Download Image quality – Conventional X-ray

Radiation physics
University hospital
Linköping
Report Radfys-02-02
Image quality – Conventional X-ray
Jonas Nilsson, Michael Sandborg
Lab Image Quality and Dose in Projection Radiography
Before the lab
Read this tutorial Image Quality – Conventional X-ray in advance so you have an idea of what
the lab is about.
During the lab
Go to the reception at Röntgenkliniken at the University hospital on level 11 in the main
building. Your lab supervisor will guide you to the viewing stations at the radiology clinic.
Join one or two other students to form a group and do the lab together. Get the correct login to
the PACS from the supervisor.
The lab consists of seven cases where one imaging or phantom parameter has been altered in
each case; for example, the tube kilovoltage, phantom thickness or focal spot size. Go through
the seven cases one after the other and assess image quality and patient dose (dose-area
product, DAP). Make sure you understand why image quality and patient dose has changed
before you go to the next case.
After the lab
Write a short report together with your fellow students and summarise for each case 1-7 how
image quality and patient dose is altered and why. Submit your lab report in lisam.
Lab: Image quality – Conventional X-ray
INTRODUCTION
You are going to study a number of image pairs during this session. Each pair contains X-ray
images produced while varying one or more X-ray system parameters, e.g. tube load or X-ray
tube voltage etc.. The images depict objects (also called test phantoms) that make it possible
to evaluate the image quality. You are supposed to work together during this session as a
group (all participants must be active), discussing and explaining why the images vary in
quality. This discussion is going to be based on the theoretical knowledge gained during
lectures, PBL group meetings, literature etc. You are going to discuss and compare the images
in terms of image resolution, contrast and dose to the patient. Since the images are digital, you
should use the possibilities (region of interest definitions, zoom possibilities etc.) offered to
you by the PACS software. At the end of this lab you will be able to describe how the X-ray
system parameters affect image quality and patient dose.
AIM:
This lab session aims at increasing your knowledge of how image quality and dose to the
patient are affected when some system parameters are altered. The parameters are:
tube kilovoltage (kV), tube current and exposure time (mAs), phantom or object-thickness,
focus spot size, x-ray beam size, degree of magnification (air gap length) and grid usage.
Since we want to concentrate on the effect of each parameter, as few parameters as possible
are going to get altered at a time. A dedicated image quality phantom has been used in order
to illustrate the different physical effects of these parameter alterations. At the end of the
laboratory session, more human like phantoms will be shown in order to demonstrate the
clinical effects in a more realistic fashion.
DEFINITIONS:
We use three concepts in order to define image quality:
Contrast: The difference in grey level between the object detail and the surrounding
background. The greater the contrast of an object the better the object stands out in the image.
Sharpness: An imaging system’s ability to image a sharp edge. In practice the concept of
spatial resolution is used meaning the system’s ability to resolve small details in the image
that are adjacent to each other or placed close together. Spatial resolution is quantified in
terms of line pairs per mm (lp/mm).
Noise: Depending on the number of photons contributing to the image formation, the image
may be perceived as being noisy to some degrees. A larger number of photons yields less
noise and possibly a better image quality. This type of noise is usually referred to as quantum
noise. There are other types of noise as well, e.g. electronic noise and detector noise.
Normally, quantum noise is the dominating noise factor, hence ‘quantum-limited’ imaging.
Dose-area product DAP is a concept that often occurs when discussing the patient dose. The
DAP is the product of the dose (in air) and the area of the x-ray beam in the same plane;
typically, just beneath the collimator below the x-ray tube. Hence the DAP depends on both
dose and beam area. (Dose in air at charge particle equilibrium is called air kerma.)
Scatter-to-primary ratio (S/P). The relationship between the amount of scattered radiation
(S) and primary radiation (P) at the image detector is often stated as the scatter-to-primary
ratio (S/P). This ratio is dependent on how the grid and/or the air gap technique is used in
order to minimize the contrast-reducing scattered radiation. It is often stated that contrast, C,
is reduced by a factor CDF=(1+S/P)-1, i.e. when S/P is high, CDF gets low and the contrast is
reduced.
Automatic exposure control AEC is used in all but the last case (case #7). This means that the
radiographer selects the tube voltage but the exposure time (s) is terminated when the x-ray
system has measured the correct dose (or “photons”) at the AEC-sensors just in front of the
image detector. This means that if the photon energy of the beam or phantom thickness is
altered the exposure time will change.
The tube currant (mA) is depending on the focal spot size. A larger tube current is possible
with the large focal spot size (larger filaments), and a smaller tube current is used with the
smaller focal spot size.
The air gap is the distance between the patient and the image detector. Typically, this
distance is small (a few cm), but sometimes the patient couch height is increased by 10 or 20
cm and the patient is positioned closer to the x-ray tube and further away from the image
detector. The anti-scatter grid is used in most examination of adults and of larger children
with mass >20 kg.
The test phantom plate that contains all the low- and high-contrast details, is positioned
above 10 cm of Plexiglas (Lucite) and then an additional 10 cm of Plexiglas is positioned on
top of the test phantom plate (see illustrations below).
The contrast is best assessed by evaluating how many of the ten circular disc (located to the
right in the images) that are visible. The sharpness of spatial resolution can be assessed by
evaluating which of the parallel lines that are separable from each other; the more line pairs
per mm the higher the spatial resolution (top left in image). The noise is best quantified by
positioning a circular region of interest in a homogeneous part of the test phantom plate and
measure the standard deviation in the pixel values.
PROCEDURE:
For each pair of images, you are supposed to assess how contrast, sharpness and DAP vary for
different values of the system parameters.
Useful, program specific advices (mouse pointer on the image under study)
1. In order to zoom continuously, hold Ctrl-button down and press the left mouse button
while moving the mouse back or forth in order to zoom in and out respectively.
2. In order to change the grey scale mapping of the image continuously hold the middle
button (wheel) of the mouse down while moving the mouse back or forth and/or left or
right.
Case 1: Dependence on tube kilovoltage kV
Image
nr
Parameter
Contrast
Sharpness
altered
(higher/lower) (lp/mm)
Dose-area
product, DAP
(higher/lower)
Conclusions
Comparison 1
1
50 kV,
338 mAs
2
125 kV,
3,2 mAs
Comparison 2
3
70 kV,
35 mAs
4
125 kV,
3,2 mAs
Case 2: Dependence on object thickness
Image
nr
Dose-area
Parameter
Contrast
Sharpness
product, DAP
altered
(higher/lower) (lp/mm)
(higher/lower)
Comparison 3
1
70 kV,
4,1 mAs,
10 cm lucite
2
70 kV,
35 mAs, 20
cm lucite
Comparison 4
3
70 kV,
35 mAs,
20 cm lucite
4
70 kV,
104 mAs,
25 cm lucite
Conclusions
Case 3: Dependence on field size
Image
nr
Dose-area
Parameter
Contrast
Sharpness
product, DAP
altered
(higher/lower) (lp/mm)
(higher/lower)
Conclusions
Comparison 5
1
70 kV,
35 mAs,
20x20 cm2
field
70 kV,
45 mAs,
2
10x10 cm2
field
Case 4: Dependence on grid
Image
nr
Dose-area
Parameter
Contrast
Sharpness
product, DAP
altered
(higher/lower) (lp/mm)
(higher/lower)
Conclusions
Comparison 6
1
70 kV,
35 mAs,
with grid
2
70 kV,
6,9 mAs,
without grid
Case 5: Dependence on focal spot size
Image
nr
Parameter
altered
Comparison 7
1
70 kV,
35 mAs,
large focal
spot
2
70 kV,
36 mAs,
small focal
spot
Contrast
(higher/lower)
Sharpness
(lp/mm)
Dose-area
product, DAP
(higher/lower)
Conclusions
Case 6: Dependence on air gap length or geometric magnification
Image
nr
Parameter
altered
Comparison 8
1
70 kV,
37,7 mAs,
10 cm
air gap,
large
focus
2
70 kV,
39 mAs,
10 cm
air gap,
small
focus
Comparison 9
3
70 kV,
36 mAs,
20 cm
airgap,
large
focus
4
70 kV,
37 mAs,
20 cm
air gap,
small
focus
Contrast
(higher/lower)
Sharpness
(lp/mm)
Dose-area
product, DAP
(higher/lower)
Conclusions
Case 7: Dependence on tube load, quantum noise and patient dose
Image
nr
Parameter
altered
Comparison 10
1
70 kV,
1 mAs
2
70 kV,
2 mAs
3
70 kV,
4 mAs
4
70 kV,
8 mAs
5
70 kV,
16 mAs
6
70 kV,
32 mAs
7
70 kV,
63 mAs
8
70 kV,
125 mAs
Contrast
Sharpness
(higher/lower) (lp/mm)
Dose-area
product, DAP
(higher/lower)
Conclusions