牙科放射線學 (2)

1. 牙科放射線學(2) Introduction to Computerized Tomography 電腦斷層介紹 陳玉昆副教授: 高雄醫學大學 口腔病理科 07-3121101~2755 yukkwa@kmu.edu.tw

2. 學 習 目 標 References for the present lecture Historical perspective CT scanner components Generations of CT machine CT number Window level and width Use of contrast medium

3. References for the present lecture 2001, November 1. 沈茂忠et al, 電腦斷層攝影診斷學, 3rd edition, Chapter 1, p. 1-18 2. Basic Principles of Computed Tomography, Edwin T. Parks, P. 547-567 3. Hofer M, CT teaching manual, 1st edition, p. 1-13

4. Historical perspective Godfrey Hounsfield: built the first CT scannerin 1972 Alan Cormack: devised the mathematicalcomputations (1964) by which an image could be reconstructed from the data generated from the CT scanner They were awarded the 1979 Nobel Prize for Medicine

5. Gantry : (can tilt up to 30o) Detector array Patient support couch X-ray source Computer Data acquisition time Reconstruction time Operating console Gantry Aperture Table feed per rotation (not per sec) Slice thickness Pitch = Patient support couch Computed tomographic scanner components (Table) Crystal scintillation detector (CsI, CdWO4) -100%; can’t pack together Gas-filled detector (Xe or Xe/Krypton) -50% efficiency; pack together Refs. 2, 3

6. Only accommodate a human head Scan time for a single slice: 6 min (4.5 min for image acquisition, 1.5 min for image reconstruction) (構成一圖像需108次橫移) First – generation machine Translate-rotate scanner Detector 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) Gantry Gantry Aperture The first scanner: A 13 mm slice with 3 line pairs/cm spatial resolution and used an 80x80 image matrix X ray tube Ref. 1

7. The second scanner: drawback Linear arrangement of detectors: Detectors in the middle of array were a different distance from the radiation source than those at the ends Increase the scatter radiation and degrade the image quality Can accommodate the whole body Scan time for a single slice: 20 second (構成一圖像需18次橫移) Second– generation machine Translate-rotate scanner Detectors Gantry 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 Gantry Aperture X ray tube Ref. 1

8. X-ray source Fan beam (Curved) Detector array Third-generation CT-scanner Ref. 2

9. Can accommodate the whole body Scan time for a single slice:  one second Curved detectors solve the differential magnification problem of linear detectors Ring artifact- if a single detector in the array was defective X-ray tube Third – generation machine Rotate-rotate scanner The third scanner: Contain  30 detectors and cover between 30o~60o with a single projection 第三代gantry只須做旋轉式的移動，不須做橫的移動，且detectors也增多，因此構圖速度更快 Detectors Gantry Gantry Aperture X ray tube Ref. 1

10. Can accommodate the whole body Scan time for a single slice: < one second Fourth – generation machine Rotate-only scanner The fourth scanner: Detector array consisted of several thousand elements & provided 360o of coverage – avoid the ring artifact Detectors 360o排滿了gantry之四周，且X光tube可旋轉 Detectors Patient ‘s radiation dose is increased Gantry Gantry Aperture X ray tube Ref. 1

11. Angulation Gantry X-ray tube 90o 270o Table movement Detector 180o Aperture Conventional CT machine- 4th generation Ref. 3

12. X-ray tube Gantry Rotation Step-wise table movement 3rd scan level 2nd scan level 1st scan level Slices for conventional CT 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 Ref. 3

13. Principal difference: patient couch moves continuously during image taking This movement produces image data for a portion of a spiral Scan time is further decreased by increasing the pitch – affect image quality Development of slip ringallows for continuous movement of X-ray source – scan time is further decreased because X-ray source can rotate faster without the heavy cables Fourth – generation machine modification Helical or Spiral CT

14. Slices for spiral CT 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 is complete and uninterrupted X-ray tube Imaging volume Gantry This technique is helpful when data are reformatted to create other 2D views: sagittal, oblique, coronal or 3D Continous table movement Ref. 3

15. 1st generation CT 2nd generation CT Multiple pencil beam Pencil beam Multiple detectors Single detector Summaries of generation of CT machines Third generation CT scanner: Both the X-ray tube & detector array 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 White & Pharoah: Oral Radiology: principle & interpretation, 5th edition, Chapter 17

16. Advantage of spiral technique Conventional CT Liver Spiral CT Advantage of spiral technique Lesions smaller than the conventional thickness of a slice can be detected Small liver metastases (7) will be not being included in the section The metastases would appear in reconstructions from the dataset of the helical technique Ref. 3

17. CT number – 影像的明暗度 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 ) Ref. 1

18. X-ray source Fan beam 不同組織 不同 attenuation coefficients Grey scale (明暗度) Hounsfield Scale (CT no.) (Curved) Detector array Attenuation Coefficient X光透過人體 會被減弱 (Attenuation) Attenuation coefficient Ref. 2

19. 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)，如用但CT no.去測量，則永遠不變 Ref. 1

20. Attenuation coefficient and CT no. for biological tissues at 60 keV Attenuation Coefficient  (cm-1) Tissue CT Number (HU) Ref. 2

21. CT No. scale (Hounsfield scale) Ref. 1

22. Different gray scale of substances under CT Ref. 3

23. Different Gray scale of substances under CT Ref. 3

24. 顱底CT說明CT No.對影像判讀之重要 Front 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 Ref. 1

25. 脊椎CT說明CT No.對影像判讀之重要 Front (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

26. 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 Ref. 1

27. Window level (center) & width Window level: CT no; Width: range between 2 CT no Modern equipment has a capacity of 4096 gray tones, which represent different density levels in HUs. (The 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 Densities of human tissues 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 Ref. 3

28. Density levels of different types of tissues Density parenchymal organs & fluids Density of all tissues Window level (center) & width Ref. 3

29. 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 Soft tissue window 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) Ref. 3

30. Window level and width Density levels of different types of tissues The meandensity levelof 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 Ref. 3

31. Window (366 Hu) Window (122 Hu) Small tumors Brain edema 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 Ref. 1

32. Lung window 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 Ref. 3

33. Brain window 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 Ref. 3

34. Bone window Brain window Bone window Bone window should have a much higher center, at about +300HU, and a sufficient width of ~1500HU Metastases in the occipital bone would only be visible in the appropriate bone window but not in the brain window Brain is invisible in the bone window: small cerebral metastases would not be detected Ref. 3

35. 200HU 300HU Summary of concept of CT window 400HU 100HU Window width 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) Internet data, anymomus, Ref. 1

36. Window level (center) & width Density levels of different types of tissues Density levels of almost all soft-tissue organs lie within a narrow range between 10 and 90 HUS The only exception is the lung and this requires a special window setting (lung window) For hemorrhage Density level of recently coagulated blood lies about 30HU above that of fresh blood. This density drops again in older hemorrhages or liquefied thromboses. Ref. 3

37. 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 Window width & window level setting

38. 1 2 3 Contrast medium - IV 對比劑為含碘水溶性對比劑 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 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 使用30%300ml瓶裝之點滴(總含碘量40gm)邊滴邊切 Ref. 1

39. Before CM After CM Oral administration of contrast media 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) Ref. 3

40. Summaries Knowing basic knowledge of CT: Historical perspective CT scanner components Generations of CT machine CT number Window level and width Use of contrast medium

41. Your constructive comments are appreciated 謝 謝