Computed Tomography Knowledge Questions and An Flashcards

What does the term "windowing" refer to in CT image analysis?

Answer: Changing the display brightness and contrast

Windowing in CT imaging refers to the process of adjusting the range of Hounsfield units (CT numbers) displayed on the monitor, which directly controls the brightness (window level) and contrast (window width) of the image. This allows radiologists to optimize the visualization of specific tissue types, such as bone, soft tissue, or lung, by highlighting their particular density ranges. It's a crucial post-processing technique for diagnostic interpretation.

Which factor contributes to the occurrence of motion artifacts in CT imaging?

Answer: Fast scan times

While fast scan times are primarily implemented to *reduce* the likelihood of motion artifacts by minimizing the time available for patient movement, they are often employed in imaging scenarios where involuntary motion (e.g., cardiac or respiratory motion) is inherently present. In these dynamic situations, even rapid data acquisition may not entirely eliminate the effects of motion, leading to residual artifacts. Thus, fast scan times are a response to, rather than a direct contributor to, the *occurrence* of motion, but motion can still be a challenge despite them.

What is the purpose of the CT bowtie filter?

Answer: To shape the X-ray beam

A bowtie filter is a specialized filter used in CT scanners that is thicker at the edges and thinner in the center. Its primary purpose is to shape the X-ray beam, making it more intense at the center and less intense at the periphery. This design helps to reduce the dose to the patient's periphery while maintaining a more uniform signal reaching the detectors, which is crucial for image quality.

Which parameter is controlled by the milliampere-seconds (mAs) setting in CT scanning?

Answer: Radiation exposure

Milliampere-seconds (mAs) is a key parameter in CT that directly controls the quantity of X-ray photons produced by the tube. A higher mAs setting means more X-ray photons are generated, leading to increased radiation exposure for the patient. Consequently, mAs directly influences the image noise level and the patient's radiation dose.

What type of CT scan is often used for assessing coronary artery disease and cardiac anatomy?

Answer: Cardiac CT

Cardiac CT, specifically CT angiography (CTA) of the coronary arteries, is a specialized application of CT imaging. It is designed to visualize the heart and its vessels, making it ideal for assessing coronary artery disease, congenital heart anomalies, and other cardiac pathologies. This technique often involves precise timing of contrast injection and ECG gating to minimize motion artifacts from the beating heart.

Which imaging modality is typically combined with CT to provide functional and metabolic information?

Answer: Positron Emission Tomography (PET)

PET/CT is a hybrid imaging modality that combines the anatomical detail of CT with the functional and metabolic information from PET. PET uses radiotracers to detect metabolic activity, while CT provides precise anatomical localization of these active areas. This combination is particularly valuable in oncology for cancer staging, treatment planning, and monitoring response to therapy.

What is the purpose of the CT localizer (scout) scan?

Answer: To define the scanning region

A CT localizer, also known as a scout or topogram, is a preliminary low-dose X-ray image acquired before the diagnostic scan. Its main purpose is to provide a frontal and/or lateral projection image of the patient. Radiographers use this image to accurately define the anatomical boundaries and precise start and end points for the subsequent diagnostic CT scan, ensuring the correct area is imaged.

What is the primary advantage of using a higher pitch value in helical CT scans?

Answer: Faster scan speed

In helical CT, pitch is the ratio of table movement per 360-degree rotation of the X-ray tube to the total beam collimation. A higher pitch value means the patient table moves faster through the gantry during each rotation. This results in a quicker overall scan time, which is beneficial for reducing motion artifacts and scanning large anatomical regions efficiently, though it can sometimes compromise Z-axis resolution.

What is the purpose of contrast bolus tracking in CT angiography?

Answer: To monitor contrast agent injection

Contrast bolus tracking is a technique used in CT angiography to optimize the timing of image acquisition relative to contrast agent injection. It involves monitoring the arrival of the contrast bolus in a target vessel or organ using real-time low-dose scans. This ensures that the diagnostic scan is performed precisely when the contrast enhancement is at its peak, maximizing vessel opacification and image quality.

Which artifact appears as dark streaks in a CT image, often caused by dense structures like bones?

Answer: Streak artifact

Streak artifacts are characterized by dark lines or bands that appear across a CT image, often originating from highly attenuating objects. While beam hardening is a common cause of streaks, especially from dense bone, the general term for these linear obscurations is "streak artifact." Metallic objects also cause severe streak artifacts due to their high attenuation and beam hardening effects.

What is the purpose of CT perfusion imaging?

Answer: To evaluate tissue blood flow

CT perfusion imaging is a functional imaging technique that assesses the dynamic distribution of an intravenously injected contrast agent within tissues over time. By analyzing the uptake and washout curves, it provides quantitative information about blood flow, blood volume, and mean transit time in various organs. This is crucial for evaluating conditions like stroke, tumors, and myocardial ischemia.

Which material is used in CT phantoms to simulate human tissue?

Answer: Water

Water is widely used in CT phantoms because its X-ray attenuation properties are very similar to those of soft tissue in the human body. Phantoms filled with water or water-equivalent materials are essential for quality assurance and calibration of CT scanners. They allow for consistent measurements of image quality parameters like noise, uniformity, and CT number accuracy.

What is the role of the CT gantry in the imaging process?

Answer: To rotate around the patient

The CT gantry is the large, donut-shaped component of the CT scanner that houses the X-ray tube and the detector array. Its primary role is to continuously rotate these components around the patient, who lies on the movable table. This rotation allows for the acquisition of X-ray projections from multiple angles, which are then used to reconstruct cross-sectional images.

What is the purpose of CT image post-processing?

Answer: To enhance image quality and visualization

CT image post-processing involves applying various computational techniques to the reconstructed images to improve their diagnostic utility. This includes multiplanar reconstructions (MPR), 3D volume rendering, maximum intensity projections (MIP), and windowing adjustments. These techniques allow radiologists to better visualize anatomical structures, identify pathologies, and tailor the images for specific clinical questions.

What is the term for the mathematical process used to convert raw projection data into cross-sectional images?

Answer: Reconstruction

Reconstruction is the complex mathematical process that transforms the raw X-ray projection data, acquired from multiple angles around the patient, into a cross-sectional CT image. Algorithms like filtered back projection or iterative reconstruction are employed to calculate the attenuation coefficients for each voxel, thereby creating the detailed grayscale image that radiologists interpret.

Which term refers to the ability of a CT scanner to differentiate between objects with similar densities?

Answer: Contrast resolution

Contrast resolution is the ability of a CT system to distinguish between tissues that have only slightly different X-ray attenuation coefficients (i.e., similar densities). High contrast resolution is crucial for detecting subtle lesions or differentiating between soft tissues like gray and white matter in the brain. It is influenced by factors such as mAs, slice thickness, and reconstruction algorithms.