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.