Instead of using the more conventional cone beam approach, a scan using North Star’s proprietary vorteX feature utilizes a helical acquisition. Whilst the part rotates, the detector and source simultaneously travel vertically relative to the part. Usually, the part will perform multiple rotations over the course of the scan and is not required to fit entirely on the detector at a given point in time.
When utilized in the right application, vorteX can have significant benefits over conventional cone beam CT. Knowing these advantages greatly expands the versatility of the CT system.
Benefits of Using vorteX
1. Reduction of Cone Beam Related Artifacts
X-rays are emitted from a small focal spot in a CT system. They then pass through the item and are detected by a Digital Detector Array (DDA) (see Fig.1). Throughout a conventional cone beam scan, X-rays above and below the center line pass through the part at an angle and will not align perfectly with horizontal flat surfaces in the other sections of the part.
Figure 1: Setup for a CT scan. X-rays are emitted from a small source, pass through the part, and are detected by the DDA. X-Ray above and below center pass through the part at an angle.
In Fig.2 the manifestation of the cone beam artifact in a reconstruction of a battery scanned in a vertical orientation can be observed. Often, tilting the part can decrease these artifacts but there are circumstances where tilting the item requires the sacrifice of other desirable scanning characteristics or is physically impractical.
Figure 2: Reconstruction of a AA battery. The top and bottom flat surfaces are blurred due to the cone beam effect.
The part moves vertically relative to the source and detector (or the tube and detector travel vertically in a different system configuration) during a vorteX scan. Every location of the part will align with the center of the X-ray beam at some point, making it feasible to clearly resolve flat surfaces throughout the entire part. A spindle of CDs is one example because, essentially, it is a stack of parts with many flat surfaces.
Figure 3: (a) vorteX scan performed on a stack of CDs. (b) Conventional cone beam scan performed on the same set of CDs. The vorteX scan provides a much crisper definition of the edges of the CDs.
In Figure 3 the results of scans completed utilizing vorteX (a) and employing a conventional cone beam scan (b) are shown. Clearly differentiating the boundaries in the vorteX reconstruction is possible whilst they are blurred together above and below the center of the X-ray beam in the conventional cone beam reconstruction. Furthermore, streaking artifacts are much more noticeable in the conventional cone beam scan (b).
2. Ability to Cover a Larger Vertical Part Area with a Smaller Pixel Size
A long and narrow part will not fit easily on a detector because of the aspect ratio. A significant drop in magnification is required for a slender small item and large items simply will not fit. Multiple scans can provide a potential solution.
Yet, this multi-scan method can be time consuming, needing a number of set-ups and reconstructions. In addition, it is often advantageous to possess a single reconstruction output for data review purposes. vorteX gives a vertical scan envelope which is only limited by the travel of the CT system.
Creating a CT method is normally a balancing act between ensuring coverage of the entire region of interest and achieving enough resolution to see features of interest. A pen for example, could be imaged in a single cone beam scan (Fig. 4a shows the projection image), and that is an excellent means to observe the overall assembly if the lower resolution is acceptable.
To examine a particular area of interest, such as the tip (Fig. 4b), a focused, higher magnification scan could be utilized. But what if the aim is to analyze the entire pen for pores as small as 50 µm?
Figure 4: (a) Pen projected to fit entirely on detector; 1.6x magnification yields an effective pixel pitch of 80µm. (b) Tip region of pen magnified to fill detector; 15x magnification yields 8.5µm effective pixel pitch. A vorteX scan can provide the coverage in (a) with the resolution in (b).
A full pen projection would only give an effective pixel pitch of 80 µm, which is not small enough to detect 50 µm pores. The focused region scan would give enough resolution but would need multiple conventional cone beam scans to cover the entire pen. The advantage of vorteX in this instance is to supply a technique which combines higher resolution with vertical coverage with a single scan.
A Few Other Factors to Consider
In contrast to a conventional cone beam scan, a vorteX reconstruction will not utilize information from every projection to establish the value of a given voxel, because that area of the part will not always be on the detector (or near the center of the detector in the case of a vorteX scan with a small helical set height).
The amount of radiographs in a vorteX scan must be increased accordingly, given that a similar number of projections contributing information are still needed to produce a reconstruction of comparable quality. This means a single vorteX scan will likely have more data acquired, a longer acquisition time, and increased reconstruction processing time.
Furthermore, the reconstructions have the possibility to be much bigger in data size, especially when vorteX is employed to combine vertical coverage with higher resolution. vorteX has a number of software settings that can be fine-tuned depending on the aims of the scan.
An item with flat surfaces that are closely spaced, such as the stack of CDs in Fig.3, will need many revolutions to eliminate the cone beam artifact. For an item such as a pen (Fig.4), it is usually possible to scan with a wider helical pitch.
vorteX provides a number of benefits that can expand the capability of a CT system. The reduction of cone beam artifacts can have a big improvement in a part with many parallel flat surfaces and the vertical travel provides further choices for elongated parts. Not all items can benefit from vorteX, but it is one more tool North Star supplies for optimizing CT.
This information has been sourced, reviewed and adapted from materials provided by North Star Imaging, Inc.
For more information on this source, please visit North Star Imaging, Inc.