How and When to Perform CT Reconstructions for Veterinary Patients

How and When to Perform CT Reconstructions for Veterinary Patients

CT reconstructions are a vital tool in veterinary diagnostics, providing detailed images for accurate diagnosis and treatment planning. Ensuring high-quality CT images is crucial, as poor image quality can lead to misdiagnosis or the need for repeated scans, which can be costly and stressful for both the patient and the veterinary team. The following guide on how and when to perform CT reconstructions is designed to help you make the best use of this modality in practice.

Understanding the CT Imaging Process

CT imaging involves three main steps: acquisition, processing, and viewing. Each of these stages is distinct and critical for producing usable images.

1. Acquisition: This step occurs in the CT room and involves positioning the patient and setting parameters such as KV, mAs, and table speed. It is important to optimise these settings because errors at this stage cannot be corrected later. During acquisition, raw data is collected as a vast set of binary data that needs further processing to be translated into interpretable images.

2. Processing: This step transforms the raw data into images using the CT scanner's built-in computer. This can be done after the patient has left, provided the raw data is available on the machine. Processing parameters include slice thickness, filters, and display field of view (DFOV). Common errors in processing can be corrected as long as the raw data is intact.

3. Viewing: This involves manipulating the processed images on a workstation using various windowing techniques to highlight different anatomical features. Windowing adjusts the image contrast and brightness to make different tissues more visible.

Key Concepts of Image Quality

To assess and optimise CT image quality, it’s important to consider three main attributes:

1. Spatial Resolution: This refers to the ability to distinguish small objects that are close together. High spatial resolution is crucial for identifying fine anatomical details, such as trabecular bone structure and small blood vessels.

2. Contrast Resolution: This is the ability to distinguish between structures with similar densities. It is vital for differentiating soft tissues. Higher contrast resolution helps in identifying subtle differences in tissue density, which is crucial for detecting lesions or abnormalities.

3. Image Noise: Noise (or ‘graininess’) appears as random variations in image density and can obscure fine details. Reducing noise is essential for enhancing both spatial and contrast resolution. This can be managed through proper acquisition parameters and the use of filters during processing.

Filters and Windowing

Filters and windowing are two techniques used to optimise the visibility of different tissues in CT images.

• Filters: These are applied during the image processing stage to either enhance or smooth out details. Sharp or high-frequency filters are used to examine bones and other high-contrast structures such as lungs, while soft or low-frequency filters are used for soft tissues. It is essential to use both types of filters for comprehensive imaging, as each provides different insights.
• Windowing: This technique is used during the viewing stage to adjust the displayed range of densities on the screen. Different windows are applied for different tissues: for example, a bone window for viewing skeletal structures and a soft tissue window for organs and muscles. Proper windowing ensures that the relevant details are visible and distinguishable.

Protocols for CT Imaging:

Establishing and following specific protocols for different types of scans ensures consistency and reliability in imaging.

1. Head CT Protocol: For head scans, typically perform both pre-contrast and post-contrast series. Use a sharp filter for bone details and a soft filter for brain and other soft tissues. Keep the acquisition and processing parameters consistent across pre- and post-contrast series to accurately compare images.

2. Thorax CT Protocol: A sharp filter is required to evaluate the lungs and musculoskeletal structures in detail. Using a soft filter pre- and post-contrast highlights the soft tissue resolution for imaging areas such as the cardiovascular system and the mediastinum.

3. Abdominal CT Protocol: In abdominal scans, both pre-contrast and multiple post-contrast series (e.g., arterial and portal phases) may be necessary. Each series should be processed using a a soft tissue filter. A bone filter should also be applied, at least to the pre-contrast series, to visualize skeletal structures, subtle areas of calcification, small gas bubbles, and other details within the abdomen.

4. Orthopaedic CT Protocol: For orthopaedic imaging, such as evaluating elbow dysplasia, use a sharp filter with a small field of view for bone details. A soft tissue series with a larger field of view can help visualise surrounding soft tissues.

Performing Quick Quality Checks

Before interpreting CT images, perform a quick quality check to ensure the images are diagnostic:

1. Check for required series: Confirm that all necessary series are available. For all body areas these will generally include at least the following:

2. Assess spatial resolution: Zoom into areas with fine anatomical details, such as trabecular bone or small blood vessels, to ensure they are clearly visible. Perform and review multiplanar reconstructions to confirm spatial resolution is maintained across planes.

3. Evaluate contrast resolution: Check that soft tissues and fluids are distinguishable from each other in the precontrast series. For example, the myocardium or liver parenchyma should be clearly distinguishable from the blood inside the heart and hepatic veins, respectively. In the head, the pre-contrast series should enable visualization of the distinct attenuations between brain and muscle, or between grey and white matter.

By systematically following these steps and protocols, you can ensure high-quality CT images that provide valuable diagnostic information, ultimately improving patient care and outcomes.

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