After the invention of CT itself and moving from first generation CT to third generation CT the incorporation of slip rings into modern CT is the most import enabler of improved scanning. It has enabled a number of clinical applications which were hypothesized when the Nobel Prize was granted for CT, but not possible until slip-ring CT was a reality.
Slip-ring CT is the technology which enables power and data to be transferred without physical cables connecting the stationary and rotating portions of the CT gantry. The slip-ring enables continuous gantry rotation which in tern enables multiple acquisition types including: helical CT, cine CT, cardiac CT, CT perfusion, gated lung imaging (i.e. 4D CT) and CT Fluoroscopy.
Slip Ring a Major CT Enabler
Before slip rings were around, CT was making good images of body parts that weren’t moving. For example, brains in well positioned patients could have high image quality even on early CT scanners (i.e. before slip rings). In cases of moving organs however CT was challenged as only narrow collimation axial scanning was possible. Narrow collimation axial scanning had long volumetric acquisition times and therefore a higher likelihood for motion artifacts. Scan times before slip rings were longer than a reasonable breath hold and no gating scanning for cardiac or respiratory motion was clinically feasible.
As we noted above there was also development on the different generations of CT scanners and the third generation CT geometry became the dominant configuration. Thus, most of the work on slip rings for clinical scanners occurred on third generation CT geometries. And the introduction of the slip-ring was a major enabler of multiple clinical acquisitions.
In this figure we’re only showing the rotating side here. There’s also a stationary ring that would be going around the rotating ring.
We need to communicate both power and data the divide between the stationary and the rotating sides. In this figure the patient table would be coming out of the page (or screen). Just to re-iterate the ring that we’re showing represents the rotating side, and there would be a ring around that which is the stationary side.
The era of cables (Before Slip Rings)
Before the slip rings, we had to pass the data and power across physical cables as shown in the figure above? Those cables could get wrapped around multiple times (i.e. like your hose reel at home). Then after you ran out of cable length you would have to reverse the direction such that you didn’t break the physical cables. This was true for both the power connection to the rotating side and getting the measured data off of the rotating side. You can see how this was a major limitation on the utility of CT scanners.
Cutting the cable (Slip Ring CT)
After the slip ring was invented, the actual acquisition became more straightforward as the gantry can just rotate continuously and there is no need for the physical cables. This removed many of the limitations which were present for CT before the introduction of the slip ring.
In this figure the lines on the rotating side represent multiple electrical conductive regions. The dark blue is the stable connection from the stationary side to the rotating side.
If you think about having brushes that sit on the stationary side at an angle (i.e. rotating side ->| <-stationary side). If the brush is positioned at an angle and the other side is rotating, there will be continuous connection between the brush and the conductive tract on the rotating side. This type of continuous electrical connection can be used to transfer power or data across the two rings and because the rings can move or slip with respect to one another it is called a slip ring.
This is usually done with what’s called a brush in classic slip ring technology, and then in newer technology we can do it with induction, basically with electromagnetic fields. In that case, you can have transfer of data, over a slip ring without any physical contact between the stationary and rotating sides.
The advantage of contactless slip rings a quieter system because you don’t actually have to have physical brushes which do make some noise when the rotating side is moving. As we mentioned, you can have multiple parallel lines in order to transfer multiple signals of data and multiple power signals, if needed.
Acquisitions enabled by Slip Rings
We will now list some of the acquisitions that are enabled by slip ring CT.
Rapid volumetric scanning, very commonly used in modern CT scanners
For instance, a major enabled technology is helical CT. As we discussed this is where the table with be moving into or out of the gantry at the same time that the gantry is rotating. This is highly preferable compared with pre slip ring CT where the system would have to stop and rotate in the opposite direction before the length of physical cable runs out.
When continuous rotation became possible the ability to achieve volumetric coverage was significantly improved with helical scanning where the volume can be built up continuously rather than in small chunks. That is why helical scanning is a bread and butter acquisition type for modern CT imaging.
Multiple rotations (with or without cardiac gating)
Another method that was enabled by slip ring CT is Cine scanning. This comes from the same root word as cinema and means that we are using the CT scanner to make a movie of the body over time.
One example is if you want to image joints moving like a hand or a foot. The CT scanner can take multiple time frames to make the movie of that object. Slip ring CT makes this type of acquisition very straightforward.
Obviously, the coverage of your CT scanner is important as in cine CT you are typically making an image volume with a number of axial acquisitions and the size of the joint that you can study will be limited by the z (SI) coverage of your CT detector.
Multiple rotations (with or without cardiac gating)
A technique which is related to Cine scanning is cardiac CT. In cardiac CT there are typically a variety of acquisition modes but they all rely on a couple basic inputs:
- Fast continuous rotation (slip ring enabled)
- ECG input to provide the gating signal for when to acquire data and which timepoint(s) to make the images.
In this respect we can view cardiac scanning as a subset of cine scanning, where gating is added based on the ECG signal. The slip ring is a key enabler for cardiac scanning as well since the gantry needs to be rotating quickly and ready to turn on the x-rays when the heart is at it’s most stationary phase. Cardiac scanning is actually one of the most complicated acquisitions on modern CT and there are typically many options, but at a high level this is enabled by the introduction of slip rings into CT scanning.
Today’s state-of-the art CT systems can cover the entire heart with one axial scan, but the slip ring is at the heart of the design since the gantry must be rotating many times before the x-rays are turned on.
Cardiac CT was actually envisioned and discussed years before it came into clinical practice. For instance in Hounsfield’s Nobel prize lecture in 1979 he described how CT could be used to image the heart as well. So, even at that time, he was envisioning Cardiac CT despite the fact that the technology at that time could not support cardiac scanning.
Multiple time points, post processing to compute tissue dyamics
Another dynamic based CT technology that was envisioned before the scanners could support it is dynamic perfusion measurements from CT scanners. Dynamic perfusion measurements were demonstrated in theory in 1980 by Leon Axel. Again, this was before the slip rings were integrated in CT scanners so the first demonstration was based on simulations.
The concept of dynamic CT perfusion measurements is that subtle changes in the CT attenuation over time can be measured and calculations from those measurements can be used to calculated properties of the flow rate through the tissue, the average time of the blood through the tissue and the volume of the microvascular in the tissue.
These perfusion techniques were first developed for the brain tissue and have subsequently been applied to other organs such as the liver and the heart as well. Each of these techniques typically require multiple time points for the calculations, which means multiple CT acquisitions taken in about one minute or less. Therefore, slip ring CT has been an enabler of dynamic CT perfusion as well.
Gated Lung Imaging (i.e. 4D CT)
Multiple time phase reconstruction during respiratory cycle
Just as the heart is moving and images can be made of the heart at the different phases images can also be made of the lungs.
- Relatively fast continuous rotation (slip ring enabled)
- Respiratory signal input to provide the gating signal for when to acquire data and which timepoint(s) to make the images.
You can see that this is similar to the case of cardiac imaging above. One difference for these gated lung imaging studies is that these images are often used as input to respiratory gating models for external beam radiation therapy, rather than standard diagnostic imaging procedures.
Since the respiratory motion is not as fast as the movement of the heart the scanner requirements to make gated lung imaging are generally lower than the requirements for gated cardiac imaging.
4D movies of all the phases can be made in the case of cardiac imaging as well and in standard non-gated cine imaging 4D images are also being generated. This is why the naming of gated lung imaging as 4D CT was never my favorite since it isn’t the first type of 4D images made by the CT scanner. However, in the radiation therapy community the name 4DCT is very much linked to gated lung imaging so we will keep it here for completeness.
As with these other applications here gated lung imaging requires multiple rotations of the gantry at a relatively high speed and therefore was not able to be performed before slip rings were incorporated into CT scanners.
Realtime 3D imaging for guiding minimally invasive procedures such as biopsies and ablations
As in X-ray imaging fluoroscopy can be performed on CT scanners where the aim is to make multiple subsequent images at a relatively low radiation dose. While the frame rate is not as high in CT fluoroscopy, there is the ability to visualize structures in 3D, whereas typical fluoroscopy has the drawback of 2D only imaging with overlapping anatomy.
CT Fluoroscopy is useful for imaging procedures such as:
- Microwave ablation procedures
- Cryoablation procedures
In each of these procedures it is useful to have 3D guidance so that you can easily visualize that your needle or probe has reached the appropriate position.
The important points for CT Fluoroscopy are a relatively fast acquisition and the ability trigger, reconstruct and view images from the tableside. As with each of the acquisitions above this on demand acquisition relies on many rapid rotations and is enabled by slip ring CT.
Rad Take-home Point: There are many CT acquisitions which have been enabled by slip ring CT, primarily the ability for multiple continuous rotations and subsequently for these rotations to be very fast (about a quarter of a second). These slip ring enabled acquisitions include:
- Helical CT
- Cine CT
- Cardiac CT
- Perfusion CT
- Gated Lung CT (i.e. 4DCT)
- CT Fluoroscopy
Summary of Acquisitions enabled by Slip rings
|Scan Mode||Optimal Use Case|
|Helical CT (Spiral CT)||Rapid volumetric scanning. Very commonly used in modern CT scanners especially for abdominal imaging.|
|Cine CT||Multiple rotations typically not gated, e.g. moving joint imaging|
|Cardiac CT||Gated scanning focused on the best portion of the ECG signal|
|Perfusion CT||Multiple time points acquired after one after another|
Post Processing to compute tissue dynamics
|Gated Lung CT (i.e. 4DCT)||Multiple time phase reconstruction during respiratory cycle for radiation treatment planning.|
|CT Fluoroscopy||Needle guidance for biopsies and ablation procedures|