牙科放射線學 (2)

陳玉昆教授 : 高雄醫學大學 口腔病理科 07-3121101~2755 yukkwa@kmu.edu.tw

Introduction to Computed Tomography

電腦斷層介紹

References for the present lectureHistorical perspectiveCT scanner componentsGenerations of CT machineCT numberWindow level and widthUse of contrast medium

學 習 目 標學 習 目 標

References for the present lecture

1. 沈茂忠 et al, 電腦斷層攝影診斷學 , 3rd edition, Chapter 1, p. 1-18

3. Hofer M, CT teaching manual, 1st edition, p. 1-13

2001, November

2. Basic Principles of Computed Tomography, Edwin T. Parks, P. 547-567

Historical perspective

Godfrey Hounsfield Built the first CT scanner in 1972

Allan M Cormack Devised the mathematical computations (1964)by which an image could bereconstructed from the data generatedfrom the CT scanner

They were awarded the 1979 Nobel Prize for Medicine

Ref: www.nobelprize.org/nobel_prizes/medicine/laureates/1979/cormack-facts.htmlRef: www.nobelprize.org/nobel_prizes/medicine/laureates/1979/cormack-facts.html

Ref: geni.comRef: geni.com

Computed tomographicscanner components

Gantry

Aperture

Patient support couch(Table)

Crystal scintillation detector (CsI, CdWO4) -100%; cannot pack togetherGas-filled detector (Xe or Xe/Krypton) -50% efficiency; pack together

Gantry:(can tilt up to 30o) Detector arrayPatient support couchX-ray sourceComputer:Data acquisition timeReconstruction timeOperating consolePitch = Table feed per rotation (not per sec)

Slice thickness

Refs. 2, 3

The first scanner: A 13 mm slice with 3 line pairs/cmspatial resolution and used an 80x80 image matrix

First – generation machine

Only accommodate a human headScan time for a single slice: 6 min (4.5 min for image acquisition, 1.5 min for image reconstruction)

Ref. 1

( 構成一圖像需 108 次 橫移 )

Detector

Gantry

GantryAperture

X ray tube

第一代 gantry 含 pencil thin X ray beam 及 detectorGantry 可做橫向移動 ( 直箭頭 )及旋轉移動 ( 曲線箭頭 )

Translate (X-ray tube)-rotate (detector) scanner

Translate (X-ray tube)-rotate (detector) scanner

Second – generation machine

Can accommodate the whole bodyScan time for a single slice: 20 second

The second scanner: drawbackLinear arrangement of detectors:Detectors in the middle of array were a different distance fromthe radiation source than thoseat the ends Increase the scatter radiation and degrade the image quality

第二代 gantry 與第一代 gantry不同處乃 detectors 增多X 光源為扇形，且旋轉角度加大

( 構成一圖像需 18 次橫移 ) Ref. 1

Detectors

GantryGantryAperture

X ray tube

Third - generation CT-scanner

X-ray source

Fan beam

Detector array(Curved)

Ref. 2

Rotate (X-ray tube)-rotate (detector) scanner

Third – generation machine

Can accommodate the whole bodyScan time for a single slice: one second

The third scanner: Contain 30 detectors andcover between 30o~60o witha single projection

Ref. 1

X-ray tube

Curved detectors solve the differential magnification problem of linear detectors

Ring artifact- if a single detector in the array was defective

第三代 gantry 只須做旋轉式的移動，不需做橫的移動，且detectors 也增多，因此構圖速度更快Detectors

Gantry

GantryAperture

X ray tube

Rotate-only (X-ray tube) scanner

Fourth – generation machine

Can accommodate the whole bodyScan time for a single slice: < one second

The fourth scanner: Detector array consisted ofseveral thousand elements &provided 360o of coverage – avoid the ring artifact

Patient ‘s radiation dose is increased

Detectors 360o 排滿了gantry 之四周，且 X 光tube 可旋轉

Ref. 1

X raytube

Detectors

GantryGantryAperture

Conventional CT machine-4th generation

Gantry

Angulation

Aperture

X-ray tube

TablemovementDetector 180o

90o

270o

Ref. 3

Gantry

X-ray tube

Rotation

Step-wisetablemovement

1st scan level2nd scan level3rd scan level

Slices for conventional CT

Ref. 3

In conventional CT, a series of equally spaced is required sequentially through a specific region, e.g. the head.

There is a short pause after each section in order to advance the patient table to the next preset position.

The section thickness & overlap/ intersection gap are selected at the outset.

The raw data for each image level is stored separately.

The short pause between sections allows the conscious patient to breathe without causing major respiratory artifacts

modificationFourth – generation machine

Helical or Spiral CTPrincipal difference: patient couch (table) movescontinuously during image taking

This movement produces image data for aportion of a spiral

Scan time is further decreased byincreasing the pitch – affect image quality

Development of slip ring allows for continuous movement of X-ray source – scan time is furtherdecreased because X-ray source can rotate fasterwithout the heavy cables

Slices for spiral CT

X-ray tube

Gantry

Continuoustablemovement

Imagingvolume

Ref. 3

In spiral CT, images are acquired continuously while the patient table is advanced through the gantry.The x-ray tube describes an apparent helical path around the patient.If table advance is coordinated with the time required for a 3600 rotation (pitch factor), data acquisition iscomplete and uninterrupted

This technique is helpful when data are reformatted to create other 2D views: sagittal, oblique, coronal or 3D

Summaries of generations ofCT machines

Ref: White & Pharoah: Oral Radiology: principle & interpretation, 5th edition, Chapter 17

Third generation CT scanner:Both the X-ray tube & detectorarray rotate around the patient

Fourth generation CT scanner:The X-ray tube rotates within a stationary ring of the detectors

Spiral CT: The X-ray tube & detectors move in a continuous spiral motion around the patient as the patient moves continuously into the gantry in the direction of the red solid arrows

1st generation CT 2nd generation CT

Pencil beam

Single detector

Multiplepencil beam

Multipledetectors

Spiral CT

Advantage of spiral technique

Liver

Conventional CT

Ref. 3

Advantage of spiral technique

Lesions smaller than the conventional thickness of a slice can be detectedSmall liver metastases (7) will be not being included in the sectionThe metastases would appear in reconstructions from the dataset of the helical technique

CT number – 影像的明暗度

Ref. 1

當 X 光源透過人體到 detector ，被 detector 接受時， X 光的部份光子（ photons ）會被人體吸收此稱為 X 光衰減（ X-ray attenuation ），而人體內每一種構造對 X 光的衰減並不相同，這種物質的 X 光衰減可用衰減係數（ attenuation coefficient ）來描述。人體內以骨質之 X 光衰減係數最大，為 0.400u ，水的衰減係數為 0.203u ，與衰減係數相對照的是 明 暗 度 表 （ gray scale ） ， 亦 稱 為Hounsfield 表（ scale ）

當 X 光源透過人體到 detector ，被 detector 接受時， X 光的部份光子（ photons ）會被人體吸收此稱為 X 光衰減（ X-ray attenuation ），而人體內每一種構造對 X 光的衰減並不相同，這種物質的 X 光衰減可用衰減係數（ attenuation coefficient ）來描述。人體內以骨質之 X 光衰減係數最大，為 0.400u ，水的衰減係數為 0.203u ，與衰減係數相對照的是 明 暗 度 表 （ gray scale ） ， 亦 稱 為Hounsfield 表（ scale ）

( Attenuation coefficient )

X-ray source

Fan beam

Detector array(Curved)

X 光透過人體會被減弱

(Attenuation)

Attenuationcoefficient

Grey scale( 明暗度 )

HounsfieldScale

(CT no.)

不同組織不同 attenuation

coefficients

Ref. 2

Attenuation Coefficient

CT number – 影像的明暗度 人體內各種組織之 X 光衰減係數： CT 係數 (CT number) 此係數以水為 0 Hu ， bone 為 +1000 Hu

CT no. 對 CT 的判讀非常重要，因為在 CT 影像上，由於 gray scale 的不同，各種組織的明暗度會改變( 會隨著不同 kVP 而明暗度會改變 ) ，如用 CT no. 測量，則永遠不變

Ref. 1

Attenuation coefficient and CT no. for biological tissues at 60 keV

CT Number (HU)Tissue

AttenuationCoefficient

(cm-1)

Ref. 2

CT No. scale (Hounsfield scale)

Ref. 1

Different gray scale of substances under CT

Ref. 3

Air (black) Fat /CSF (almost black)

Muscle (dark gray) Bone (white)

Blood vessels (light gray) Pancreas, salivary glands

in the trachea in the colon

longitudinalsection

transverse section

trabeculae

cortical

longitudinal section

transverse section

Different Gray scale of substances under CT

Ref. 3

Front顱底 CT 說明 CT No. 對影像判讀之重要顱底 CT 說明 CT No. 對影像判讀之重要

Ref. 1

Frontal sinus，外耳道，mastoid air cells等，含空氣，CT number -1000Hu深黑色

眼框內脂肪，皮下脂肪，CT number -80Hu，極黑

大，小腦組織及枕部肌肉是soft tissue ， CT number 40Hu，為灰色

骨頭， CT number800-1000Hu，白色

Front

脊椎 CT 說明 CT No. 對影像判讀之重要

Ref. 1

(B):L5-S1間椎體為骨質構造，CT number 800-1000Hu，呈白色

(D):椎間盤為纖維組織， CT number 70Hu，呈灰白色

(M):脊椎後肌肉為軟組織， CT number 45Hu，呈灰色

(S):脊椎腔內肖腔含 CSF為體液， CT number 6Hu，呈黑色

(F):脊椎腔內脂肪與椎體外脂肪， CT number -80Hu，呈深黑色

Ref. 1

CT No. 對影像判讀之重要性出血性中風及基底核鈣化 右側 putamen內有一大片白色區，左右基底核也各有小片白色區，憑肉眼難予判斷鈣化或出血。 用 CT number測量， putamen 為69Hu，因此是新鮮血塊，雙側基底核之白點為 120Hu，因此是鈣化

孔洞腦 (porencephalic cyst) 左半球之吸收值 (density)相當低，其 CT number 為 6Hu，因此是水的成份

Window level: CT no; Width: range between 2 CT no

Window level (center) & width

Ref. 3

Modern equipment has a capacity of 4096 gray tones, which represent different density levels in HUs. (Density of water was arbitrarily set a 0 HU and that of air at -1000 HU)

Monitor can display a maximum of 256 gray tones Human eye is able to discriminate only ~20 Human tissues densities extend over a fairly narrow range (a window) of the total spectrum (10-90HU), it is possible to select a window setting to represent the density of the tissue of interest

Density levels of different types of tissues

Density of all tissuesDensity parenchymalorgans & fluids

Window level (center) & width

Ref. 3

Soft tissue window

All tissues with a density <-125 HU (e.g. lung) are black Those density levels >+225 appear white and internal structural features cannot be differentiated

Ref. 3

Density values centered at 50HU with a width of about 350HU. The result is a representation of density values from -125HU (50-350/2) up to +225HU (50+350/2)

Density levels of different types of tissues

Window level and width

Ref. 3

The mean density level of the window should be set as close as possible to the density level of the tissue to be examined.

The lung, with its high air content, is best examined at a low HU window setting

Bone require an adjustment to high levels

The width of the window influences the contrast of the images: the narrower the window, the greater the contrast

Lung window

Bone window

Window (122 Hu)

Brain edema

Window (366 Hu)

Small tumors

Ref. 1

Window level and width

左圖 window width 為尋常的 125Hu，不易看到頭顱骨折，但可清楚看到大腦間的出血 ( )右圖同一影像，但 window width 調到 1000Hu，此時可清楚看到頭顱骨折 ( )，但完全無法觀察顱內構造

Lung window

Ref. 3

If lung parenchyma is to be examined, e.g. when scanning for nodules, the window center will be lower at about -200HU, & the window width (2000HU). Low density pulmonary structures can be much more clearly differentiated

Brain window

Ref. 3

Density values of gray & white matter differ only slightly. The brain window must be very narrow (80-100HU-> high contrast) and the center must lie close to the mean density of cerebral tissue (35HU) to demonstrate these slight differences

Bone window

Bone window should have a much higher center, at about +300HU, and a sufficient width of ~1500HUMetastases in the occipital bone would only be visible in the appropriate bone windowbut not in the brain windowBrain is invisible in the bone window: small cerebral metastases would not be detected

Ref. 3

Bone window

Brain window

Summary of concept of CT windowSummary of concept of CT window

Window width100HU 400HU200HU 300HU

Internet data, anymomus, Ref. 1

人 體各種器官、組織，在 CT 上之 CT number 以及在尋常 window width (100-400, center 20-50) 下的明暗度 ( 黑白顏色 )

Density levels of different types of tissues

For hemorrhageDensity level of recently coagulated blood lies about 30HU above that of fresh blood.

This density drops again in older hemorrhages or liquefied thromboses.

Window level (center) & width

Ref. 3

Density levels of almost all soft-tissue organs lie withina narrow range between 10 and 90 HU

The only exception is the lung and this requires a specialwindow setting (lung window)

Density levels of almost all soft-tissue organs lie withina narrow range between 10 and 90 HU

The only exception is the lung and this requires a specialwindow setting (lung window)

Window width & window level setting

每一個影像在 CT上都可以在監視器螢幕上根據 gray scale調整明暗度。Window width與各種組織的解像對比（ sensitive-contrast resolution）有關， window窄對比越清楚。 Window level（有些機器稱 center）則與影像黑白度有關， level越高，影像越白， level越低影像越黑。觀察腦室的層次時，此層次各種構造以 CSF （ 4 HU）及brain tissue （ 36 HU）較重要，兩者 resolution contrast不大，因此窗寬設定在 100 ～ 125 HU較適宜，此時頭顱不重要，呈白色

在顱底時，其構造較複雜有 brain （ 36 HU），眼眶（含脂肪， -100 HU），骨質構造也重要（ +1000 HU），所以各種構造之 contrast之差異較大，此時窗寬應設在 200 ～ 400 HU

觀察頭顱之骨質，甚至窗寬須設定在 1000 HU，但此時就犧牲 brain tissue 了 至於 window level則通常於 window width setting好之後再視片子的明暗度調整

Ref. 1

Contrast medium - IV

Ref. 1

最常用的是每 100ml 含碘 30gm 之 60% ， 100ml瓶 裝對比劑，做 CT 時以快速靜脈點滴，通常於滴入2/3 時開始切 CT ，再邊滴邊切

1

先以 50ml 空針快速注射 50ml 對比劑後，同時再滴入 100ml 對比劑，再邊滴邊切2

用 30%300ml 瓶裝之點滴 ( 總含碘量 40gm) 邊滴邊切3

對比劑為含碘水溶性對比劑

Oral administration of contrast media

Before CM

AfterCM

Ref. 3

Without contrast medium (CM), it is difficult to distinguish between the duodenum (130) & the head of the pancreas (131, right figure) & other parts of the intestinal tract (140) would also be very similar to neighboring structures

After an oral CM, both the duodenum & the pancreas can be well delineated (Figures below)

SummariesSummariesKnowing basic knowledge of CT:

1. Historical perspective2. CT scanner components3. Generations of CT machine4. CT number5. Window level and width6. Use of contrast medium

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