Author: Ahmed Alsultan

Maarten Smits en Evert-jan Vonken, UMC Utrecht

 

What is Cone beam CT?

The c-arm in the angio-suite can be used to perform CT imaging by rotating the c-arm around the patient during imaging. During this rotation, an isotropic (i.e. equally sized voxels) volume is generated from which CT images can be obtained through multiplanar reconstruction (MPR). 
 Synonyms for cone beam CT are:
- C-arm CT
- Rotational CT
- Rotational angiography
- Flat-panel volume CT
Commercial names are often used as well, for instance:
- DynaCT [Siemens]
- XperCT [Philips]
- 3D imaging [Toshiba]
- Innova CT HD [GE Healthcare]

Fan-shaped beam in conventional CT

The cone-shaped beam of cone beam CT
This article gives an overview of the technique, hardware and possible applications of cone beam CT in the angio suite.
Cone beam CT can be performed during a regular angiography or any other intervention in the angiosuite. The body part in the isocenter of the C-arm is imaged during a 180 – 360 degrees rotation.
Just like with standard CT, the images can be viewed in orthogonal planes or in a multiplanar reconstruction. In addition, the images can be fused with fluoroscopy to provide the interventionalist with extra information

Hardware – C-arms

There are several kinds of C-arms available. These can be floor- or ceiling mounted and have different degrees of freedom.
Cone beam CT is an available option on most of the current systems.


Pros and cons of cone beam CT regarding image quality

Image quality

The image quality of cone beam CT is in general inferior to the quality of conventional CT scans. Some important characteristics are listed on the left.

Types of rotation

Systems can rotate either in a propellor-fashion (from the head-end of the table) or in a roll-fashion (from the side of the table) or both, see figure.

Video showing a double-sweep off-center rotation to create an extended field of view cone beam CT


Extended FoV obtained by using a double off-center rotation (source: https://youtu.be/GOoErthM3w4)
Certain systems can perform more advanced rotations, such as:
Double off-center rotations to obtain a larger field of view (see video).
Dual phase scanning by scanning during the forward rotation and during the backward rotation. This way, a dual phase CT can be acquired with for instance contrast in arterial and venous phase.

Set-up with CT in the angio suite, Infinix-I 4DCT [Toshiba]

The typical, limited field of view (FoV) of a cone beam CT covers only a part of the abdomen. Angio-CT has a far larger FoV. 

Using 4DCT for perfusion imaging. Excerpt from a presentation by Dr. F. Irani (see full video below).

Full lecture on Angio-CT by Dr. F. Irani (Singapore General Hospital)

Hardware – Angio CT

Angio suites can be equipped with a ‘real’ CT-scanner in the room. This allows for fast imaging with a large field of view and even the ability to perform 4D imaging (figure). These systems are still quite rare with only a few systems installed world wide.

Applications

There are several ways in which cone beam CT can be helpful for interventions:
  Combining CT-guided puncture with angiography
  Assessing vascular territories
  Identifying feeding vessels
  Evaluation of complex vascular structures
  Radiotherapy planning

Direct puncture of an aortic aneurysm sac with glue embolization

 Combining CT-Guided Puncture with angiography

It is already common practice to combine interventions in the angio suite with ultrasound. Access to for instance a vessel or an abscess can be obtained with ultrasound, after which the procedure can be further guided by fluoroscopy.
Combining angiography with CT can be very useful as well. Examples are:
  • Direct endoleak puncture and embolisation
    Finding a route to puncture the aneurysm sac (preferably the part of the sac that is filling with contrast), and then switch to angiography to find and embolize feeding vessels.
  • Challenging abscess drainage or for instance Percutaneous Radiographically placed Gastrostomy (PRG)
    Finding a save route to puncture the gastric wall in difficult cases (e.g. overlying bowel)
  • Vertebroplasty
    Navigating the needles in the correct position. The cementing can be followed with fluoroscopy.

 Assessing vascular territories

A major advantage of cone beam CT is that images can be acquired while injecting contrast through the catheter into a selected region.


Fused cone beam CT of a left (red) and right (blue/green) hepatic artery injection shows complete tumor coverage.


Contrast enhancement of the stomach during work-up for radioembolization. This branch had to be coil-embolized.


Selective contrast injection allows for exact delineation and calculation of the perfused volume for radioembolization.
 
  • Prostate embolization or radioembolization or chemoembolization of liver tumors
    • Is the tumor or target organ fully targeted from the current injection position?
    • Are there any non-target areas/organs perfused from this injection position?
    • What is the volume of the targeted area? This is necessary for calculating the required activity in radioembolization.

Tumors can be (partly) vascularized by aberrant or parasitic vessels. Cone-beam CT can help to check if each tumor is completely covered from the intended injection positions.

 

Fusion of two cone beam CT's using XperCT software [Philips Healthcare]

Cone beam CT can be used to find the vessel (red) supplying a target tumor (in blue)

The guidance can be used as an overlay for real-time guidance (different patient).

 Identifying feeding vessels

In angiography, vessels can be projected on top of each other which makes it hard to identify which vessel is leading to what part of the organ or tumor. Cone beam CT can help to identify the target vessels.
This is for example useful in:
  • Radioembolization / TACE / vascular malformations:
    For selectively targeting a tumor in one of the liver segments, cone beam CT helps to identify the right branch. Most vendors offer software packages for this purpose. The target area can be selected (drawing a region of interest around it) and the software will identify the main feeding vessels. This sort of ‘roadmap’ can be projected on top of the real time angio images for ease of navigation.
  • Hemorrhage:
    It can be hard to determine the exact branch that is leading to the defect or blush. Identifying the feeding vessel with cone beam CT can save time by obviating the need for selective catheterization of all daughter branches.

 Evaluation of complex vascular structures

Cone beam CT with per-catheter contrast injection can yield high-resolution images with high contrast between vascular structures and surrounding tissue. These images can be used to better evaluate complex vascular structures such as cerebral aneurysms and AVMs.

 

Cone beam CT for planning stent placement for an ophthalmic artery aneurysm
Used for:
  • Aneurysm and AVM embolization: measuring dimensions, evaluating feeding vessels and choosing the optimal projections. The projections can be chosen on the 3D reconstruction of the cone-beam CT and can then be stored for automatic positioning of the C-arm for DSA.

Angio of a cerebral AVM with the patient wearing the radiotherapy mask.
Cone beam CT images enable clear delineation of the AVM.
Dose planning software showing the dose to the AVM and to surrounding structures (see dose volume histogram in the right upper corner). Case courtesy dr. Ernst Smid, UMC Utrecht

 Radiotherapy planning

It can be difficult to outline the exact position and extent of cerebral AVMs when planning stereotactic radiotherapy. AVMs may not be clearly delineable on MRI or CT. Cone-beam CT with selective contrast injection in the feeding vessels is useful for this indication. The selective contrast injection increases the contrast between the AVM and the surrounding tissue.
Scanning is performed with the patient’s head in the radiotherapy mask (figure).
Used for:
  • Cerebral AVMs
    Stereotactic radiation of cerebral AVMs requires precise delineation of the target and non-target volumes. Unfortunately, AVMs are really hard to delineate on standard imaging (MRI or CT). The selective contrast injection during angiography yields a good visibility of these malformations. 
    By performing the angiography in the radiation mask, the cone-beam CT can be used to delineate the AVM (see figures).

Formula 1 pit stop. A well-trained team makes this a smooth procedure. 

Tips and tricks

Integrating cone-beam CT in the workflow of the angio suite may be challenging at first. The angio team may be out of their comfort zone when asked to prepare for a cone-beam CT. 
Performing a cone-beam CT can be compared with performing a pit stop in Formula 1 racing. It is a team effort in which each team member knows what to do. The technicians/nurses need to be actively involved in preparing the patient.
The first rounds of cone beam CT may be time consuming and cumbersome. But after a few times, it becomes routine. Our advice is therefore to go through all the steps with your team and write a protocol that team members can follow. Then, start using this feature as soon as possible.

Diluting the contrast agent results in less artifacts
 
Other tips and tricks:
  • Patient positioning: In case of abdominal cone-beam CT, the arms of the patient should be raised over his/her head to reduce attenuation in lateral projection and make space for the rotating c-arm. A technician or nurse should help the patient to place the arms over the head. Any other objects that may interfere with the rotation of the C-arm must be removed (e.g. infusion pole, monitors).
  • Contrast: per-catheter contrast injections should be performed with contrast agent diluted with 50% saline. Pure contrast will result in inferior image quality with streak artifacts surrounding the arteries. The injection rate depends on the type of vessel that is catheterized.
  • Timing: depending on the desired imaging phase, one should choose a delay and scan time. For liver imaging for instance, a variable delay (based on the time to parenchymal enhancement seen on digital subtraction angiography) and a 10 sec scan time have proven to yield good results.

  • Post-processing: familiarize yourself with the viewing and post-processing software provided by the vendor of your system. Reconstructing the images or activating certain tools may not be intuitive and may cause stress when faced with this during an intervention.
  • Reviewing images: once the cone-beam CT has been made, one should take time to review the acquired images. Most systems have a viewing station in the control room outside the angio suite. The interventionalists are adviced to take off their gloves and gowns and take a few minutes to scroll through the scans. Important decisions are made based on these scans, so these few minutes are well spent.

References

  • van den Hoven et al. Use of C-Arm Cone Beam CT During Hepatic Radioembolization: Protocol Optimization for Extrahepatic Shunting and Parenchymal Enhancement. CVIR 2015
  • Tognolini et al. C-arm computed tomography for hepatic interventions: a practical guide. JVIR 2010

Lorem ipsum

"Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."

CT MIP hepatic artery
Aberrant left hepatic artery arising from the left gastric artery (arrow)

Work-up procedure in the angiosuite.

99mTc-MAA SPECT and corresponding 90Y-SPECT. Images reproduced from Smits et al. Eur J Pharm 2013, with permission from Elsevier.