Thursday, May 18, 2017

CT scans and role of Contrast enhancement


Contrast enhancement and it's role in CT scan
The concept of Contrast enhancement in radiology is not new and it has been in practice even before the Advent of CT scans.
CT scan as a modality of imaging was invented by a British engineer Godfrey Hounsfield in the year 1972.

Purpose of Contrast enhancement

Contrast enhancement is a method of exaggerating  the visible difference between adjacent structures on scan by administrating contrast agents.The term Contrast enhancement in CT scan includes usage of radio opaque substances for better visualization of the anatomic structures as well as better localization and characterization of the pathologies, better differentiation of the pathology from the normal surrounding structures.

Principle of Contrast enhancement

The diffusion of contrast agents from the blood stream to the body tissue is physiologically limited. In pathologies such as cancer, blood vessels grow (angioneogenesis) with increased leaking of contrast agents resulting in lesions much more visible on Contrast enhanced scans.
In CNS, contrast diffusion is limited by Blood brain barrier. Disruption of BBB lead to enhancement after administration of contrast agents.

Indications of Non Contrast CT (NCCT )
For detection of
1.Stones in kidney,ureter, cbd
2.Calcification
3. Fat in various tumors
4. Head injury
5. Acute hemorrhage
6. Stroke
7. SAH


CECT

The pathologic lesions show enhancement or attenuation depending upon the phase of contrast enhancement. So if you are looking for a particular pathology,it is important to know in which phase of CECT to look for.
For that purpose,I've enumerated the phase in which CT scan is done and can be recorded.

1. Non enhanced phase (NECT)
Uses are same as those of Ncct. Many a times this scan is done before administration of the dye to compare pre and post contrast enhancement study.
Calcification, fat in tumors, inflammation and infarction can be seen in this phase well.

2. Early arterial phase (15-20 secs post injection)
When contrast is still in the arteries, it has not enhanced the organs.
This phase is useful to look for vascular abnormalities such as aneurysms, vascular stenosis, etc

3. Late arterial phase (35-40 secs post injection)
Sometimes known as arterial phase.
All the structures that get their blood supply from arteries will show optimal enhancement in this phase.

4. Hepatic or late portal phase (70-80 secs post injection)
Liver parenchyma enhance trough blood supply by portal vein and some enhancement of hepatic veins.

5. Nephrogenic phase (100 secs post injection)
This is when all of the renal parenchyma including medulla enhances. Particularly helpful for small renal cell carcinoma which are otherwise missed.

6. Delayed phase (6-10 mins post injection) called as wash out phase or equilibrium phase
Washout of contrast in all abdominal structures except for fibrotic tissues which become relatively more dense in this phase.

Factors affecting CECT
The timings depend on
1. Organs to be scanned and focussed
2. Type of CT machine available, number of slice
3. Amount of contrast given depending upon the body weight of the patient
4. Injection rate of the contrast
5. Route by which contrast given. (Mainly IV but can be oral,rectal too)

Lesions / pathologies visualized on CECT
1. Liver tumors
Due to it's dual blood supply, 80% by portal vein and 20% by hepatic artery normal parenchymal enhancement maximally in hepatic phase . On the contrary, all all liver tumors are supplied 100% by hepatic artery. So hyper vascular tumors are best seen in late arterial phase. Hypovascular tumors on the other hand are better seen in hepatic phase.
2. Fibrotic lesions
Fibrotic lesions like cholangiocarcinoma and fibrotic mets hold contrast much longer than normal parenchyma hence best seen in delayed phase.
3. Pancreatic tumors most of them being hypovascular are seen best in late arterial phase. In cases of acute pancreatitis, late arterial phase best detects necrosis. Remember chronic pancreatitis can be very well appreciated on NCCT due to calcification.
4. Anastomosis leakage 
CECT done in post op patients to check anastomosis leakage. Oral contrast play a role here for check scans done in post op bowel anastomosis.

5. Pulmonary embolism - 
Good quality scans are required to delineate the emboli in the pulmonary vasculature.
6.CT angiography 
For vascular studies.

Dr. Shil Pill

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