CT imaging: Where are we going? (Proceedings)

Computed tomography is becoming more and more readily available to general practices, either as a local referral practice or in-house in larger practices. Typically this procedure is performed only on the most complex cases and only under general anesthesia. Recent developments have allowed faster, safer and higher quality imaging to all come together in a clinical setting.

CT technology

The original computed tomographic systems were single slice sequential imaging machines. The gantry rotated about the patient in a few seconds then the patient couch moved forward a distance dictating the slice thickness and the gantry rotated again. These sequential systems were very slow. Often the imaging of a dog thorax required 60+ minutes for data acquisition and computer analysis for final display of the images. The bad old days of CT.

All newer systems provide helical imaging capability. With helical (=spiral") imaging the patient moves continuously, without pause for computer analysis, and the entire batch of data is then analyzed after the scanning is completed. This provides faster imaging then sequential systems. However with single detector CT systems the scanning is still limited by rotation time (up to 1 second) times the thickness of each slice imaged (preferable <1mm) times the length of the body part scanned. So a 20cm long thorax might still require minutes to scan (i.e. 1 sec x 1 mm x 200mm = 200 seconds sec, = 3+ minutes). We can't stop the motion under these circumstances and patients are still anesthetized with single slice helical systems.

Newer CT system are multidetector; 2, 4, 8, 16, 64, etc. With these systems we significantly decrease the time of imaging by 1) collecting many slices of data concurrently, and 2) faster rotation time (<0.5 seconds) resulting in dramatic shorter imaging times (0.5sec/slice x 1.0mm slice thickness x 200mm/16slices/rotation = 6 sec). Most of the imaging in dogs and cats can be performed in 5-15 seconds with a 16 slice helical CT protocol. Scanning is best performed with sub-millimeter slice thickness with overlap of the imaging sets. This will produce isotropic imaging.

Isotropic imaging means that the reconstructed images have the same image resolution as the original imaging plane. This makes MPR, 3-D, and virtual endoscopy CT imaging possible.

New technology

Virtual CT imaging is possible for an array of hollow tubular organs. Because contemporary CT scanners offer isotropic, or near isotropic, resolution, display of images does not need to be restricted to the conventional axial images. Instead, it is possible for a software program to build a volume by 'stacking' the individual slices one on top of the other. The program may then display the volume in an alternative manner.

MPR reconstruction

Multiplanar reconstruction (MPR) is the simplest method of reconstruction. A volume is built by stacking the axial slices. The software then cuts slices through the volume in a different plane (usually orthogonal). MPR is frequently used for examining the spine. Axial images through the spine will only show one vertebral body at a time and cannot reliably show the intervertebral discs. By reformatting the volume, it becomes much easier to visualize the position of one vertebral body in relation to the others.

Modern software allows reconstruction in non-orthogonal (oblique) planes so that the optimal plane can be chosen to display an anatomical structure. This may be particularly useful for visualizing the structure of the bronchi as these do not lie orthogonal to the direction of the scan.

For vascular imaging, curved-plane reconstruction can be performed. This allows bends in a vessel to be "straightened" so that the entire length can be visualised on one image, or a short series of images. Once a vessel has been "straightened" in this way, quantitative measurements of length and cross sectional area can be made, so that surgery or interventional treatment can be planned.

3D rendering techniques

Surface rendering

A threshold value of radiodensity is chosen by the operator. A threshold level is set, using edge detection image processing algorithms. From this, a three-dimensional model can be constructed and displayed on screen. Multiple models can be constructed from various different thresholds, allowing different colors to represent each anatomical component such as bone, muscle, and cartilage. However, the interior structure of each element is not visible in this mode of operation.

Volume rendering

Surface rendering is limited in that it will only display surfaces which meet a threshold density, and will only display the surface that is closest to the imaginary viewer. In volume rendering, transparency and colors are used to allow a better representation of the volume to be shown in a single image—e.g. the bones of the pelvis could be displayed as semi-transparent, so that even at an oblique angle, one part of the image does not conceal another.

Where different structures have similar radiodensity, it can become impossible to separate them simply by adjusting volume rendering parameters. The solution is called segmentation, a manual or automatic procedure that can remove the unwanted structures from the image.

Virtual CT endoscopy

Virtual endoscopy of the upper airway, trachea, and central bronchi has been reported in a number of clinical settings. The images produced by the scanner must be processed into a "fly through" 3D image, using a cine program which allows the user move through the bowel as if performing a normal endoscopic procedure. Anticipated improvements in CT resolution with the new multislice helical scanners should enhance the resolution of virtual endoscopy. The longitudinal resolution of multislice helical CT can be three times greater than that of single-slice helical CT. This will enable depiction of more distal airways.

Clinical case discussion

In this session we will discuss newer CT technology using case examples. Clinical cases will be introduced, using conventional imaging or physical examination findings as the basis for comparison to the subsequent CT findings.

Summary

Newer CT technologies offer the opportunity for faster, higher resolution imaging. This improvement will often result a set of CT images that can be reconstructed into different planes, surface rendered 3-D or virtual endoscopic image sets. The goal of all these new imaging technology is to provide a diagnosis with as little cost, morbidity and time as possible.

References

Schwarz T, Johnson V. BSAVA Manual of Canine and Feline Thoracic Imaging. 2008. 408pp.