This document discusses techniques for computed tomography (CT) angiography. It covers advances in CT technology that have improved angiography, including faster scan speeds and thinner slices. Optimal CT angiography depends on scan technique factors like protocol and contrast injection, as well as image post-processing techniques. Newer multi-detector CT machines allow covering volumes more quickly and with higher resolution. Methods like multi-planar reformation and volume rendering help visualize vascular structures from CT image data.

1. Ehab Abou Elfotouh Elaryan.
Lecture Of Radio-diagnosis.
Al-Azhar University.

2.  In recent years, rapid advances in computed tomographic
(CT) technology and image postprocessing software have
been made.
 CT angiography was improved substantially by increasing
scan speed and decreasing section thickness and emerged
as a powerful tool in neurovascular imaging.
 Nowadays, spiral CT systems with acquisition capabilities
of up to 64 sections per gantry rotation are introduced in
clinical practice.
 Assessment of vascular studies based on axial images
alone is not straightforward; two-dimensional (2D) and
three-dimensional (3D) visualization methods are
routinely used to obtained images comparable to those
acquired with catheter angiography.

3.  Optimal image quality depends on two factors:
A- CT angiography technique (scan protocol,
contrast material injection protocol, image
reconstruction methods).
B- Data visualization technique (image post-
processing).

4.  A- Technique:
 Scan speed :
 For evaluation of the basal intracranial arteries, a scan
range of approximately 100 mm needs to be covered.
 Examination of the whole length of the carotid arteries
from the aortic arch to the circle of Willis requires a
scan range of approximately 250 mm.
 The arterio-venous transit time in cerebral bed equal
about 5 second.

5.  A- Technique:
 Scan speed :
 With four–detector row CT at a collimated section
width of 1 mm, a pitch of 1.5, and a gantry rotation
time of 0.5 second, the volume of cerebral artery can
be covered in about 9 seconds. This is not fast enough
to avoid venous overlay.
 With 16–detector row CT at a collimated section width
of 0.75 mm, a pitch of 1.5, and a rotation time of 0.5
second, the same range can be covered in 3 seconds,
well beyond the arterio-venous transit time.

6.  A- Technique:
 Scan speed:
 At examination of the whole length of the carotid
arteries from the aortic arch to the circle of Willis:
 The scan time would be 21 seconds for four–
detector row CT.
 7 seconds for 16–detector row CT.
 4 seconds for 64–detector row CT (64 × 0.6 mm,
pitch of 1.3, 0.33-second rotation time).

7.  Spatial Resolution:
 The smallest distance between two points in the object
that can be differentiated as separate details in the image,
generally indicated as a length or a number of black and
white line pairs per mm (lp/mm).
 The small caliber of cervical and intracranial vessels
requires the highest spatial resolution in all three
dimensions.
 In-plane spatial resolution is predominantly determined
by detector geometry and the convolution kernel.
 It is not substantially improved in scanners with increasing
detector row numbers.

8.  Spatial Resolution:
 The major advantage of more detector rows is higher
through-plane resolution by reducing the width of a single
detector row from 1–1.25 mm (four–detector row CT) to
0.5–0.6 mm (64–detector row CT).
 Typical in-plane resolution with application of a CT
angiography protocol:
 16 × 0.75-mm detector configuration, 120 kV, 100 mAs
field of view of 120 mm, medium sharp convolution kernel
is 0.6 mm and through-plane resolution is 0.7 mm.
 64 × 0.6-mm detector configuration, 120 kV, 140 mAs
field of view of 120 mm, medium sharp convolution kernel
is 0.6–0.7 mm and through-plane resolution is 0.5 mm.

9.  Contrast Material Injection:
 In order, to obtain high-quality CTA images, high
concentration of contrast in the vessels is necessary.
 Short scan times require short contrast material
injection.
 Technique-related factor: To deliver an appropriate
amount of iodine, injection rates of 4–5 mL/sec and
highly concentrated contrast medium (iodine, 350–
370 mmol/mL) are preferable.
 Type of injection and volume of contrast material
may also effect Ct contrast enhancement.

10.  TYPE OF INJECTION:
 I- Intra-venous contrast agent administration, including
three methods:
A- Fixed scan delay technique (15-45s).
B- Test bolus injection technique.
C- Automated bolus-tracking technique (Smart
Prep, CARE Bolus, and Sure Start).
*Individual timing of contrast material injection (bolus
tracking or test bolus injection) is mandatory to take
advantage of phase-resolved image acquisition.

13.  TYPE OF INJECTION:
 I- Intra-arterial contrast agent administration:
 Invasive method.
 Performed with a combined angiography and CT
unite.
 High concentration of contrast material can be
obtained in intra-cranial arteries without
consideration the appropriate timing of injection.
 Need small amount of contrast material.

14.  Image Reconstruction:
 To reduce image noise, images may be reconstructed
slightly thicker than the detector collimation, for example
with a 0.75-mm section thickness from a data set acquired
with 0.6-mm detector collimation.
 Overlapping image reconstruction should always be
performed to improve 3D post-processing.
 The reconstruction algorithm influences the spatial
resolution in plane.
 The ideal algorithm would combine low image noise and
sharp edge definition, maintaining good low-contrast
resolution.

15.  Image Reconstruction:
 Soft algorithm reduce image noise and allow
smooth surfaces with rendering techniques,
improving the visualization of aneurysms and
vascular malformations.
 Sharper algorithm improve edge definition and
reduce blooming effects from calcifications,
necessary for stenosis measurements, at the
expense of higher image noise.

16.  Image Post-processing Techniques:
 Several image processing techniques for CT
angiography are currently being used clinically.
 Image processing involves traditional operations
such as:
A- Multi-planar reformation (MPR) .
B- Maximum intensity projection (MIP).
C- Surface and volume rendering associated with
bone subtraction.

17.  Multi-planar Reformation
( MPR ):
 MPR creates views in different
planes without loss of original
CT information.
 Only 2D views can be
generated.
 If the CT data meet the
requirements of isotropy,
spatial resolution is similar to
the original source images.
 Both diameter reduction and
area reduction can be
measured, and no information
is suppressed in the final
image.

18.  Multi-planar Reformation
( MPR ):
 A variant of MPR is curved
planar reformation.
 Curved planar reformation
provides a 2D image that is
created by sampling CT
volume data along a
predefined curved plane.
 This technique is employed
to display tortuous
structures.

19.  Maximum Intensity
Projection ( MIP ):
 MIP images are created by
displaying only the highest
attenuation value.
 The depth information along
the projection ray is lost to
visualize the spatial
relationship of various
structures.
 The volume has to be rotated
and viewed from different
angles.

21.  Volume Rendering:
 is a visualization technique that
creates a 3D impression and
provides densitometric
information.
 Visualization of CT angiography
data with volume rendering is
based on transfer functions that
map measured intensities to
colors and opacities.
 Opacity values on a spectrum
from 0% to 100% (total
transparency to total opacity)
are assigned along artificial rays
that pass through the data.

22.  Volume Rendering:
 Separation of different tissue types (ie, bone, contrast-
enhanced vessels, soft tissue) can be performed and can be
color encoded.