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Understanding Computed Tomography: Part 1

Vet CT are pleased to announce a series of articles in which we will provide a short oversight of important features of a CT scanner and how they influence the imaging technique and quality. We hope this will enable you better to understand and apply CT settings.

PART 1: Understanding the effect of slice width, tube current and display window width on image noise and partial volume artifact.

Achieving a high signal to noise ratio

CT image quality is to a large degree a compromise between image noise and partial volume artifact. In CT the more X-ray photons hit the detector, the more signals are generated. The less signal, the more there is image noise. So to achieve a high signal-to-noise ratio one can:

1. Increase the tube current: The higher the mA, the more photons are generated, the better the signal-to-noise ratio. Problem: This prolongs cooling times of the tube. It is generally difficult to apply high tube current (200 mA or more) over large areas of patient cover.

2. Increase the slice width: The bigger the area of detector fields exposed, the more X-ray photons can hit it, and the better the SNR. Problem: With thick slices, all densities within the slice width area are averaged, creating image blur, called partial volume artifact.

3. Select a low frequency reconstruction kernel: The kernel is the mathematical method to create a matrix image from all the angular density measurements. Low frequency kernels smoothen edges and paint over areas into homogenous looking structures. Problem: Low frequency kernels are only useful for soft tissue assessment. Evaluation of bone, lung and nasal turbinates require a high frequency kernel.

4. Set the display window width very wide (more than 2000 HU). With such settings the noise just blends in with the general grayness of the image and is not visible as such. These window settings are applicable for the nasal turbinates, lungs and bone. Problem: Narrow window settings are needed to evaluate soft tissue structures.

Achieving optimal image quality

To achieve the optimal image quality soft tissue and bone/nose/lung images are therefore treated differently:

  • Trabecular bone, nasal turbinates and lungs contain wide range of densities but are also important to assess for very small lesions. Therefore a thin slice width is essential (0.5-2mm) minimizing partial volume artefact. A high-frequency reconstruction kernel is chosen to maximize spatial resolution, but this will increase noise. In order to minimize image noise related to the thin slice width, the tube current is chosen relatively high. Images are only viewed with a very wide window with which the noise is not very apparent. Interestingly, bone, nose and lungs are not fundamentally different and can all be reconstructed in the same way. This allows simultaneous assessment of for instance the vertebral column and the lungs, without the need of another reconstruction.
  • Soft tissue structures such as muscles, the brain, spinal cord and abdomen have a narrow range of densities. In order to maximize the contrast of this narrow range of densities, noise must be kept at a minimum and this is achieved by increasing the slice width (3-5mm), selecting a low frequency kernel and choosing a narrow display window. The tube current can then be kept relatively low.
  • More problematic are the soft tissues for which partial volume artefact is a problem, such as the spinal cord.Here we really want to assess every millimeter of the cord in maximal detail to evaluate for instance the presence of a disk protrusion. The only way to achieve this is then to selecta thin slice width with a high tube current.

Multidetector CT

In multi-slice CT units, several reconstructions with different slice thickness combinations can be reconstructed from the raw data and this is usually implemented in the image protocol. For most anatomic regions a pre IV contrast series should be dual-reconstructed with:

  • A thin slice, high frequency kernel viewed with a wide window
  • A thick slice, low frequency kernel, viewed with a narrow window

Post IV contrast series aim solely to show contrast uptake in soft tissue structures. This requires soft tissue settings only. Therefore only a thick slice, low frequency kernel combination viewed with a narrow window is useful.