The document discusses radiation risks from medical imaging and provides strategies to reduce radiation dose during computed tomography (CT) scans. It includes 27 figures showing comparisons of CT images from optimized low-dose protocols versus standard-dose protocols. The low-dose protocols utilized techniques such as lowering kV, adjusting the tube current based on patient size, and limiting the scan range. These techniques achieved radiation doses reductions of 30-80% while maintaining diagnostic image quality.

1. 1
Medical Physics: Radiation Risks
DR. Muhammad Bin Zulfiqar
PGR-FCPS III SIMS/SHL
GRAINGER & ALLISON’S DIAGNOSTIC RADIOLOGY

2. • FIGURE 1-1 ■ The generally accepted LNT model
implies that all exposures, even if they are very
low, carry a certain risk. It is sometimes suggested
that there may be some protective effect of
radiation at very low doses but other studies
show hyperradiosensitivity at very low doses

3. • FIGURE 1-2 ■ Dose distribution during
fluoroscopy at the operator’s position 80 cm from
a water phantom placed on the table for under-
and overcouch X-ray tube configurations.
Operating conditions: 125 kVp, 1 mA.

4. • FIGURE 1-3 ■ Chart representing the relationship between dose and relative average diameter as
measured by the scout view (topogram) used by AEC systems to adapt the dose to the patient’s
weight. The strongest increase in dose is expected with AEC warranting a constant noise (GE and
Toshiba CT systems), whereas the dose increase may be moderate with AEC tolerating more noise
in obese patients (Siemens and Philips CT systems).

5. • FIGURE 1-4
• ■ Brain CT performed in two consecutive acquisitions obtained at
120 kV (64 × 0.6 mm) in a 70-year-old man with acute stroke.
Arrows show a hypoattenuated area in the left parietotemporal
region. Image thickness is 3 mm. (A) was obtained at 120 kV and
300 mAseff with AEC switched off and the corresponding CTDIvol is
63 mGy. (B) was obtained at 120 kV and 280 mAseff (quality
reference index) with AEC switched on. The tube current–time
product is lowered at 198 mGy by the AEC system and the
corresponding CTDIvol is 42 mGy. The reconstruction filtered back
projection kernel is H20, slightly smoother than the filtered back
projection H30 used in (B).

6. • FIGURE 1-5 ■ Two consecutive brain CT examinations in a 42-
year-old man who had a motor vehicle accident. Acquisitions
were obtained at 120 kV and with a collimation of 16 × 0.6 mm.
Image thickness is 3 mm. (A) was obtained with fixed tube current
at 450 mAseff and resulted in a CTDIvol of 61 mGy (reconstruction
filtered back projection kernel is H30). (B) was obtained 3 months
after (A) with AEC switched ‘on’, a quality reference mAs at 380 mAs
and mean effective tube current–time product of 312 mAs,
resulting in a CTDIvol reduced to 42 mGy (reconstruction filtered
back projection kernel H20). An arachnoid cyst is seen in the left
temporal region. ;

7. • FIGURE 1-6 ■ Optimised
brain CT acquisition
obtained with 40 × 0.6
mm collimation, 140 kV,
AEC switched ‘on’, a
quality reference mAs at
170. The resulting
delivered tube current–
time product is 114
mAseff and the CTDIvol is
30 mGy. DLP is 442
mGy.cm, approximately
one-half of the RDL for
brain CT that is usually set
at 1000 mGy.cm. (A) Axial
2-mm slice and (B) coronal
3-mm slice reconstructed
with filtered back
projection kernel H20.

8. • FIGURE 1-7 ■ Optimised brain CT acquisition in an 8-
year-old boy with 100 kV, activated AEC and quality
reference tube current–time product at 300 mAseff.
The resultant tube current– time product is 237
mAseff and the CTDIvol at 20.7 mGy. Slice thickness is 3
mm and reconstruction filtered back projection kernel
is H20.

9. • FIGURE 1-8 ■ Comparison
of two consecutive CT
images of sinunasal cavities
obtained from a 34-year-
old woman, including axial
and coronal 2-mm-thick CT
slices. The first examination
(A) was acquired in 2010 on
a 16-slice MDCT system
with a CTDIvol of 6.75 mGy
and a DLP of 66 mGy.cm.
The second examination (B)
was acquired on a 2 × 64-
slice MDCT system with a
CTDIvol of 3.3 mGy and a
DLP of 40 mGy.cm. Kernel is
a filtered back projection
algorithm for both
examinations (H50).

10. • FIGURE 1-9 ■ Axial 3-mm slices at the level of lung bases obtained
in two male patients weighing 85 kg, with similar chest wall
thickness and 33 cm in lateral chest diameter. On the left, the
acquisition parameters were not optimised and the noise index
specific to the AEC system was set at 11 HU. The resulting CTDIvol is
of 28.3 mGy and exceeds the French national DRL (set at 15 mGy for
chest CT). On the right, acquisition parameters were optimised and
the AEC system tolerated a noise of 15 HU and limited the dose
increase from 80 to 100 mAseff. The resulting CTDIvol is 7.7 mGy.

11. • FIGURE 1-10 ■ Consecutive chest
CT examinations in the same
patient with stable body weight
of 65 kg obtained in 2010 and in
2011. A pulmonary lesion
appeared in 2011 in the left
upper lobe. The 2010 acquisition
was obtained with 120 kV and
80 mAs, whereas the second CT
was obtained at 140 kV and 38
mAs default setting. The CTDIvol
was reduced from 4.24 to 2.36
mGy (–45%). Image quality is
very similar between both
acquisitions, as shown in
mediastinal window in (A), and in
pulmonary windows in coronal
(B), axial (C) and sagittal views
(D). C D A B

12. • FIGURE 1-10 ■ Consecutive
chest CT examinations in the
same patient with stable body
weight of 65 kg obtained in
2010 and in 2011. A pulmonary
lesion appeared in 2011 in the
left upper lobe. The 2010
acquisition was obtained with
120 kV and 80 mAs, whereas
the second CT was obtained at
140 kV and 38 mAs default
setting. The CTDIvol was
reduced from 4.24 to 2.36 mGy
(–45%). Image quality is very
similar between both
acquisitions, as shown in
mediastinal window in (A), and
in pulmonary windows in
coronal (B), axial (C) and sagittal
views (D). C D A B

13. • FIGURE 1-11 ■ CTPA examination in a patient weighing
55 kg obtained at 80 kV and with a quality reference
mAs AEC setting at 12 mAs, reduced by the AEC to 56
mAseff in the present patient. The CTDIvol of this
acquisition is 1.1 mGy and the DLP of the entire
examination is 39 mGy.cm. The use of a 80-kV tube
potential reduces the dose and makes it possible to
obtain a perfect vessel opacification.

14. • FIGURE 1-12 ■ CTPA examination in a patient
weighing 115 kg obtained at 120 kV. The CTDIvol of
this acquisition is 10.5 mGy and the DLP 394 mGy.cm.
The vessel enhancement is not excellent, probably
because of tube potential at 120 kV. A pulmonary
embolism is seen in the right upper lobe. Note that this
obese patient was exposed to a DLP that does not
exceed the RDL of a standard patient.

15. • FIGURE 1-13 ■ Low-dose chest CT examination acquisition obtained in a 23-year-old woman with
suspected tuberculosis. Tube potential is 140 kV, quality reference mAs (and the tube current is 9
mAs) of AEC is set at 9 mAs and the CTDIvol is 0.97 mGy, whereas the DLP is 33 mGy.cm. Slice
thickness is 3 mm and reconstruction kernel uses iterative technique. As shown in Fig. 1-14, in a
standard patient weighing 70 kg, such low-dose chest CT protocol delivers a DLP not higher than 20
mGy.cm. Using 0.017 mSv/mGy.cm as conversion factor, the effective dose is 0.34 mSv, equivalent
to the dose of a two-view radiographic examination.

16. • FIGURE 1-14 ■ Low-dose chest CT examination
obtained in a patient weighing 65 kg with the
same tube potential and quality reference mAS
as in Fig. 1-13. AEC system reduced the tube
current from 9 to 5 mAseff. The CTDIvol is of 0.57
mGy and the DLP of 18 mGy.cm. Note the
tracheobronchial diverticuli.

17. • FIGURE 1-15 ■ Optimised dose unenhanced MDCT
obtained in an obese patient weigting 135 kg, with
acute appendicitis. The high tube potential and low
tube current strategy is used (140 kV–120 mAs).
CTDIvol is 15 mGy and dose–length product is 747
mGy.cm. Average abdominal diameter is 40 cm.

18. • FIGURE 1-16 ■ Optimised dose 16-detector row CT (Emotions 16,
Siemens, Forchheim, Germany) in a 68-year-old man weighing 75
kg (standard size patient) and with right lower quadrant pain.
Image noise measured in inferior vena cava is 16 HU. Tube
potential is 130 kV, quality reference mAs at 80 mAseff and is
lowered to 50 mAseff by the AEC. The subsequent CTDIvol is 5.6
mGy. Acquisition height is 37 cm and the corresponding DLP is 233
mGy.cm.

19. • FIGURE 117 ■ Reduction of tube potential in a patient weighing 67 kg
with chronic pancreatitis and a pseudocyst in the head of the pancreas.
On the left, 120-kV MDCT with default 120-mAs quality reference tube
current. The CTDIvol is 4.2 mGy and the DLP is 194 mGy.cm. On the right,
tube potential was set at 100 kV with unchanged quality reference tube
current (120 mAs). The CTDIvol is reduced to 2.5 mGy and the DLP is 101
mGy.cm. For the examinations, the AEC system has adjusted the tube
current from 120 to 62 mAseff.

20. • FIGURE 1-18 ■ Low-dose unenhanced MDCT of the
abdomen in an extremely obese 22-year-old woman
with right iliac fossa pain. Tube potential is 140 kV
and the quality reference mAs at 30 mAseff. AEC
system adapted the tube current–time product to 57
mAs. The CDTIvol is 5.9 mGy, whereas the DLP is 290
mGy.cm. Arrow shows the normal appendix.

21. • FIGURE 1-19 ■ Low-dose unenhanced MDCT of the abdomen in an obese
21-year-old, 1.78-m-tall man, weighing 105 kg referred for acute pain in
the right iliac fossa. Tube potential is 140 kV and quality reference tube
current is set at 20 mAseff. AEC increased this default tube current to 35
mASeff. The CDTIvol is 3.7 mGy, whereas the DLP is 131 mGy.cm. Note that
this low DLP was achieved because the acquisition height has been limited
to the lower three-quarters of the abdomen. A standard acquisition would
have delivered 900 mGy.cm in this patient.

22. • FIGURE 1-20 ■ Low-dose unenhanced MDCT of the
abdomen in a 21-year-old man weighing 90 kg and 1.78 m
tall, with suspected acute left renal colic. Tube potential is
140 kV and quality reference tube current is set at 30
mAseff. AEC adapted the tubecurrent–time product to 32
mAseff. The CDTIvol is 3.4 mGy and the DLP is127 mGy.cm.
A 2-mm large calculus is seen in thedistal left ureter.

23. • FIGURE 1-21 ■ Low-dose iodine-enhanced MDCT of the
upperabdomen with a 21-cm acquisition height performed in
order to confirm acute right pyelonephritis (arrow) in a 22-year-
old woman. Tube potential is lowered at 110 kV and the quality
reference tube current is set at 60 mAs, reduced to 28 mAs b the
AEC. The applied CTDIvol is 2 mGy and the DLP is limited to 42
mGy.cm, equivalent to the risk of an abdominal plain radiograph.

24. • FIGURE 1-22 ■ Iodine-enhanced abdominal MDCT in a
12-year-old boy weighing 45 kg complaining of right
iliac fossa pain. No sign of appendicitis was found by
CT. Tube potential is 100 kV and quality reference tube
current is 120 mAseff. The AEC reduced this tube
current to 57 mAseff. The resulting CTDIvol is 2.6 mGy
and the DLP is 101 mGy.cm. (A) A representative 3-mm
axial view and (B) coronal and sagittal 3-mm views.

25. • FIGURE 1-23 ■ Low-dose unenhanced MDCT of
the abdomen obtained in a 11-year-old boy
weighing 40 kg, referred for right iliac fossa pain
after inconclusive US examination. Tube
potential is 100 kV and quality reference tube
current at 70 mAseff. This preset is reduced to 37
mAseff by the AEC system. The resulting CTDIvol
is 1.7 mGy and the DLP is 51 mGy.cm. The
appendix (arrows) is normal in axial (A) and in
coronal and sagittal orientations (B).

26. • FIGURE 1-24 ■ Unenhanced MDCT obtained at 80
kV in a 9-yearold boy weighing 34 kg after
inconclusive ultrasound examination showing an
acute appendicitis (arrow) while delivering 1.4
mGy (CTDIvol) and 50 mGy.cm (DLP).

27. • FIGURE 1-25 ■ Comparison of two consecutive CT-
enhanced acquisitions of the abdomen with a dose
reduction of 33% in an 87-yearold woman weightng
88 kg. The first one is performed with 120 kV and 150
mAseff, and the second one with the same tube
potential but 100 mAs. Slice thickness is 3 mm. Image
noise is slightly higher in the 100-mAs axial (A) and
coronal (B) orientations, but the 100-mAs images are
of acceptable quality.

28. • FIGURE 1-26 ■ Comparison of two consecutive unenhanced MDCT examinations
in a 48-year-old man weighing 75 kg who had a sigmoid perforation on a foreign
body that proved to be a swallowed toothpick. The toothpick is visible on the
right-sided coronal reformat. The first CT was not optimised and at 120 kV and a
noise index of 10 UH for 1.25-mm slices. The resultant CTDIvol is approximately 20
mGy and the DLP is 1009 mGy.cm. The second CT displayed on the left was
acquired after endoscopic removal of the toothpick at 140 kV and a quality
reference tube current of 40 mAseff, reduced to 36 mAseff by AEC. The
corresponding CTDIvol was 3.8 mGy and the DLP was 148 mGy.cm, the dose being
reduced by a factor of 7.

29. • FIGURE 1-27 ■ Side-by-side comparison of two
consecutive unenhanced MDCT of the lumbar spine
obtained in a 67-year-old patient weighing 92 kg, with
stage IV colon carcinoma and complaining of low-back
pain. Two acquisitions are obtained, one at standard
dose of 68 mGy (CTDIvol, displayed on the right), and
the second at an optimised dose of 36 mGy (CTDIvol,
displayed on the left). (A) Sagittal reformats in soft-
tissue algorithm and window, (B) axial slices and (C)
sagittal reformats with bone algorithm and window.

30. • FIGURE 1-27 ■ Side-by-side comparison of two
consecutive unenhanced MDCT of the lumbar spine
obtained in a 67-year-old patient weighing 92 kg, with
stage IV colon carcinoma and complaining of low-back
pain. Two acquisitions are obtained, one at standard
dose of 68 mGy (CTDIvol, displayed on the right), and
the second at an optimised dose of 36 mGy (CTDIvol,
displayed on the left). (A) Sagittal reformats in soft-
tissue algorithm and window, (B) axial slices and (C)
sagittal reformats with bone algorithm and window.

31. • FIGURE 1-28 ■ Side-by-side comparison of two
consecutive unenhanced MDCT of the lumbar spine
obtained in a 72-year-old man weighing 71 kg, with stage
III non-small cell lung carcinoma who complained of low-
back pain. Two acquisitions are obtained, one at a
standard dose of 38 mGy (CTDIvol, displayed on the right),
and the second at an optimised dose of 23 mGy (CTDIvol,
displayed on the left). (A) Axial slices at the level of L5–S1
disc showing a disc herniation. (B) The same herniation
(arrow) in left parasagittal orientation. Sagittal reformats in
soft-tissue algorithm and window.

32. • FIGURE 129 ■ Side-by-side comparison of sagittal
reformats of the lumbar spine showing the potential
benefit of iterative reconstruction technique for imaging
low-back pain with CT. The right one is obtained with
iterative reconstruction and the left one with usual filtered
back projection technique. Noise seen in FBP reformat is
significantly reduced by the iterative technique.