4 computed tomography Dr. Muhammad Bin Zulfiqar Here we will discuss Basic Principles of Ct scan We will discuss how we can perform it.

1. 4Computed Tomography
DR MUHAMMAD BIN ZULFIQAR
PGR III FCPS Services institute of Medical
Sciences/ Services Hospital Lahore
GRAINGER & ALLISON’S DIAGNOSTIC RADIOLOGY

2. • FIGURE 4-1 ■ CT performance. Performance
doubled every two years. This trend has
stopped around 2010.

3. • FIGURE 4-2 ■ (A) Multidetector (MDCT)
Principle. Modern multidetector CT
machines use a third-generation setup
with a rotating tube–detector
combination. The detector is split into
multiple thin parallel detector rows. (B)
Dual-source CT. Two tube– detector
units are combined in one machine. This
requires only a quarter rotation to
complete a 180° half-scan. (C) z-Flying
focal spot (from Flohr et al.16). A z-FFS
rapidly toggles the position of the focal
spot in the z-direction. This produces
twice the number of projections per
rotation but does not increase detector
width, image acquisition speed or
number of available detector rows. In
addition, the tissue covered by the two
beams is separated towards the tube
and overlaps towards the detector.

4. • FIGURE 4-3 ■ Two pairs of images demonstrating the
effect of using different reconstruction kernels. Images
through the thorax on lung windows at the level of the
aortic arch using (A) soft tissue and (B) lung algorithms
show the latter to give a much sharper image of the focal
lesion within the left upper lobe and of the background
emphysema. Images through the upper lumbar spine on
bone windows using (C) soft tissue and (D) bone algorithms
show the latter to give a much sharper image of the focal
metastatic deposit within the vertebral body.

5. • FIGURE 4-3 ■ Two pairs of images demonstrating the
effect of using different reconstruction kernels. Images
through the thorax on lung windows at the level of the
aortic arch using (A) soft tissue and (B) lung algorithms
show the latter to give a much sharper image of the focal
lesion within the left upper lobe and of the background
emphysema. Images through the upper lumbar spine on
bone windows using (C) soft tissue and (D) bone algorithms
show the latter to give a much sharper image of the focal
metastatic deposit within the vertebral body.

6. • FIGURE 4-4 ■ Comparison of filtered back projection
(FBP). (A) and (C) and iterative reconstructions (IR; (B)
and (D)). If the filter settings of IR are set to maximum
noise reduction, a plastic look with isolated noise pixels
may arise (B). For appropriate settings, image quality
can be substantially improved, especially in areas of
high absorption, such as the shoulders, or high
contrast, such as the lungs (D).

7. • FIGURE 4-4 ■ Comparison of filtered back projection
(FBP). (A) and (C) and iterative reconstructions (IR; (B)
and (D)). If the filter settings of IR are set to maximum
noise reduction, a plastic look with isolated noise pixels
may arise (B). For appropriate settings, image quality
can be substantially improved, especially in areas of
high absorption, such as the shoulders, or high
contrast, such as the lungs (D).

8. • FIGURE 4-5 ■ Effect of imaging and reconstruction
thickness. Sagittal images through the lumbar spine using
(A) 1-mm imaging reconstructed in 1-mm-thick slices, (B) 1-
mm-thick slices reconstructed in 5-mm-thick slices and (C)
5-mm-thick images reconstructed in 5-mm thick slices.
Note the progressive deterioration in image detail,
development of a step artefact and blurring of the images.

9. • FIGURE 4-5 ■ Effect of
imaging and reconstruction
thickness. Sagittal images
through the lumbar spine
using (A) 1-mm imaging
reconstructed in 1-mm-
thick slices, (B) 1-mm-thick
slices reconstructed in 5-
mm-thick slices and (C) 5-
mm-thick images
reconstructed in 5-mm
thick slices. Note the
progressive deterioration in
image detail, development
of a step artefact and
blurring of the images.

10. • FIGURE 4-6 ■ CT-guided
drainage procedure. The
diagnostic study (A)
demonstrates an abscess
within the sigmoid mesentery
following an episode of
diverticulitis. Under CT
guidance (B) a drainage
catheter is inserted from the
left side of the anterior
abdominal wall and the
abscess completely drained.

11. • FIGURE 4-7 ■ ECG-synchronised CT acquisitions. Spiral CT with
retrospective gating (A) keeps the tube current constant throughout the
entire data acquisition process while only a small portion (8–20%) of the
data (grey) is used for image reconstruction. ECG dose modulation (B)
reduces the tube current during systole and has a higher dose efficiency of
16–40%. Prospective triggering (C) with a stop-and-start technique uses
50–100% of the data, depending on the amount of padding. ‘Flash
scanning’ (D) uses a very fast spiral data acquisition process with
prospective triggering, and has a dose efficiency of nearly 100%.

12. • FIGURE 4-8 ■ Effect of cardiac gating.
Conventional axial CT image (A) through the left
atrium in a patient with a history of ischaemic
stroke shows extensive motion artefacts.
However, retrospective gating of the images (B)
clearly demonstrates thrombus within the left
atrial appendage.

13. • FIGURE 4-9 ■ (A) The Hounsfield units of different
structures vary with the energy of the X-ray photons and,
more importantly, so does the rate of change. Dual-energy
imaging exploits this to identify individual chemical
components within a structure. Imaging of a thoracic aortic
dissection with (B) conventional unenhanced and (C)
arterial phase imaging. A ‘virtual unenhanced’ image (D),
created by subtracting the iodine from the arterial phase
study, enhances the conspicuity of the dissection flap and
removes the need for the unenhanced phase.

14. • FIGURE 4-9 ■ (A) The Hounsfield units of different structures vary
with the energy of the X-ray photons and, more importantly, so
does the rate of change. Dual-energy imaging exploits this to
identify individual chemical components within a structure. Imaging
of a thoracic aortic dissection with (B) conventional unenhanced
and (C) arterial phase imaging. A ‘virtual unenhanced’ image (D),
created by subtracting the iodine from the arterial phase study,
enhances the conspicuity of the dissection flap and removes the
need for the unenhanced phase.

15. • FIGURE 4-10 ■ Perfusion CT in left middle cerebral artery
territory infarct. Mean transit time (MTT) map (A) shows
an area of delayed MTT in the posterior part of the left
middle cerebral artery territory. Cerebral blood flow (CBF)
map (B) shows a larger area of reduced CBF indicating
ischaemic and infarcted tissue. Cerebral blood volume CBV
map (C) shows a small area of reduced CBV in the left
parietal convexity corresponding to core infarct. The area of
mismatch between the regions of reduced CBF and CBV is
potentially salvageable ischaemic penumbra.

16. • FIGURE 4-10 ■ Perfusion CT in left middle cerebral artery territory
infarct. Mean transit time (MTT) map (A) shows an area of
delayed MTT in the posterior part of the left middle cerebral artery
territory. Cerebral blood flow (CBF) map (B) shows a larger area of
reduced CBF indicating ischaemic and infarcted tissue. Cerebral
blood volume CBV map (C) shows a small area of reduced CBV in
the left parietal convexity corresponding to core infarct. The area of
mismatch between the regions of reduced CBF and CBV is
potentially salvageable ischaemic penumbra.