Chất lượng hình ảnh

Factors affecting image quality Image quality Contrast Tương phản Distortion & artifact Ảnh giả Noise Nhiễu Blur or Unsharpness Phân giải...  Độ tương phản là sự khác nhau về thang xám

Image quality

Trường Đại Học

Nguyễn Tất Thành

Ngành kỹ thuật y sinh

 Vận dụng được mối liên hệ giữa các thông

số liên quan đến chất lượng hình ảnh

trong chụp ảnh tia X trên thực tế

Factors affecting image quality

Image quality

Contrast Tương phản

Distortion

& artifact Ảnh giả

Noise Nhiễu

Blur or Unsharpness Phân giải

Image quality evaluators/descriptors

NOISE

RESOLUTION CONTRAST

Wiener spectra

Modulation transfer Function MTF

Signal-to-noise

Ratio S/N

Contrast detail analysis Rose model ROC analysis

Contrast

 Độ tương phản là sự khác nhau về thang xám giữa những điểm nằm cạnh nhau.

properties that makes an object (or its representation in an image)

distinguishable from other objects and the background

Contrast

Low Contrast

Medium Contrast

High Contrast

Image contrast refers to the fractional difference in optical density of brightness between two regions of an image

Contrast

Weber contrast

Michelson contrast

Subject contrast

Do A>B nên 0<Cs<1 Nếu z=0  Cs = 0 Khi z càng tăng thì độ tương phản càng tăng

Some factors influencing contrast

 Radiographic or subject contrast

 Tissue thickness

 Tissue density

 Tissue electron density

 Effective atomic number Z

Subject contrast

 Peak voltage value has an influence on

the beam hardness (beam quality)

 It has to be related to medical question

 What is the anatomical structure to investigate?

 What is the contrast level needed?

 For a thorax examination : 130 - 150 kV is

suitable to visualize the lung structure

 While only 65 kV is necessary to see bone

structure

Technique factors (1)

Technique factors (2)

 The higher the energy, the greater the

penetrating power of X Rays

 At very high energy levels, the difference

between bone and soft tissue decreases and both become equally transparent

 Image contrast can be enhanced by choosing

a lower kVp so that photoelectric interactions are increased

 Higher kVp is required when the contrast is high (chest)

X Ray penetration in human tissues

60 kV - 50 mAs 70 kV - 50 mAs 80 kV - 50 mAs

 Second level

 Third level

 Fourth level

 Fifth level

Technique factors (3)

 The mAs controls the quantity of X Rays

(intensity or number of X Rays)

 X Ray intensity is directly proportional to the mAs

 Over or under-exposure can be controlled by adjusting the mAs

 If the film is too “white”, increasing the mAs will bring up the intensity and optical

density

X Ray penetration in human

tissues

70 kV - 25 mAs 70 kV - 50 mAs 70 kV - 80 mAs

Detector contrast

Radiographic contrast(screen film)

chỉnh hoặc tăng cường mà phụ thuộc vào bề dày của bệnh nhân

Display contrast

Digital image contrast(contrast to noise ratio)

quan đến độ tương phản là CNR

ảnh hơn là tương phản chính nó

δ

Resolution

 Smallest distance that two objects can be separated and still appear distinct

 Example of limits

 Film screen: 0.01 mm

 CT: 5 mm

 Other definition: “Point-spread” function

 Characteristic of a “point” object

 Point object expected to be point in image

 Blurring due to imperfections of imaging system

 Measurement: full-width-at-half-maximum FWHM

 Khi 2 điểm quá gần nhau đến nỗi chúng

xuất hiện như 1 điểm,ta nói ảnh mất độ

phân giải

Factors affecting image sharpness

Image Unsharpness

Geometric Unsharpness

Subject Unsharpness

Motion Unsharpness

Subject Unsharpness

Geometric blur

 Another cause of lack of geometric sharpness

is the distance of the receptor from the object

 Moving the receptor away from the object

results in an increased lack of sharpness

 N.B.: The smaller the focal size and closer the contact between the object and the film (or

receptor), the better the image quality as a

result of a reduction in the geometric

sharpness

Physical mechanisms of Blurring

Lack of sharpness in the subject

 Not all structures in the body have

well-defined boundaries (superimposition

essentially present in most situations)

 The organs do not have square or rectangular boundaries

 The fidelity with which details in the object are required to be imaged is an essential

requirement of any imaging system

 The absence of sharpness, in the

subject/object is reflected in the image

Lack of sharpness due to motion (1)

imaging

 uncooperative child

 organ contraction or relaxation

 heart beating, breathing etc.

examination time short and asking the patient to remain still during the examination

Lack of sharpness due to motion (2)

 Shorter exposure times are achieved by the use of fast intensifying screens

 N.B.: Faster screens result in loss of details (receptor sharpness)

 Further, the use of shorter exposure time

has to be compensated with increased mA

to achieve a good image

 This often implies use of large focal spot

(geometric sharpness)

Lack of receptor sharpness

 The intensifying screen in radiography has a

crystal size which is larger than that of the

emulsion on the film

 An image obtained without the screen will be sharper than that obtained with the screen, but

will require much more dose

 The thickness of the screen further results in

degradation of sharpness

 On digital imaging, the image displayed at a

higher matrix with a finer pixel size has better clarity

Distortion and artifacts

 Unequal magnification of various anatomical structures

 Inability to give an accurate impression of the real size, shape and relative positions

 Grid artifact (grid visualized on the film)

 Light spot simulating microcalcifications (dust

on the screen)

 Bad film screen contact, bad patient

positioning (breast)

Distortion and artifacts

Distortion and artifacts

Pixel resolution

in height has a total of 2048×1536 =

3,145,728 pixels or 3.1 megapixels

Pixel resolution

Spatial resolution

Frequency domain

chia ra với chu kì nhỏ hơn,ta nói sóng đó

Spatial resolution

Temporal resolution

 (frames/s), while high-speed cameras may resolve 100 to 1000 frames/s

Image magnification

có độ phóng đại <1

Image magnification

phóng đại vật được tính bằng công thức:

phóng đại sẽ làm tăng blur

nhỏ(0.1-0.3mm),sự phóng đại có thể làm cải

thiện độ phân giải không gian

Image magnification

Noise

Noise

 3 ảnh có cùng độ tương phản nhưng nhiễu khác nhau

Quantum noise

mà ta có thể đo hoặc đếm được

tích(ta không đếm số lượng tia X vì nó có tính lượng tử) và đầu thu hình vuông(số pixel vuông) thì nhiễu được tính bởi CT:

Quantum noise

 Nhiễu khi được quan sát bằng mắt gọi là nhiễu tương đối (relative noise) hoặc hệ số biến thiên (coefficient of variation COV)

 Tỉ số tín hiệu trên nhiễu (signal to noise ratio)

 Khi N tăng thì nhiễu tăng và liều bệnh nhân cũng tăng

Quantum noise

SNR

Others sources of image noise

(fluoroscopy) sử dụng bầu tăng sáng (image insentify), tia X dược biến đổi nhiều lần để tạo ảnh  có nhiều nhiễu

Noise in film

any uniform region of the film

Image noise

 Information that is not useful is noise

 Different components of noise are:

 Radiation noise (“heel effect”)

 Structure noise (Compton scattering)

 Receptor noise (non-uniform response to a uniform X Ray beam)

 Quantum mottle (low photon flux)

Contrast detail curves

detail)

hơn so với ở dưới bên trái

Contrast detail curves

nhưng độ tương phản thấp hơn B và

ngược lại

Tài liệu tham khảo

Imaging

 IAEA Training Material on Radiation

Protection in Diagnostic and Interventional Radiology