Reviews of the Medtronic Emprint HP Ablation System | Which Medical Device

 

 

The new Emprint Microwave ablation system from Covidien/Medtronic is an advance in microwave technology. There are certain differences between microwave and radiofrequency ablation although both cause cell and therefore tumour death by heating.

RFA is current based and unipolar systems (most of them) require grounding pads to form a complete circuit. Heating is dependent on tissue impedance which rises as the tissue is ablated and the current rapidly drops off with distance. Machine algorithms varying power with distance can counteract this to a certain extent.
Microwave technology is field based. The electromagnetic field around the probe heats tissue through vibrating the water molecules. The field shape is determined by flow of electrons within the probe and material close to the probe, ie the targeted tissue.

When tissues are heated there is an active and passive zone. The active zone is directly heated by the RF energy or microwave whereas the passive zone is heated by conduction from the active zone.

Heat can be conducted away from the tumour by ‘thermal sinks’ such as blood vessels. This affects both microwave and RF technology. There is also an ‘electrical sink’ whereby RF current travels along vessels and other tissue planes ‘short circuiting’ the current and causing unpredictable ablation volumes. Microwave systems are not affected by this.

Covidien call their technology ‘Thermosphere technology’. This consists of three components that aim to produce a more spherical and therefore predictable zone of ablation.

These are:

Thermal control

Field control

Wavelength control

 

Thermal Control

Coaxial waveguides heat up as the microwaves are directed to the probe tip so cooling is needed. Thermal control is cooling of the needle almost all the way to the tip of the device.

 

Field Control

Field control restricts the radiating portion of the antenna to the distal end with an RF choke.

 

Wavelength control

As tissue heats the tissue desiccates. This lowers the dielectric constant. This in turn increases the wavelength. This leads to oval ablation zones. Wavelength control ‘hides’ the radiating metal from the tissue by the circulating fluid. This gives a constant favourable radiating environment.

 

 

Results in Practice

This is a small right upper pole RCC I recently treated using the Emprint system.

 

This image shows the microwave needle inserted into the tumour

 

You can see the relatively rounded ablation zone on the post procedure CT at 3 months.

 

The other interesting feature of this device is the extreme ease of visibility of the needle/antenna using ultrasound. I think this may be due to the water jacket for cooling.

You can see this demonstrated really well in the video tab.

 

The following 2 pictures show a larger RCC treated using this system.

 

 

Area of ablation post microwave therapy:

 

Using The Device

 

The device itself is extremely easy to use and is well shown in the video tab showing a full microwave ablation.

You insert the needle/antenna into the tumour using US/CT guidance. Then set the time and power for the ablation. You can then perform track ablation by slowly withdrawing the needle whilst active.

The theoretical ablation sizes are in the table below:

This shows the variation with time and power.

My only slight criticism of the system is the slightly larger needle size compared with RFA, however this is made up for by its visibility and rapid more predicatble ablation zone.

 

Dr Phil Haslam

Consultant Interventional Radiologist

Freeman Hospital, Newcastle upon Tyne

UK