A number of techniques are available for improving image quality of CTAP include:
1. Temporal resolution
A combination of increased number of multi-detector rows up to 320,
dual tube configuration,
faster rotation time and higher table pitch in modern CT scanners produces superior temporal resolution and shortens scanning time.
The improved temporal resolution helps limiting the breathing and body motion artifact,
as well as vascular pulsation artifact,
resulting in better delineation of organ and lesion margins (Figure 1).
2. Water equivalent diameter (WED)
WED is an automated dose measurement tool accounting for different patient body habita by referring to a circular water phantom size that provides the cross-sectional attenuation characteristic.
This allows appropriate dose index and tube current modulation (2,3,4).
It has the benefit of reducing radiation dose and maintains image quality,
irrespective of variation of body contours and internal tissue characteristics (Figure 2).
WED eliminates the need for BMI calculation and manual measurement of body size for radiation dose adjustment.
3. Lowering KVp
The K-edge of iodine is 33.2 keV.
Obviously,
the maximum iodinated contrast enhancement in CTAP could be obtained by using a monochromatic beam of radiation of energy just above 33.2 keV.
This is,
however,
not practicable in the body scanning (5,6). With the aid of iterative reconstruction,
CT acquisition can now be lowered to 80-100KVp from the standard 120KVp that results in contrast optimization within the organs and facilitates lesion detections (Figure 3).
4. Model-based iterative reconstruction (MBIR) and knowledge-based iterative reconstruction (KBIR)
The latest advanced MBIR and KBIR not only reduce the radiation dose in CT scanning,
but are able to reduce image noise and improve low subject contrast detectability (7-11).
These help delineate the organs and lesions (Figures 4,
5,
6 &7).
5. Fine-focal spot CT scanning
Two (standard and fine) focal spots are found in x-ray tube of CT scanners. The advancement in tube technology and better cooling system allow the employment of fine focal spot for CTA scanning. The fine-focal spot in x-ray tube minimizes the penumbra effect of x-ray (12,13,14,15) (Figure 8) and,
therefore,
improve clarity of organ and lesion margin (Figure 9,10). It is also able to minimize the calcium blooming artifacts.
6. Liver detection algorithm
The liver has low intrinsic soft tissue subject contrast,
and therefore,
hypovascular lesions may be difficult to detect where there is a lot of background image noise. The liver detection algorithm is a novel technique that targets more radiation dose to the upper abdomen containing liver and spleen,
but less dose to the pelvis in such a manner that the overall radiation dose is not increased (16). This enables better image quality of the important low subject contrast organs and allows detection of subtle liver and splenic lesions (b) (Figure 11,
12).
7. Dual energy scanning
In dual-energy CT,
two CT datasets are acquired with different x-ray spectra,
which are generated using different tube potentials.
Several technical approaches,
such as sequential acquisition,
rapid voltage switching,
dual-source CT,
layer detector,
quantum-counting detector,
can offer different spectral contrast and therefore dual energy acquisition (17). Spectral information is then obtained.
Dual energy can optimize contrast opacification in vessel lumen by lowering KeV (Figure 13 & 14),
and remove calcium and metal artefacts by raising KeV (Figure 15).
It can also provide an added benefit of an iodine/perfusion map of organs that may aid the diagnosis.
8. Single photon metal artifact reduction technique
Metal within the computed tomography (CT) field of view causes streak artifact that degrades the diagnostic quality of the processed images.
This is related to the high Z-number of most metals and is physically due to a combination of beam hardening,
scatter,
edge effects and photon starvation. The recently developed single photon metal artifact reduction software technique removes metal artifact from coils,
clips and adjacent prosthesis to allow improved view of abdominal organs,
adjacent soft tissue and bone details (Figure 16).
9. Image fusion
Some lesions,
eg.
hepatoma,
can only be seen on certain contrast phase of CTAP,
but not on the non-contrast CT which limited the biopsy and ablation procedure. Image fusion is the recent CT technological development which allows for visualisation of target lesion that is otherwise of similar density to surrounding tissue on non-contrast phase. The previous contrast CT or MRI can be fused with non-contrast image to provide interventional guidance (Figure 17).
10. Colour image display according to attenuation values
Some structures or lesions may be obscured by the adjacent tissues or blood vessels.
By applying different colour display according to the attenuaton values help distinguishing the extent of pathology and its relationship to blood vessels (Figure 18 & 19).
11. 4-Dimensional volume CT
When dynamic CTAP is performed on wide area detector CT scanner,
it renders dynamic information of structural movement and flow details for vascular pathology (18) (Figure 20). It also allows better assessment of structures and vessels that courses through or around bones or dense calcifications.