helps in distinguishing between different tissues and identifying abnormalities in CT images.
| 2. What are the different variants of CT scans? | |
There are several variants of CT scans, each designed for specific purposes. Some commonly used variants include:
- Contrast-enhanced CT: This involves the administration of a contrast dye to highlight blood vessels, organs, or abnormalities in the body.
- Cardiac CT: It focuses specifically on the heart and its blood vessels, providing detailed images to assess heart function and detect coronary artery disease.
- Virtual colonoscopy: It is a non-invasive alternative to traditional colonoscopy, using CT scans to visualize the colon and detect polyps or tumors.
- CT angiography: It examines blood vessels in various parts of the body, such as the brain, lungs, or extremities, to diagnose conditions like aneurysms or blockages.
- CT-guided biopsy: It involves using CT imaging to assist in performing a biopsy, where a small tissue sample is taken for further analysis.
| 3. What are the limitations of cardiac CT scans? | |
While cardiac CT scans are useful in evaluating heart health, they do have some limitations. These include:
- Radiation exposure: CT scans involve the use of ionizing radiation, which can be a concern, particularly in repeated or long-term imaging.
- Limited assessment of coronary artery disease: While cardiac CT can detect significant blockages in the coronary arteries, it may not accurately identify mild or moderate disease.
- Difficulty in imaging certain patients: Patients with high heart rates, irregular heart rhythms, or obesity may pose challenges in obtaining clear and accurate images.
- Allergies or adverse reactions to contrast dye: Some individuals may be allergic to the contrast dye used in cardiac CT, leading to potential complications.
- Cost and availability: Cardiac CT scans can be expensive and may not be widely available in all healthcare facilities.
| 4. What is the purpose of computed tomography (CT) scans? | |
Computed tomography (CT) scans are used for various purposes in medical diagnostics. Some common reasons to perform CT scans include:
- Detecting and diagnosing diseases or injuries: CT scans provide detailed images of internal structures, helping doctors identify abnormalities such as tumors, fractures, or infections.
- Guiding treatment and intervention: CT scans can assist in planning and guiding surgical procedures, radiation therapy, or minimally invasive interventions like biopsies or drain placements.
- Monitoring treatment progress: By repeating CT scans over time, doctors can assess the effectiveness of treatments or surgeries and make necessary adjustments.
- Screening for certain conditions: In some cases, CT scans may be used for screening purposes, such as lung cancer screening in high-risk individuals or detecting abdominal aortic aneurysms.
- Emergency evaluation: CT scans are often used in emergency departments to quickly assess and diagnose traumatic injuries or acute conditions.
| 5. How does the CT-HU scale help in interpreting CT images? | |
The CT-HU scale plays a crucial role in interpreting CT images by providing quantitative information about the radiodensity of different tissues. It allows radiologists and clinicians to differentiate between various structures and identify abnormalities based on their density values.
For example, in a chest CT scan, the CT-HU scale helps in distinguishing between air-filled lungs (low density), blood vessels (medium density), and dense structures like bones or tumors (high density). By analyzing the Hounsfield units, radiologists can identify potential issues such as lung nodules, blood clots, or fractures.
The CT-HU scale also helps in assessing the effectiveness of treatments. For instance, in a contrast-enhanced abdominal CT, the scale can show if a tumor has decreased in size or if blood flow has increased in a particular area.
Overall, the CT-HU scale provides crucial information for accurate diagnosis, treatment planning, and monitoring of patients undergoing CT imaging.
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Nov 21, 2024 Last updated |
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