A number of techniques are available for improving image quality of CTA include:
1. Temporal resolution
Increased number of multi-detector rows up to 320,
dual tube configuration and faster rotation time in modern CT scanners produce superior temporal resolution which can be down to 66 msec.
Latest development of soft-ware motion correction technique,
eg.
Snapshot freeze (GE Healthcare,
Milwaukee,
USA),
may further lower the temporal resolution down to 28 msec (1)(Figure 2).
All these can produce superior temporal resolution and help correcting motion artifact from cardiac and aortic pulsations,
breathing and body motion With these techniques,
the vessel definitions can be defined down to small vessel caliber of 1mm.
2. Lowering KVp
The K-edge of iodine is 33.2 keV.
Obviously,
the maximum contrast in CTA using iodinated contrast 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 (2). With the aid of iterative reconstruction,
CT acquistion can now be lowered to 80KVp from the standard 100-120KVp that result in contrast optimization within the vessel lumen and facilitates the detection of luminal and vessel wall pathology and contrast leak (Figures 3 & 4).
3. ECG-gating
Most modern CT scanners have ECG-gating facility that limits the pulsation artifact from heart and aorta leading to superior vessel wall and luminal clarity (3,4) (Figure 5). Vessel clarity is of high importance in the examination,
such as CTPA,
as motion artifact degrades vessel borders and may lead to erroneous diagnosis of pulmonary embolism (5).
This ECG gating in particular enhances the clarity of the subsegmental branches of the pulmonary arteries in the medio-basal segments of both lower lobes adjacent to the heart border (Figure 6).
4. Model-based iterative reconstruction (MBIR) and knowledge-based iterative reconstruction (KBIR)
The latest MBIR and KBIR not only reduce the radiation dose in CT scanning,
but are able to reduce image noise,
improve low subject contrast detectability and minimize some of the blooming artifact from dense calcified plaques in vessel walls (6-10).
These better delineate the vessel luminal and adjacent soft tissue details (Figures 7 & 8).
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 (11,12,13,14) (Figure 9) and,
therefore,
improves vessel wall clarity and reduces calcium blooming artifact (Figure 10 & 11).
6. 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 (15). Spectral information is then obtained.
Dual energy can optimize contrast opacification in vessel lumen by lowering KeV (Figure 12),
and remove calcium and metal artefacts (Figure 13).
It can also provide an added benefit of an iodine/perfusion map that may aid the diagnosis.
7. 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 adjacent vessels,
soft tissue and bone details (16-18) (Figure 14).
This software technique also helps to remove blooming artefacts from calcified plaques in vessel walls.
8. Colour image display according to attenuation value
The arteries may be obscured by the adjacent veins,
or vice versa,
in CTA due to un-intended early or late contrast enhancement. By applying different colour display according to the attenuaton values of contrast in vessel lumen help distinguishing the vessel of interests from other non-interested vessels (Figure 15).
9. CT-Digital Subtraction Angiography (CT-DSA)
When advanced subtraction technique is applied on dynamic CTA on wide area detector scanner. It produces CT DSA and renders dynamic information of vascular pathology including the direction of flow (Figure 16),
and arterial supply and venous drainage of a lesion (14). It also allows better assessment of vessel lumen,
in particular,
those vessel segments that courses through bones or obscured by calcified plaques (Figure 17).