Dual energy CT is a technique based on the attenuation of photons from different x-ray spectra transmitted through the body.
Multiple dataset are acquired using different enery spectra.
The attenuation of a single voxel depends on the material composition in terms of atomic number Z and electron density and on the energy of the x-ray beam.
Due to photoelectric interactions,
the attenuation increases at lower energies, especially in elements with higher atomic number such as iodine and calcium.
The attenuation value at two different energies within a voxel can be analyzed using material decomposition techniques,
and post-processing of images can provide additional diagnostic value compared to single energy conventional CT. Dual-energy computed tomography (DECT) enables specific tissue characterization providing virtual monochromatic images (VM),
electron density map ρ(e) and effective atomic number Z(eff).
monochromatic synthetic images at lower energies may support iodine dose reduction.
Because the x-ray absorption of iodine increases near its k-edge (33.2 keV),
DECT images displayed at lower monochromatic energies have substantially increased intravascular attenuation.
VMI have important vascular applications as virtual unenhanced imaging, bone mask imaging,
optimization of image quality,
and metal artifact reduction.
Dual energy Technique:
The Dual energy CT is a new technology based on the principle that involves the acquisision of multiple dataset with different x-ray energy spectra
For this purpose different approaches have been developed over the years,
but the clinically available DECT systems are dual source CT (DSDE),
a single-source (SSDE) CT system with fast switching of tube voltage and a SSDE with dual-layer detector system.
DSCT consists of two tubes placed at 90 degrees that work simultaneously with different voltages and the respective detectors.
The advantages are the possibility of modifying parameters as current and voltage for each single tube to obtain an optimal contrast and the simultaneous acquisition of data.
In SSDE CT system with fast switching voltage,
the tube voltage alternates between a high and low value.
It allows nearly simultaneous acquisition of dataset without limiting the scan field of view.
One alternate technique involves a single voltage tube emitting the spectrum x-ray that is detected by “sandwich” layer detector system; the superficial detector layer absorbs the lower energy photons while the deep detector layer absorbs the higher energy photons.
In vascular application CT angiography is an important tool to study cardiovascular disease.
The aim is to obtain the maximum intra-luminal opacification (HU > 200) to better evaluate vessels course and patency.
The degree of intravascular contrast enhancement is fundamental to achieve diagnostic images.
The adjustable parameters are contrast medium volume (injection duration x rate) and concentration. High injection rate,
without increasing contrast material volume,
is particularly useful when attempting to achieve high arterial enhancement because there is a faster accumulation of contrast medium.
Contrast media with high iodine concentration contributes to high rate of iodine delivery to maximize the arterial enhancement,
and help to reduce contrast rate.
Obviously a fast iodine delivery can be achieved also with smaller volume of contrast medium and high concentration injected at a fixed injection rate.
In this case the arterial opacification will be greater but in shorter time.
Optimal balance between concentration and flow rate (Iodine delivery rate) is important to obtain maximum opacification in vascular application.
Adjustment of iodine dose according to BMI and cardiac output should always be performed.
A combination of highly concentrated iodinated contrast material and a high flow rate is a possible approach to achieve high vessel signal in CTA by increasing the iodine delivery rate,
but the use of CTA can be limited in patients with decrease renal function.
Reducing iodine concentration decreases the risk of contrast-induced nephropathy (CIN).
In literature there isn’t data to support a dose-toxicity relationship for intravenous iodinadeted contrast medium.
The risk of CIN varies between 5% and 20% with eGFR > 45 ml/min and increase in frail patient with other risk factors as diabetic nefropathy,
cardiac heart failure,
age > 70 yr,
use of nephrotoxic drugs.
Furthermore in oncologic patients,
whose veins are severely compromised,
using low rates becomes an important diagnostic advantage.
We propose our experience about applying dual energy for vascular evaluation in frail patients.