CT成像原理介绍(英文版)

1、ct 成像原理介紹,computed tomography,ct basics principle of spiral ct scan parameter & image quality optimizing injection protocols clinical applications,x-ray discovery,x-ray was discovered by a german scientist roentgen 100 years ago. this made people for the first time be able to view the anatomy struct

2、ure of human body without operation,but its superimposed and we couldnt view soft tissue,my name is godfrey hounsfield i work for the central research labs. of emi, ltd in england i developed the the first clinically useful ct scanner in 1971,history of computed tomography,early 1970s,1963 - alan co

3、rmack developed a mathematical method of reconstructing images from x-ray projections,for the first time we could view:,ct broke the barrier,- tomographic or “slice” anatomy- density difference,but its time consuming and resolution needs to be improved,concept of x-ray attenuation,an x-ray beam pass

4、ing through the body is attenuated (loses its energy) by : absorption scattering,incident x-ray,transmitted ray,scattered x-rays,body tissue,absorption by the tissue is proportional to the density,less dense tissue,more dense tissue,more attenuation,less attenuation,how does ct work?,x-ray goes thro

5、ugh collimator therefore penetrate only an axial layer of the object, called slice,how does ct work?,patient is placed in the center of the measurement field x-ray is passed through the patients slice from many direction along a 360 path the transmitted beams are captured by the detectors which digi

6、tizes these signals these digitized signals called raw data are sent to a computer which create the ct image,how does ct work?,the object slice is divided into small volume elements called voxels. each voxel is assigned a value which is dependent on the average amount of attenuation,how is ct image

7、generated?,the attenuation values are transferred to the computer where they are coded & used to create a slice image,how is ct image generated?,ct generations & design,“generation” is used to label ct tube-detector designs,3rd generation design rotating tube & detector,4th generation design fixed r

8、ing detector,slip-ring technology,power is transmitted through parallel sets of conductive rings instead of electrical cables continuous gantry rotation prerequisite for spiral ct,non slip-ring scanner,slip-ring scanner,computed tomography,ct basics principle of spiral ct scan parameter & image qual

9、ity optimizing injection protocols clinical applications,what is spiral scan? - just 4“c”,continuously rotating tube/detector system continuously generating x-ray continuously table feed continuously data acquisition,reconstruction of arbitrary slices (either contiguous or overlapping) within the sc

10、anned volume distance between the slices is called increment,a,b,volume data,continuous data acquisition,increment,slice thickness,increment = slice thickness no overlap no gaps,contiguous image reconstruction,increment,overlap,slice,thickness,overlapping image reconstruction,increment slice thickne

11、ss overlap of slices closer image interval more images created,increment slice thickness gaps between slices images are further apart less images created,image reconstruction with gaps,increment,slice thickness,standard ct / slice imaging,deep inspiration,shallow inspiration,misregistration due to d

12、ifferent respiratory levels between slices,partial volume effect,unable to resconstruct images at arbitrary position,slice imaging is slow,scan the whole region of interest in one breath hold,reconstruction of overlapping images without additional dose,retrospective reconstruction of slices in arbit

13、rary position within the scanned volume,no gaps since radiation always transmits the whole volume,spiral ct / volume imaging,computed tomography,ct basics principle of spiral ct scan parameter & image quality optimizing injection protocols clinical applications,scan parameters,x-ray tube voltage (kv

14、p) x-ray tube current (ma) scan time (s) slice thickness or collimation (mm),table speed (mm/rot) or feed per 360 rotation pitch interpolation process increment (mm),table speed & pitch,table speed is defined as distance traveled in mm per 360 rotation,pitch =,table feed per rotation collimation,10m

15、m p1,10mm p2,30s,30s,more coverage in the same time with extended pitch!,pitch 2 covers 2x distance as pitch 1,scan range = 300mm,10mm p1 10 mm/s,10mm p2 20 mm/s,30s,15s,cover the same volume in shorter time with extended pitch,to reduce artifacts due to table motion during spiral scanning, we use a

16、 special reconstruction process called interpolation,interpolation,converts volume data into slice images,interpolation algorithm,wide algorithm,slim algorithm,2 x 360 = 720 raw data,2 (180+52) = 464 raw data,wide algorithm produces a broader image thickness wide algorithm uses more raw data = less

17、image noise,pitch 2,pitch 1,pitch 2 scanning produces a broader image thickness pitch 2 scanning does not increase image noise,30% increase in image thickness with pitch 2,slice sensitivity profile ( ssp ),ssp describes the effective slice thickness of an image and to what extent anatomy within that

18、 slice contribute to the signal,ssp,resolution,all points within the slice contribute equally & points outside of the slice do not contribute to the image at all .,image signal,ideal ssp,z-axis (mm),collimation = width of x-ray beam =slice profile,slice profile (sp),effective slice thickness of an i

19、mage,slice profileresolution,factors influencing slice profile collimation pitch interpolation algorithm (360 or 180),factors influencing ssp,collimator width collimation = ssp spiral ct table speed or pitch interpolation algorithm = mathematical process required to reconstruct axial images from the

20、 spiral volume data set,pitch & slice profile,slim vs wide ssp comparison,slice profile slim %broaden wide %braden pitch one 5.0 mm 0 6.3 mm 26 pitch two 6.5 mm 30 10.8 mm 116,wide 720 degree more photons,slim 464 degree less photons,ssp spatial resolution,ssp spatial resolution,smoother image,noisi

21、er image,slim - advantages,improved z resolution reduced partial volume artifacts slim + extended pitch longer coverage same coverage with shorter scan time or thinner slices less radiation dose,wide - advantages,noise reduction smoother image useful for scanning huge patient only for scanning at pi

22、tch one,slice profile comparison,lesion smaller than 1cm,scan range = 150mm,10/10/10 (15s),5/10/5 (15s),slice profile = 10mm,slice profile = 6.5mm,optimizing the scanning parameters,smallest possible effective slice thickness,scan length (mm),scan duration (s),table speed (mm/s),smallest collimation

23、 (mm),scan duration,depends on the scan length & patients breath-hold compliance,table speed pitch factor,1 pitch 2 to cover the whole volume in one breath-hold,computed tomography,ct basics principle of spiral ct scan parameter & image quality optimizing injection protocols clinical applications,pe

24、ripheral vein eg. antecubital vein 19-20 gauge needle or iv catheter,volume,flow rate,concentration,injection protocols,300 mg i/ml non-ionic contrast,2 - 5 ml/s cardiac output,80 - 150 ml patients weight & region of interest,site,scan delay,delay between injection initiation & the start of the scan

25、 sequence,tailoring scan & injection protocols,injection duration must be equal to or greater than scan time,50,100,150,200,250,times,hu,contrast,50,100,150,200,250,times,hu,contrast,nacl,bolus duration scan time insufficient, inhomogeneous opacification,bolus duration = scan time uniform, maximum o

26、pacification,enhancement curve of the target region,50,100,150,200,250,times,hu,optimal window,early,time-density curve of the target region,late,contrast bolus timing,determines optimal scan delay for spiral cta sequence,contrast,nacl,test bolus procedure,imaging volume for spiral cta,10-20 ml of contrast is injected at the chosen rate for spiral,after a delay of 8-10s, low-dose, single-level axial images are acquired every 2s at the starting point of the imaging volume,dynamic scans at this position,dynamic evaluation to generate a time-density cu