Imaging is a process that is essential to understand the dynamics of transplantation. It also reveals the intricate biological aspects that determine graft survival. Dr Sonal Asthana, HPB and Multiorgan Transplant Surgeon, Aster Integrated Liver Care group, Aster CMI Hospital, Bengaluru speaks to M Neelam Kachhap about the trends in solid organ imaging
What are the recent trends/ new technologies for imaging solid organs?
Broadly, imaging for solid organs can be static or dynamic. Ultrasound is an older tool, but is extremely useful in experienced hands. Recent advances include the use of contrast enhanced ultrasound (CEUS), which uses microbubbles to better define the blood flow in the organs studied. An ultrasound variant called a fibroscan gives us an assessment of liver stiffness (fibrosis) by using shear stress. The most common modalities of cross sectional imaging are CT scans, MR scans and their variants (PET-CT and fMRI).
Which modalities are used for evaluation of liver?
All three modalities mentioned are useful for liver imaging. CT is more commonly used because of familiarity of interpretation. Most clinical specialists are well versed with CT interpretation. MRI scans are more useful, but less commonly used.
What are the challenges during imaging of paediatric patients?
There are very specific concerns that need to be kept in mind when dealing with paediatric patients. Firstly, radiation exposure should be minimised. Because they have more rapidly dividing cells than adults and have longer life expectancy, the odds that children will develop cancers from X-ray radiation may be significantly higher than adults. It has been estimated that children less than 10 years of age are several times more sensitive to radiation than middle-aged adults. CT scans can subject patients to 10-800 times the amount of radiation compared to an X-ray, hence need to be used with caution. Secondly, imaging tools which have long processing times are not suitable for very small children. Children often need sedation or anaesthesia for imaging procedures such as MRI due to long scan times.
Which modalities are preferred for pediatric patients as compared to adult patients?
As I mentioned earlier, radiation exposure is a primary concern while dealing with paediatric patients and hence measures that do not subject children to ionising radiation like ultrasound and MRI are preferred. However, MRI scans are noisy and take a long time to process images. So, it is difficult to work with childeren around this. They get scared and they move so children may need to be sedated for the process. Incidently, the newer generation MRIs have tried to work around these problems with virtual reality headsets (to avoid claustrophobia). They also offer faster processing times and are better.
Are MRI scans better than MDCT while evaluating hepatic disease?
Yes. MRI scans can evaluate fat, fibrosis and general state of the liver. They are much more useful in obese patients and provide much better imaging cover of the liver as compared to MDCT. In fact, MR specialists can distinguish between benign and malignant liver tumours with more than 95 per cent accuracy, without the need for a biopsy.
How has imaging changed for transplant procedures in recent years?
Several new technologies have benefitted through the transplant field over the past few years. One of them is the intraoperative ultrasound with high frequency probes. This equipment helps us to check the vascular joints (anastamosis) at the time of surgery which is crucial for favourable outcomes. Another one is the portal vein flow cuffs. This is a cuff like device placed around the main vein of the liver (portal vein) after surgery. It measures the flow through the vein and alerts the surgeon in case of low flow or risk of blockage. Another important development is CT scan. Faster processing speeds and better clarity have reduced the minimum resolution to < 5mm. A new dye (cholograffin) allows the surgeon to assess the biliary anatomy, which previously could only be assessed by MRI.
There have been a large number of exciting developments in MRI. Firstly, 3T machines have much better resolution than previous 1.5 T machines. MR spectroscopy can alert us about the content of certain chemicals (e.g. choline) in a mass, which gives one an idea of whether it is benign or malignant.
MR steatography can measure the fat content in liver. MR elastography measures fibrosis of the liver in a very reliable manner. I might add fibroscan as well since it allows us to measure stiffness (fibrosis) in livers both before and after transplant. All these modalities provide useful information during surgery and are important for good clinical outcomes.