CT (Computed Tomography) scanning can be accomplished with multiple different scan modes which have different characteristics. Axial non-volumetric scanning is how CT began where one (or just a few) slices are acquired during each gantry rotation. Traditional helical scanning (some call spiral) has the advantage of being faster than non volumetric axial scanning and also uses data continuously for improved motion robustness. There are then different specialized versions of helical scanning from low pitch scanning for cardiac to high pitch helical scanning for fast volumetric acquisitions. For state of the art systems wide coverage volumetric axial scanning is also an option to cover an entire organ in one rotation. These acquisitions are particularly useful for repeated scanning such as in time resolved angiography or perfusion applications.
In the discussion below we will describe in general the different scan modes at a high level that are used on modern CT systems for general scanning (i.e. a single anatomical CT scan, not fancy temporal stuff).
One thing to note however is that there are a variety of terms used to describe the same things by different vendors of CT equipment. We will try not to be too heavily biased here but if you have a question like, ‘Is that same thing called something else on my system?’, please refer to the AAPM Lexicon.
This helpful lexicon was driven by the AAPM with chairs Cynthia McCollough and Dianna Cody. This lexicon could also be very useful if you have been trained primarily on one vendor’s equipment and you need to learn another vendor’s terms.
Anyway, with that caveat out of the way we will go ahead with a general description of the basic scan modes for third generation CT scanning.
Axial Non-Volumetric Scanning
In modern CT scanners, which are predominately 3rd generation CT scanners, there is an x-tube and an x-ray detector mounted on a gantry straight across from one another. These scanners have different scanning modes and the best choice of scanning mode depends upon the clinical task.
So called ‘classical scanning’ that could be done even 20-30 years ago is still regularly done now on third generation scanners and we will refer to it here as non-volumetric axial scanning.
This mode is used most often on scanners with relatively narrow collimation along the Z axis (patient SI direction).
With the relatively ‘short’ detector being rotated around the patient, each axial scan covers only a relatively small coverage (e.g. several mm per axial rotation). The patient table is then translated or moved in between axial scans; each axial scan is reconstructed, and then the sub-volumes are combined to form a full volume covering the total extent of the anatomy of interest in the SI direction.
We will refer to the volume that can be reconstructed from each axial rotation as a single ‘slab’. In the case of non-volumetric axial scanning to cover 160mm with 10mm slabs (assuming no overlap between the slab) we need 160/10=16 slabs.
Non-volumetric axial scanning is useful for those neuro patients who don’t move during the scan. Under these conditions we can still get very high-quality images even with a relatively ‘old’ scanning technique.
Traditional Helical Scanning (Spiral Scanning)
The primary difficulty with non-volumetric axial scanning is that when there is patient motion between acquisitions. In this case there can be significant motion between the axial ‘slabs’. This is especially problematic when many slabs are needed such as the example given above where at least 16 slabs are needed to cover 160mm. In these cases, if there is any motion between slabs it will result in inconsistencies especially in the reformat (sagittal and coronal images).
In this figure of a coronal slice of a head image we have simulated motion during axial acquisition assuming motion between slabs. This type of non-volumetric axial acquisition is susceptible to motion between slabs as many slabs are needed and there is a time delay between each slab.
This is a major why helical scanning and later full coverage axial scanning have become the default standard for imaging in anatomy where motion is common. In the abdomen for instance helical scanning can better deal with small motion in the abdomen because:
- Helical scanning is faster than non-volumetric axial scanning as the table does not need to slow down and speed up.
- Helical scanning has continuous data usage so that the detector data is used in a ‘sliding window’ manner. Compared with axial where the neighboring slabs use separate data which is more susceptible to motion between the slabs.
In most cases the table moves during the scan to achieve the helical acquisition but there are some scanners in which the whole scanner, x-ray tube and detector, moves on rails while the patient table is stationary (these are specialty scanners for interventional settings).
For typical CT scanners the table is moving during the scan, as this is equivalent to the source and detector assembly moving in the other direction during the scan.
In helical scanning an important descriptor of the acquisition is the helical pitch. The pitch is defined as the speed that the table moves with respect to the length of the detector in z (SI direction).
The pitch 1.0 setting is good for routine abdomen scanning and is used often in for standard CT. One option which is also available in helical scanning is to reduce the helical pitch.
In this figure you can see an illustration for a reduced pitch, 0.5. In this case we have more measurements, which we call higher data redundancy.
The lower pitch settings are useful if you need more flux, for instance, in the case of very large patients. Additionally, if you want to keep the flux the same, you can still use this scan mode and increase the speed of rotation.
This is because both the helical pitch and the rotation time linearly effect the exposure.
Low Pitch Helical Cardiac Scanning
Even lower pitches are possible on CT systems and, for instance, in traditional cardiac CT scanning, you’re actually using a very low pitch, (eg. a pitch of ~ 0.2).
In a separate post later we will discuss the details of ECG gated cardiac and we won’t go into those detail here.
The only thing that we want to point out here is that since the cardiac images are gated by the ECG signal the redundant or extra data is not used in low pitch helical scanning. Therefore, the low pitch cardiac helical scanning is less dose efficient that traditional helical scanning. This is required for cardiac imaging as it is most important to have images of the same phase in the cardiac cycle.
So, on older systems the classical way of doing cardiac scanning was on very low pitch helical acquisitions. On these acquisitions the dose efficiency could be increased somewhat by modulating the mA according to the ECG signal, however the very low pitch gated scans are still less dose efficient than the newer methods which we will introduce below.
High Pitch Helical Scanning
We discussed pitch 1.0 in the section above and mentioned some reasons to use lower helical pitches. In this section we will describe the opposite scenarios where the desire is to go to pitches larger than 1.0.
An extreme case a helical pitch of 2.0 is shown in this figure. At the very center of the gantry (i.e. the iso-center) there is enough data to faithfully reconstruct an image even for a helical pitch of 2.0. However, for very high pitches there can be artifacts outside of a very small field of view as the data is not sufficient for reconstruction.
For some applications where a smaller field of view is acceptable high helical pitches can be used and have the advantage that they can complete the scan through the object more rapidly than lower pitch options.
Depending on what field of view you need to reconstruct accurately, you can have helical pitches on a system that are approaching 2. All of this discussion has been focused on the standard CT systems which have a single x-ray source and a single detector.
Some scanners have two x-ray tubes and two detectors. We will refer to these systems as 2T2D systems. The second tube and detector are mounted nearly 90 degrees apart from each other. With these systems one can prescribe acquisitions in the realm of pitches between 3 and 4 depending on the artifacts you are willing to tolerate as the field of view gets larger.
The advantage of these high pitches is the ability to cover regions quickly. If you have a table that has fast translation, then you can perform your scanning quickly.
In this high pitch helical mode, 2T2D systems also can be used for cardiac scanning.
If you prospectively know when to start the acquisition and the patient has a relatively low and reproducible heartbeat (i.e. no arrhythmias) then it is possible to scan the heart with a single high pitch helical acquisition.
In that case, the system will decide ahead of time when the table should start to move. Again, we won’t go into details here on the specifics of cardiac scanning but just want to point out that there are may ways to scan the heart.
Multi-slab Axial (Step and Shoot)
This is related to non-volumetric axial scanning. In the example case above we were discussing above for non volumetric axial scanning with a collimation of 10mm, in which case 16 separate acquisitions would be needed to cover 160mm.
If the coverage of each axial slab is increased from 10mm to 40-60mm, then it only takes 3-4 slabs to cover 160mm. This is the reason that we have a separate category that we are refer to as multi-slab axial.
In modern CT this multi-slab axial scanning method is commonly used especially for cardiac scanning, where each axial slab is reconstructed as a gated short-scan.
On those systems where the high pitch helical mode is an option, very often cardiac scanning is still done in the step-and-shoot type of mode and sometimes even with the low pitch helical scanning mode. The newer modes have advantages for ideal patients but multi-slab axial scans are more robust to variation in heart rate for instance than high pitch prospectively triggered scanning.
The multi-slab axial mode and the low pitch helical modes are more robust than the high pitch helical mode to changes in the heart rate during the scan, as cardiac phase padding can be added to the acquisition for each slab in multi-slab axial and is inherent in a low-pitch helical acquisition.
Depending on the characteristics of the patient (e.g. heart rate, stability of heart rate and patient size), one will determine which is the best mode to use on a 2T2D system.
The concept here with wide-cone axial is to cover the complete organ in one rotation or less. Therefore, a much larger detector is needed for this type of scanning. For instance multiple vendors have detectors with coverage at iso-center of 160mm.
The idea is that you can get the full heart in one rotation and for neuro perfusion exams or for 4D-CTA, you can cover the whole brain in one rotation. This allows the fastest volumetric scanning possible and is ideal for repeated scanning acquisitions as well since the table does not need to translate in between acquisitions.
With wide-cone axial you can do continuous scanning and monitoring as there is no need to translate the table and wait for it to speed up and slow down.
The most difficult application from the standpoint of the technical factors of a CT scanner is frequently cardiac scanning. The wide-cone axial scan mode is particularly advantageous for cardiac scanning as the whole heart is covered in one rotation, so there will not be any mis-registration at the border between slabs (as would be possible in the multi-slab axial approach described above).
We will have more details specifically on cardiac CT in another post but the wide-cone axial is also well suited for dynamic cardiac perfusion measurements as multiple scans of the heart are required for cardiac perfusion and again it is best to capture each heart beat in a single rotation.
Summary of Scan Modes
Here we will summarize the discussion above and provide a brief summary table for the different volumetric scan modes on modern CT scanners. In a separate post we will discuss cardiac gating and other scan modes such as cine scanning.
Please see the table in this section for the high level comparison between the different methods for scanning a single volume on a modern CT scanner.
At a high level each of the scan modes described above is either just axial or helical scanning:
|Scan Mode||Quick Description|
|Non-volumetric axial scanning||Rotate Gantry, translate table, repeat many times…|
|Helical Scanning||Rotate Gantry and translate table together|
Move the table faster for higher pitches
|Multi-slab Axial (Step and Shoot)||Rotate Gantry, translate table, repeat few times|
|Wide-cone Axial (Cone-beam MDCT)||Rotate Gantry|
So, if we look at all those different modes that we talked about that are used on modern CT scanners, at a high level, again we have non-volumetric axial scanning. That’s what I’m calling classical scanning. That’s good for things that aren’t moving because if the object isn’t moving it’s okay to take as long as you wish. But once you start dealing with abdomens, for instance, or hearts, you definitely want to have scans which can be happening more rapidly, otherwise, you’ll have transitions between the slabs in a non-volumetric scan (or artifacts within the slab if motion happens within the slab).
Then the next general type of scanning we discussed is helical scanning. We define the pitch, that is just the table translation distance in one rotation (mm) divided by the height of the detector (mm).
|Scan Mode||Optimal Use Case|
|Axial Non-volumetric Scanning||Non-moving anatomy such as compliant neuro exam|
|Helical Pitch 1.0||Routine abdomen scanning|
|Helical Pitch 0.5||Useful when more flux is desired|
For matched flux you can speed up gantry rotation time
|Helical very low Pitch ~ 0.2||Traditional method for cardiac scanning|
Relatively higher dose due to overlap in acquisition
|High Pitch Helical/(Single or Multi-source)||Useful for very fast acquisition|
Can be used for prospective cardiac for lower/
regular heart rates
Not greate for very large patients
|Step and Shoot Axial||Good for routine with stable heart rate|
|Wide-cone Axial||Whole organ coverage in less then one rotation|
Great for single consisient cardiac scan
We then put things into context, where a pitch of 1 is nice for a bread and butter abdomen scan. Then we discussed that the utility of lower pitch scans such as pitch 0.5 is if you need more flux. In the case that you have enough flux using a lower pitch is still an option, but in this case you want to speed up the gantry rotation speed. For a reference on the relationship between the CT technical parameters and CT dose please see our CT calculator.
Finally, very low pitches are used for traditional gated cardiac scanning. We discussed that gated imaging does not have the same relationship that all the data is used to reconstruct an image and therefore the low pitch helical modes have a low dose efficiency.
Finally, for helical scanning we came back and discussed the high pitch helical acquisition that useful in both single and multi-source systems (2T2D systems). The benefit is that you get a relatively fast acquisition. It’s not typically great for very large patients as the image quality degrades for large field of views when the pitch is very high.
Then step-and-shoot axial, this is really good for routine cardiac. It’s usually the bread and butter for routine cardiac on a system that has 40 or 60 millimeters in coverage.
Finally, wide-cone axial scans gives you a nice option for whole organ coverage in just one rotation. It’s great for a single cardiac acquisition where there is consistency throughout the whole volume as it is all acquired in a single rotation or less.