Z-axis resolution

Fig. 2. Depth discrimination (z-axis resolution) properties of a confocal microscope. The illumination and detection images in a confocal microscope are diffraction-limited and confined to a small region of the specimen (1). Only light emitted in the plane of focus and on the optical axis will pass the detector pinhole and form an image. Light emitted from other areas of the specimen does not enter the detector pinhole.

FIGURE 10.1 CARS image of yeast cell in water observed at 1215 cm. (b) Estimation of the Z-axis resolution of the CARS microscopy system. Open circles, observed data solid curve, result of curve fitting dashed curve, differential of the solid curve. 

Measurements of Applied Chemoattractant Stimulations To quantitatively measure an applied cAMP concentration in live cell experiments, Alexa 594 mixed was mixed with cAMP and imaged using a confocal microscope with a Z-axis resolution of 1-2 pm (rrrNote 1). The intensity change of Alexa 594 and cell... 

To ensure an accurate measurement, fluorescence images are usually acquired with a Z-axis resolution at about 1-2 pm. 

With SDCT the concept of collimation is a simple one to understand the size of the X-ray beam is controlled by a set of collimators placed near to the X-ray tube. Collimation in SDCT is directly related to the slice (section) thickness and narrower collimation improves z-axis resolution, though requires an increase in the mAs to overcome the additional image noise. 

Fig. 3.1. Coronal CT images reconstructed from axial data. The single slice acquisition utilized 5 mm thick sections. The resulting Z-axis resolution is poor. The 4,16 and 64 row scanners utilized 1.25,1, and 0.75 mm thick axial sections respectively. Note improvement in time of acquisition...

Fig. 3.5. Coronal reformatted image from CT colonography data sets shows supine acquisition (left) and prone acquisition (right) in same patient. CTC data obtained in supine positions was obtained with 16 x 75 mm slices and the prone acquisition with 16 x 1.5 mm slices. Obviously there is better Z-axis resolution on the supine data set. The CTDI was 14% higher for the supine acquisition...

In principle, using similar parameters on both SSCT and MDCT results in wider anatomic coverage and faster scanning time with MDCT. On the other hand, MDCT provides sub-millimeter collimation, improves z-axis resolution and generates isotropic voxels, thus resulting in better image quality of reformatted planes as well as three-dimensional reconstructions. The drawback is represented by data... 

Resolution in the z-direction is particularly important for three-dimensional reconstructions. With SSCT a compromise between slice thickness, anatomical coverage and scanning time is necessary. The result is a relative thick collimation (3-5 mm) and consequently slice thickness negatively affecting the quality of reconstructed images. A possible solution, in order to improve z-axis resolution using SSCT, is to reconstruct raw data in order to obtain overlapped slices. Usually, 60% overlap is recommended (Paul et al. 1999). 

Both single-sHce and multi-sHce spiral CT require an interpolation of the acquired measurement data in the longitudinal direction to estimate a complete CT data set at the desired plane of reconstruction. As a consequence of this interpolation, the sHce profile changes from the trapezoidal, in some cases almost rectangular shape known from axial scanning to a more beU-shaped curve see Fig. 1.8. The z-axis resolution is no longer determined by the collimated beam width S ji alone (as in axial scanning), but by the effective slice width s, which is established in the spiral interpolation process. Usu-... 

Fig. 1.12. Demonstration of z-axis resolution for a MDCT system using the z-flying focal spot technique. MPRs of a z-resolution phantom (high-resolution insert of the CATPHAN, the Phantom Laboratories, Salem,...

Axial (z-axis) resolution is measured along the optical axis of the microscope, i.e. perpendicular to the plane of focus. Axial resolution can be defined using two criteria, either the minimum distance that the diffraction images of two points can approach... 

In single-slice CT,the most important scan parameters can be provided in the form of a triplet including slice collimation (SC, in mm), table feed/rotation (TF, in mm) and reconstruction increment (Rl, in mm). Depending on clinical indications, a compromise has to be reached between z-axis resolution and required scan length. The effective slice thickness or slice width (SW) can be calculated from slice collimation and pitch, P (=TF/SC). The suggested scan parameters for single-slice spiral CT of the liver are 5/8/4 (SC/TF/Rl) and the SW is 6.2 (Uggowitzer 2003). 

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