High pitch CTPA presentation

This is my own work. Presented here for your reference. A verbatim transcript from my notes is provided below the presentation for clarity.

ASMIRT-FB-High-Pitch-CTPA-Presentation-Ready

Slide 1|

Good morning. My name is Hung and I work at… My presentation today is really a discussion on a CT classic – the CT pulmonary angiograms. No matter how you do it, every radiographer and practice will have their own special way to perform a CTPA. Today, I want to throw a new spanner into the works and present to you a different way that you may or may not have heard of before. And this is the title and topic of my presentation today. Free breathing high pitch CTPA.

Slide 2|

To begin, what are pulmonary emboli and why is it important for us to diagnose them? Pulmonary embolism is the mechanical obstruction of the pulmonary vasculature in the form of a venous thromboembolism. PE symptoms can occur almost immediately after mechanical obstruction in the form of tachycardia and tachypnoea. It is not uncommon for the patients to feel more pain when taking in a deep breath. These symptoms can also be chronic and may not be immediately noticeable.

According to one article by Halligan and colleagues, there are ten million PE cases annually which puts PE as the third most common diagnosed vascular disease behind acute myocardial infarction and stroke. When a patient present to the emergency department it is important to follow a set of clinical pathways to ensure thromboembolisms do not get overlooked. This flow chart on the screen is one of many flow charts that are in circulation. In this flow chart, the authors have chosen to immediately categorise patients with suspected PE as hemodynamically stable or unstable.

You will also notice that in this flow chart, a CTPA is only performed if a D-Dimer test returns an abnormal result. Unfortunately, I am sure we are the same consensus that this isn’t always the case. For most junior residents it almost seems like the old DR. ABCD has now become airways, breathing, CTPA, discharge.

Slide 3|

Hopefully, this is a familiar concept to everyone. To confidently diagnose a patient with a PE, a filling defect needs to be seen on contrast enhanced imaging. I’ve stolen and image of Radiopaedia to demonstrate this. In this slice you can see a large non-occlusive embolus draped over both pulmonary arteries. This is commonly called a saddle embolus.

I also want to throw a shout out to our colleagues in nuclear medicine. The nuclear medicine ventilation perfusion scans are particularly useful in patients who are contraindicated to CT contrast.

Slide 4|

Over a year ago we tendered for a new CT machine and the one that we got is capable of performing spectral imaging. In our initial apps training we revised our CTPA protocol from a pretty stock standard CTPA protocol to the one that you see on the screen. I want to draw your attention to the second line. Our CTPA scans are performed at the 90kVp and 150kVp with a tin filter. Why have we chosen to scan at 90kVp, what is the benefit?

Slide 5|

I do apologise to the people that had to suffer through my presentation on Friday. I do recycle my presentation slides and this is as funny as it will get. To everyone else, I hope you get the reference. The reason that we utilise 90kVp is to maximise contrast enhancement. At 80/90 kVp, we have a mean photon energy very close to the k-edge of iodine. As we approach the k-edge of iodine we maximise contrast attenuation and maximise its appearance on our images.

Slide 6|

After several months of this DE protocol, we reviewed our images and found that we had a small but noticeable amount of diagnostic scans. Some of them can be attributed to language barriers and the patient not being adequately prepared for the scan in one form or another.

Slide 7|

The images on the projector demonstrate the common artifacts that we usually contribute as an undiagnostic scan. The top row demonstrates a condition called interruption of the contrast column. This is easy to spot because the pulmonary trunk is washed out, the aorta is enhanced and there are still lots of contrast in the SVC. This interruption of the contrast column causes inhomogeneities that can look like thrombi. The bottom row demonstrates several breathing artifacts through the midheart, degrading image quality and impairing reliable diagnosis.

Slide 8|

If you recall, I wanted to discuss a new novel approach that we’ve started at Peter Mac and that was free-breathing high pitch CTPA. Let’s start with the high pitch bit, what does high pitch actually mean?

Slide 9|

High pitch means scanning the patient really really really fast. Usually for our Flash CTPA studies the pitch is set anywhere between 2.5 to 3.0.

Just a quick refresher on pitch. A pitch of less than one usually means there is overlap of adjacent acquisitions and this increases the amount of time you are scanning an area. Because you are scanning an area more than once, there is more information available for the computer interpolate image slices there is an inherent increased in SNR and CNR. I wanted to also point out that there is a common misconception that a pitch of less than one, you are increasing the patient’s radiation dose. Now, in theory this is true however in practice. All CT manufacturers will provide some mechanism to offset the decrease or increase in pitch to maintain constant noise level set by the user. This offset, will usually keep the radiation dose relatively similar irrespective of which pitch you use. In Siemens this mechanism Is the effective mAs, in Philips mAs per slice.

Of course, for most scanners, if you go above a certain pitch you start introducing gaps between your acquisitions and consequently interpolation artefacts.

Slide 10|

Our scanner like I said has an average pitch of 2.5 to 3 for flash acquisition and a table speed of 73.7 cm/s on turbo flash mode. The scanner also has inbuilt mechanisms to negate sampling gaps along the z-axis.

Slide 11|

One of the benefits of performing a flash scan is an increase in pulmonary trunk attenuation. In one study, they authors found that the high pitch group performed better than the low pitch group.

Slide 12|

In pretty much every study that we’ve done. This is also the case. The image on the left is a flash CTPA and the image on the right is the dual energy CTPA. These are both of the same patient about 3 months apart. You can appreciate that the image quality is very similar but the HU through the pulmonary trunk on the flash is more double that of the DE dataset.

I hypothesise that the higher HU seen in the high-pitch group can be attributed to the sub-second scan. In our dual energy study, the acquisition takes roughly 2-3 seconds. If we take into account the scan range from apices to lung bases, by the time the scanner is over the pulmonary trunk we are more likely to image the contrast that has mixed with the unopacified blood from the IVC rather than the initial solid column of contrast that triggered the ROI.

Slide 13|

And give yourself a pat on the back if you spotted the filling defect.

Slide 14|

In terms of motion artifacts. In another study by Sandfort and his team, they compared high-pitch non gated contrast enhanced ct chest and prior low pitch ct scans for the same patients. This graph summarises a component of their study, the number of studies with motion artefacts. Red indicates that the study is non-diagnostic and dark green means the study was free of motion. Although it is quite obvious that there is tremendous decrease in the number of non-diagnostic studies moving from low pitch to high pitch, I want to draw your attenuation to the substantial increase in the number of no to mild motion artifacts across the board. This suggests that high-pitch scanning allows us to further improve the scans of patients in the low-pitch group. 

Slide 15|

If you have the time, I employ you to have a read of this article by Ajlan and colleagues. It is well written and definitely the first of its kind that I could find in the scholarly databases. In this study, the authors concluded that high-pitch helical CTPA acquired during BH or in FB yields comparable image quality for the diagnosis of PE and lung pathology.

Slide 16|

It is well documented in literature that inspiration breath holds leads to a decrease in intrathoracic pressure and impacts the enhancement of the pulmonary arteries. This leads to an interruption of the contrast column and compromising our diagnosis. Patient factors like impaired hearing and in the inability to hold their breath are good reasons why a reliable CTPA with free breathing might be a good alternative.

Slide 17|

Both these images are of the same patient. The left study is a routine CT CAP with breath hold but performed in flash mode due to the patients young age and to minimise patient dose. The study of the right is a flash CTPA performed in free breathing mode. This is a great example of how free breathing and breath hold flash studies yield very compatible if not identical outcomes.

Slide 18|

Like any technique, there are draw backs. High pitch CTPAs are not fool proof and can still have artifacts. Something that I haven’t personally experienced but it is reported in literature is performing the scan in expiration by accident.

Slide 19|

I am embarrassed to say that I performed this CT study. CT study on the left is a flash study and the right is actually a DE study. We did the flash study first but it failed miserably. I can’t remember the exact reasons why we then did a DE study but as you can see it worked better than the Flash study.

Slide 20|

To summarise, free breathing high-pitch CTPA provide exceptional diagnostic images with better enhancement through the pulmonary trunk than the normal pitch CTPA studies. Sub-second scans allow us to ‘freeze’ motion. There is less cardiac and respiratory motion. Thank you

Slide 21|

References