Categories
IR subject

Cone beam CT

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
Categories
IR subject

Embolization – Materials & Technique

Maarten Smits, Max Seidensticker
UMC Utrecht and LMU Hospital Munich

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Principle of embolization

This article is about therapeutic embolization, in which a part of the blood circulation is impeded by the introduction of embolization agents.
An embolus is defined as “Undissolved material carried by the blood and impacted in some part of the vascular system (…)” (Dictionary.com).
The indications for embolization can be grossly categorized into bleeding control, occlusion of pathologic vasculature, and tissue ablation. See table 1.


a. Coaxial catheterization technique using a 4-6F catheter that accomodates a 1.5-3F microcatheter that can be used to negotiate further in the vascular system. b. Example of a microcatheter (Progreat® [Terumo]).

Catheterization

Access to the vessel to be embolized can be obtained by direct puncture (superficial varicose veins, fistula or malformation) or indirectly via an endovascular approach. We will focus on the endovascular approach. The target vessel should be catheterized with a catheter that is able to negotiate the vasculature and allow passage of the embolization agent.
Microcatheters are most suitable for this purpose. These catheters usually have a straight tip and a typical outer diameter (OD) of 1.5-3 French (F). They can be advanced coaxially through a larger supporting catheter (OD 4-6 F) in order to reach deep in the vasculature. Guiding sheaths or guiding catheters can be used for support as well.


Two types of anti-reflux catheters. a-b. Cone-shaped Surefire infusion system® [Surefire Medical]. The deployed tip is faintly visible (see radiopque markers at the arrow). c-d. Occlusafe® microcatheter featuring a balloon at the tip [Terumo].
A stable position of the entire coaxial system is key when administering the embolization agent to minimize the risk of non-target embolization. Exciting developments in this field are the anti-reflux catheters that use either a cone- or umbrella-shaped tip (e.g. Surefire infusion system®, Surefire Medical Inc.), or a balloon (Occlusafe®, Terumo) to prevent reflux of embolization agent and thus non-target embolization. These catheters are mainly used for chemoembolization or radioembolization.

Example of microcatheters with a detachable tip, Sonic Fusecath® [Balt extrusion]

When using agents like histoacryl or ethylene-vinyl alcohol (Onyx®) there is a risk that the microcatheter gets stuck in the embolus. Leaving a microcatheter inside the body (after cutting off the external part) causes surprisingly little symptoms, and the catheter will eventually be integrated in the vessel wall. This is of course not a desirable situation. To overcome this issue, manufacturers have developed catheters with a detachable tip akin to a lizard’s tail (e.g. Apollo® [Medtronic] and Sonic® [Balt]). The tip breaks off when the tip is fixed in an embolus and the rest of the catheter is pulled outwards, leaving only a small part of the catheter in situ.

Embolization agents

There is a large variety of agents available to perform embolization, ranging from autologous blood clot to devices like coils or vascular plugs, see Table 2.
Choosing the embolization agent depends on the indication and the experience at hand. Each agent has its specific advantages and disadvantages. Factors influencing the choice of material comprise the size of the target vessel, flow velocity, and duration of embolization (permanent/temporary).

Autologous blood clot

In acute situations, the patient’s own blood can be used for embolization. A small amount of blood is withdrawn and kept for a few minutes after which thrombus has formed and can be reinjected at the point of hemorrhage. This form of embolization is temporary.

Schematic display of thrombin injection in a false aneurysm.

Thrombin

Thrombin is well known for embolizing false aneurysms (aneurysm spurium), particularly in the groin after femoral arterial puncture. This technique – first described in 1986 – consists of direct, ultrasound guided injection of thrombin in the false aneurysm. The primary success rate is about 90%. Aneurysms should have a small neck to avoid spill of thrombin into the femoral artery which can cause femoral artery thrombosis (0.5-1% risk).

The neck of the aneurysm should be identified. This neck should ideally be narrow relative to the overall size of the false aneurysm to avoid spill of thrombin in the femoral artery. See video.
 
Thrombin is injected under ultrasound-guidance. The ultrasound probe can be used to compress the neck of the false aneurysm. However, a little inflow of blood is needed to evenly spread the thrombin through the aneurysm.

A variety of coil shapes: a. Nester® coil [Cook Medical], b. Complex helical coil [Boston Scientific], c. Straight Hilal® coil [Cook Medical], d. Figure 8 coil [Boston Scientific], e.  Vortx coil [Boston Scientific], f. Vortx Diamond coil [Boston Scientific], g. Multi-curled Hilal® coil [Cook Medical], h. AZUR® framing coil [Terumo]

Coils

Coils are probably the most used type of embolic agent. Coils are braided wires generally made of platinum or stainless steel that can be loaded into a catheter and will coil into shape when released out of the catheter or sleeve. Coils come in a large variety of shapes and sizes fit for specific situations. Most coils have protruding fibers to enhance thrombosis.
Common thicknesses of coils are: 0.014”, 0.018”, 0.035” and 0.038”. Microcatheters will generally only accomodate 0.014” and 0.018” coils. Other important dimensions of coils are their length (mostly in the range of 1-10cm) and their diameter when deployed (range of 1-20mm). As a general rule, the diameter of the coil should be oversized about 20% compared to the vessel diameter as measured on DSA.
Indications:
– Traumatic hemorrhage
– Aneurysms
– Varices (in certain cases)
– Arteriovenous fistulas

Case of embolization for acute hemorrhage from a branch of the left hepatic artery (arrow). This was iatrogenic injury after abdominal surgery. A combination of Gelfoam, straight- and figure-8 coils was used. No more extravasate after embolization (B).

Coil packaging [Boston Scientific].

Saline-flush Technique.

Coil delivery

Coils often come packaged in a protective sheath (see figure). The coil is inserted by placing the sheath in the back end of the catheter. Then, the plunger that comes with the coil, is pushed all the way into the back end of the sheath containing the coil. The coil is then loaded into the catheter.
From there on, coils can be pushed through the catheter using a coil pusher. Alternatively, pushing the coil with a saline bolus is a fast way to deliver coils (saline-flush technique).

Coiling techniques

Packing coils in a large calliber vessel can be challenging. Here are a few techniques that can be used.
In the standard (coaxial) technique, coils are densely packed by pushing and pulling the microcatheter forward and backward.
In the anchor technique, a side branch is used to anchor the first coil which is intentionally protruding into the mother vessel. This provides an anchor for following coils.

In the scaffold technique, an oversized coil with high radial force is placed first. This coil will stay in position due to its high radial force. The scaffold created this way can be filled with smaller and softer coils. The scaffold technique is particularly usefull in large or high-flow vessels.

a. Interlock™ system [Boston scientific], b. Concerto™ system [ev3], c. InZone® Detachement system [Stryker], d. Axium™ Instant Detacher [ev3].

Detachable coils

Conventional coils are deployed once pushed out of the catheter. Incorrectly placed coils can cause serious complications and can be hard to retrieve. Several types of detachable coils have been developed to enable more controlled coil-embolization. Detachable coils are helpful for embolization in high-flow vessels, embolization close to the origin of a vessel, and for aneurysms when dense coil packing is desired. Detachable coils can be retracted and redeployed a number of times until satisfactory placement is reached and the coil is detached.
There are several types of detachable coils that have their own method of detaching. Some coils are locked onto the pusher and are released when the coil is fully pushed out of the catheter. Most detachable coils work with some kind of detacher that mechanically or electronically activates a release mechanism (see images for examples).

a. Dense coil packing in the gastroduodenal artery prior to yttrium-90 radioembolization. b. The attempt to embolize up to the origin of the gastroduodenal artery led to a coil protruding into the proper hepatic artery (arrow). c. Embolization up to the origin of a vessel is easier controlled using detachable coils (different case then a and b, using Interlock®[Boston Scientific]).

a-c. PCOM aneurysm embolized using detachable coils

Several types of vascular plugs: A-C. Amplatzer plugs [St. Jude Medical], D. Microvascular plug system [ev3]

Deploying an Amplatzer plug

Plugs

Vascular plugs are devices made of a nitinol mesh that expand into their original shape when deployed. Plugs can be used as alternatives to coils for controlled embolization of a large vessel, vessel wall defect or pseudoaneurysm. A single plug can be used to embolize a vessel that would otherwise require multiple coils. Vascular plugs are detachable.
Indications:
– High flow vessels
– Large defects

Endoleak repair with a plug post TEVAR. a. DSA from the left subclavian artery shows a small endoleak. An Amplatz plug (14mm x 10mm) was placed in the origin of the subclavian artery (b). c. Volume rendered CTA image shows the position of the plug in relation to the TEVAR stent.

a. Case of a arteriovenous fistula of the suprascapular artery in a young patient years after a wound from a glass incident. b. A 7mm x 20mm balloon was used to obstruct the high flow fistula. c. Coils were added to fully embolize the efferent artery. D. Goldballoon® [Balt extrusion].

Detachable Balloon

The first use of detachable balloons were described by dr. Serbinenko in 1974. Detachable balloons are an excellent tool for obstructing high-flow vessels or fistulas where coils can easily dislodge. The method of detachment can differ, one system works by pushing a larger catheter over the balloon catheter wheras another systems requires gentle traction on the balloon catheter to detach the balloon.
Indications:
– High flow vessels
– Fistulas

Particles

Embolic particles can be devided into spherical and non-spherical particles.
Spherical particles, sometimes called spheres or microspheres, come in several sizes ranging from app. 40 – 1300μm (micron). Some products are advertised to have a tight size callibration which is favorable for controlled embolization. Examples of spherical particles are: Embozene® [Boston Scientific], Embosphere®/Embogold®/Quadrasphere® [Merit Medical Systems], Contour SE™ [Boston Scientific], Bead Block® [BTG], and LC Bead®/DC Bead® [AngioDynamics], see Table.
Microspheres can be used for embolization of tumors as well, especially when containing drugs or radioactive elements. TransArterial ChemoEmbolization (TACE) and Radioembolization (a.k.a. Selective Internal Radiation Therapy, SIRT) are well known examples of embolization for oncologic purposes. Tumor embolization can also be performed with inactive particles, which is referred to as ‘bland’ embolization.

Particles for embolization a-b. Embozene® microspheres [Boston Scientific] c. Embospheres® [merit Medical] d. Bead Block® [BTG-IM] e. LC Bead® [BTG-IM]  f. Contour™ PVA embolization particles [Boston Scientific].

Poly-vinyl alcohol particles (PVA) are mostly non-spherical, although spherical PVA particles are available as well.  PVA particles are irregularly shaped, can aggregate and the embolization is slightly less predictable then with spherical particles.
Non-target embolization is a great risk of particles. Since particles are very small they can shunt into non-target vessels or through arteriovenous shunts and cause severe complications and even death. Most types of particles are not visible under angiography.
Indications:
– Recurring hemorrhages, access for future embolization needs to be preserved (e.g. nasopharyngeal or bronchial hemorrhage)
– Tumors
– Hemoptysis
– Uterine fibroids

Embolization of a uterus myoma. a. DSA pre-embolization b. post-embolization with 700μm Embozene® particles.
Gelfoam® preparation. (figures courtesy of Dr. Orhan Konez)

Gelfoam®

Gelfoam® [Pfizer] is a sterile compressed sponge created from gelatin granules and purified porcine skin. Gelfoam® comes as a layer of foam that should be cut into small pieces and then mixed with contrast solution to create a very thick mixture (slurry) that can be used for embolization.
Gelfoam® is a fast, cheap and reliable embolic agent. Its key characteristic is non-permanent embolization. Gelfoam® is usually completely resorbed in a few weeks.
Indications:
– Traumatic hemorrhage
– Post-partum fluxus
– Uterine fibroids

Liquid embolics

There is a variety of liquid embolics. Liquid embolics can be applied by direct injection into the target region or transcatheter injection. Liquids can be combined with non-liquid embolic agents such as coils or plugs.

Glue

Cyanoacrylate (Histoacryl®) is similar to glue used for all kinds of purposes in and around the house. It is a monomere that polymerizes when in contact with water or blood. Histoacryl is a blue colored substance that comes in small pipets (see image).
Histoacryl can be diluted with Lipiodol for radiopacity and to prevent immediate solidification. The catheter needs to be primed with 5% glucose water before injection. Then, Histoacryl is slowly injected and a cast will form distal to the catheter tip. After a few seconds the catheter can be retracted with a firm pull. Make sure there is no reflux to prevent non-target embolization or catheter entrapment.
Indications:
– Traumatic or iatrogenic hemorrhage
– Vascular malformations
– Tumor embolization
– Portal vein embolization

a. Aethoxylerol® [Kreusler pharma] b. Dehydrated alcohol injection [Akorn]

Sclerosants

Intravascularly injected sclerosants lead to and inflammatory reaction resulting in endothelial damage and fibrotic obstruction of the vessel or vascular malformation. Sclerosants are indicated to embolize a peripheral vascular bed.
Examples are:
Polidocanol (Aethoxysklerol®), alcohol, detergents, and antibiotics (doxycycline, bleomycine).
Indications:
– Varices
– Vascular malformations
– Tumor embolization

Onyx® [ev3]

Precipitating agents

There is a category of liquid embolics consisting of ethylene vinyl alcohol copolymers that are dissolved in dimethyl sulfoxide (DMSO) and are mixed with tantalum powder for radiopacity. Examples of available products in this category are: Onyx® [Covidien], SQUID™[Emboflu], and PHIL® [Microvention].
These precipitating agents need to be shaken at least 20 minutes prior to use to obtain a homogeneous mixture of the tantalum powder. Catheters and hubs used need to be compatible with DMSO since standard plastics are dissolved by DMSO. The dead space of the catheter needs to be primed with DMSO before injection.
Once injected into the body, DMSO dissolves and the ethylene vinyl alcohol copolymeres precipitate inside the blood vessels. The embolic agent can be pushed deep into the vascular bed by slow, intermittent injections. Backflow along the catheter can be managed by pausing injection and waiting for a plug to form around the catheter tip. Consequent injections will follow the path of least resistance which is then hopefully away from the catheter and into the target vasculature. Microcatheters with a detachable tip are strongly recommended.
Indications:
– Vascular malformations
– Hemangiomas
– Aneurysms

Various embolic agents in relation to the vessel size that they are capable of embolizing

Choice of material

The choice of embolic agent depends on many factors. Safety is the main factor, therefore experience of the interventionalist with the embolic agent and risk of non-target embolization should always be weighed.
Other factors are the organ or lesion to be embolized, the desired effect of embolization (permanent or temporary), proximal or distal embolization. Availability and costs of the embolic agent are not unimportant either.
Distal embolization (using liquids or small particles) is more likely to cause tissue necrosis since end-arterioles are blocked and collaterals have no way of reaching the vascular bed. Distal embolization carries a high risk of shunting to non-target organs such as the lungs or brain.
Many embolic agents can be combined to increase efficacy.

References

  • 1. Cope C, Zeit R. Coagulation of aneurysms by direct percutaneous thrombin injection. AJR Am J Roentgenol 1986; 147: 383–387.
  • 2. Vascular Embolotherapy – Vol 1. General Principles, Chest, Abdomen and Great Vessels-2006. Springer.
Categories
IR subject

Radioembolization

Maarten Smits, UMC Utrecht

Electron microscope image of resin 90Y-microspheres

Rationale

Radioembolization (RE) is a form of internal radiation therapy for tumors in the liver.

RE is performed with microspheres containing radioactive yttrium-90 or holmium-166 that are administered in the hepatic arteries from an endovascular catheter.

The liver has a dual blood supply. The liver parenchyma relies mostly on the portal vein for blood supply. In contrast, tumors arising in the liver are predominantly fed by the hepatic artery.1,2

Table 1. Tumor types

 

Table 2. Gross selection criteria

Patient selection

RE can be performed in patients with all kinds of tumors in the liver, ranging from primary tumors such as hepatocellular carcinoma (HCC) and cholangiocarcinoma (CCC) to liver metastases from colorectal carcinoma, breast cancer, neuroendocrine tumors and melanoma, see table 1.

 

 

 

There is much discussion about the exact selection criteria for RE. Table 2 lists the gross selection criteria that many centers use.3

More information is available on the websites of the manufacturers of yttrium-90 microspheres: SIR-Spheres(R) and Therasphere(R). QuiremSpheres(R), containing holmium-166 are not commercially available yet.

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

Vascular anatomy

Before proceeding with a work-up angiography it is advisable to have a look at the patient’s vascular anatomy. The arterial anatomy of the liver region can vary widely between patients (see figure). A thin slice CT angiography viewed in MIP-mode can help to identify variant anatomy.4,5

Figures from ref 5 with permission from Springer

Work-up procedure in the angiosuite.

Digital subtraction angiography with contrast injection in the common hepatic artery. 
Injection position of 99mTc-MAA in the right hepatic artery after coil embolization of the gastroduodenal artery (long arrow) and the right gastric artery (short arrow). Subsequent injection position in the left hepatic artery. Images reproduced from Smits et al. Eur J Pharm 2013, with permission from Elsevier. 

Work-up angiography

  1. Perform a contrast run from the celiac axis and the superior mesenteric artery (for abberant vessels).
  2. Assess the vascular anatomy. The anatomy should have been assessed on CTA at this point.
  3. Embolization of non-target vessel (see subtopic)
  4. Perform C-arm CT (see subtopic)
  5. Inject 99mTc-MAA at the desired injection positions

Panel a displays a digital subtraction angiography with contrast injection in the common hepatic artery in a breast cancer liver metastases patient. Panel b: 1. common hepatic artery; 2. gastroduodenal artery; 3. right gastric artery; 4. left hepatic artery, 5. right hepatic artery. Tumorous areas are marked with ‘T’, catheter as a white dotted line.

Embolization of non-target vessels

Traditionally, all non-target vessels arising near to the injection position are coil-embolized. These vessels typically include the gastroduodenal artery (GDA) and right gastric artery (RGA). Currently, most centers decide whether or not to close these vessels based on the vascular anatomy (e.g. Where is the origin of the GDA? Is there retrograde flow in the GDA?) and intended injection position(s). Also, the use of an anti-reflux catheter may decrease the risk of back-flow and thus the need of closing the non-target vessels.

 

Cystic artery

The cystic artery can pose a dilemma. The cystic artery often arises from the right hepatic artery and closing this vessel to prevent radiation cholecystitis may be tempting. Closure may however cause ischemic cholecystitis. The consensus of the available literature on this topic seems to be that in case of visually (so no dosimetry needed) high 99mTc-MAA deposition in the gallbladder, one should seek a more distal injection position. If that is not possible, one can choose to (partially) close the cystic artery with coils or gelfoam, preferably at the time of the treatment angio to avoid formation of new collateral vessels.6,7

C-arm CT used to ensure complete tumor coverage. Depicted is a neuroendocrine tumor, supplied by the left- (in red) and right hepatic artery (in blue/green).

Cone-beam CT

One can use the C-arm at the angio suite to acquire cross-sectional CT images. This technique is called C-arm CT or cone-beam CT. The combination of CT with contrast injected via an endovascular catheter can be useful in several ways:

  1. Non-target embolization detection:
    Is there any enhancement of non-target organs?
  2. Selecting the target vessel supplying a tumor
    In case of multiple vessels projecting on top of each other on 2D-angiography, the target vessel can be identified using C-arm CT.
  3. Assessment of the target volume:
    What is the size of the volume that is reached from a certain injection position?
  4. Tumor coverage:
    Is the tumor completely covered? A tumor may be supplied by multiple vessels from within the liver or outside the liver A large variety of parasitic vessels have been described, particularly in HCC. Incomplete tumor coverage will lead to inadequate treatment.

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

99mTc-MAA imaging

After injection of 99mTc-MAA at the work-up angio, the distribution is assessed using nuclear imaging.

 

Non-target embolization

Distribution of 99mTc-MAA anywhere outside the liver can be detected with 99mTc-MAA-SPECT and should be resolved before treatment. Regions of interest are the duodenum, stomach, pancreas, falciform ligament and umbilicus. Free pertechnetate can cause diffuse high activity in the stomach simulating non-target embolization. This can be recognized by its characteristic pattern and accompanying high uptake in the thyroid gland. With the growing experience of many IR-teams and the growing use of C-arm CT, non-target embolization is now more and more frequently detected already during angio. Some experts have even gone as far as stating that 99mTc-MAA is only necessary for lung shunt detection.

 

Lung shunt

Hepatopulmonary shunting can be evaluated on planar imaging. In fact, the limit of a maximum 20% lung shunt fraction is based on studies using planar imaging. However, it is clear that attenuation-corrected Single Photon Emission Computed Tomography (SPECT) is far more accurate than planar imaging. Even when using SPECT, it seems that 99mTc-MAA tends to overestimate the amount of lungshunting when compared with the actual microspheres.8

 

 

a. Administration of 90Y-microspheres in the angio-suite. b. Therasphere administration system

Treatment angiography

After the work-up angiography and 99mTc-MAA imaging, the patient returns for the actual treatment procedure. This is generally performed 1 or 2 weeks after the work-up angiography.

In the angiosuite, a microcatheter is placed in the exact same position(s) as where the 99mTc-MAA has been injected. Next, the administration system holding a vial that contains the radioactive microspheres is attached to the microcatheter. The administration system consists of a perspex box for SirSpheres, Theraspheres, and Quiremspheres (perspex absorbes most beta radiation), see images.

The microspheres are brought into suspension and pushed forward through the tubing system and into the microcatheter. Depending on the type of microspheres and type of tumor, one should monitor the forward flow distal to the catheter to avoid reflux.

Post-treatment dosimetry

Post treatment imaging to visualize the distribution of the radioactive microspheres can be useful in order to confirm an adequate dose to the tumors and no non-target deposition.

Post treatment imaging for yttrium-90 microspheres can be performed with Bremsstrahlung SPECT (indirect gamma-emission from decelerating beta-particles) and with PET (32 positrons per million decays), see figure A.

Holmium-166 microspheres on the other hand can be visualized with SPECT (direct gamma-emission) and with MRI (holmium is a paramagnetic element). See figure B.

References

  • 1. Bierman HR, Byron RL, Jr., Kelley KH, et al. Studies on the blood supply of tumors in man. III. Vascular patterns of the liver by hepatic arteriography in vivo. Journal of the National Cancer Institute 1951; 12(1)
  • 2. Dezso K, Bugyik E, Papp V, et al. Development of arterial blood supply in experimental liver metastases. The American journal of pathology 2009; 175(2)
  • 3. Coldwell D, Sangro B, Wasan H, et al. General selection criteria of patients for radioembolization of liver tumors: an international working group report. American journal of clinical oncology 2011; 34(3)
  • 4. van den Hoven AF, Smits ML, de Keizer B, et al. Identifying aberrant hepatic arteries prior to intra-arterial radioembolization. Cardiovascular and interventional radiology 2014; 37(6)
  • 5. van den Hoven AF, van Leeuwen MS, Lam MG, et al. Hepatic arterial configuration in relation to the segmental anatomy of the liver; observations on MDCT and DSA relevant to radioembolization treatment. Cardiovascular and interventional radiology 2015; 38(1)
  • 6. Prince JF, van den Hoven AF, van den Bosch MA, et al. Radiation-induced cholecystitis after hepatic radioembolization: do we need to take precautionary measures? Journal of vascular and interventional radiology : JVIR 2014; 25(11)
  • 7. McWilliams JP, Kee ST, Loh CT, et al. Prophylactic embolization of the cystic artery before radioembolization: feasibility, safety, and outcomes. Cardiovascular and interventional radiology 2011; 34(4)
  • 8. Elschot M, Nijsen JF, Lam MG, et al. ((9)(9)m)Tc-MAA overestimates the absorbed dose to the lungs in radioembolization: a quantitative evaluation in patients treated with (1)(6)(6)Ho-microspheres. European journal of nuclear medicine and molecular imaging 2014; 41(10)

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