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Advanced Biomedical Imaging Dr. Azza Helal A. Prof. of Medical Physics Faculty of Medicine Alexandria University Lecture 7 Factors affect CT image quality.

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Presentation on theme: "Advanced Biomedical Imaging Dr. Azza Helal A. Prof. of Medical Physics Faculty of Medicine Alexandria University Lecture 7 Factors affect CT image quality."— Presentation transcript:

1 Advanced Biomedical Imaging Dr. Azza Helal A. Prof. of Medical Physics Faculty of Medicine Alexandria University Lecture 7 Factors affect CT image quality & Advanced CT machines

2 Points to be covered  Image quality  Noise, contrast, spatial resolution.  Radiation dose from different CT scans  Artifacts  Advances in CT

3 1. Image quality Image quality also depends on: Nature of x-ray source & detectors, Number & speed of measurements. Method of reconstruction & method of displaying the image. The more measurements the more accurate the reconstructions. Ability to reconstruct axial images into coronal, sagittal or oblique planes

4 Kanal A pixel is 2D element of the image A voxel is 3 D element of the image Slice thickness is the 3 rd dimension of the voxel.

5 Noise (Grainy image) variation in no of x ray photons absorbed / voxel. Causes: Any cause decreases no of photons  Zoom enlargement and narrow window.  The decrease in slice thickness and scan time  Obese/ Patient thickness and high attenuation materials as bone and prosthesis in slice.

6  It limits image quality (decreases CR & SR). Noise is reduced by Any cause increases photons no absorbed / voxel. S/N ratio  kv  MA or scan time (patient dose )  FOV, slice thickness or pixel size, SR is worse

7 Spatial resolution (SR) Minimum distance between two points that imaging system distinguishes them as being separate. SR is improved by using (scanner design) Smaller focal Spot & detector width Smaller slice thickness, pixel size & FOV (small voxels) More projections & lower Pitch Reconstruction & decreased Patient motion.

8 Contrast resolution ability to detect small difference in HU of adjacent structures.  It is superior to plain film due to smaller scatter & removal of superimposed anatomy, very fine x-ray beam, double collimation & Windowing Contrast Resolution is improved by  Causes increase no of photons (dose increases) as:  More mAs,  increasing FOV, pixel size, slice thick.  Patient size (thin patient). low KV (U α Z 3 /E 3 )

9  Not possible to achieve good CR & SR at same time except by deliver high unaccepted pt dose.  To improve contrast by 2 pt dose increases by 4.  To improve SR by 2 patient dose increases by 8.  To half slice thickness pt dose increases by 2  To ½ noise, dose increases by 4  Typical effective dose is in range of 5-10msv.

10 140KV120KV80KVParameter PoorIntermediateBestContrast LeastAverageMostNoise MostAverageLeastPenetration lowestintermediatehighestPatient dose Factors affect patient dose 1. KVp Low Kv increase dose & contrast 2. mAs Doubling mAs double dose, 3. Pitch Dose α mAs/pitch. 4. Slice thickness thin slice increases dose 5. Scan time Increase time increase dose 2. Radiation dose U α Z 3 /E 3

11 How to reduce the dose in CT scan?  Decrease mA or current  Increase Kv  Increase slice width  Decrease the time, 1/2 the time, 1/2 the dose.  Double the pitch 1/2 the dose. Reducing patient dose AFFECT image quality  Increase noise  Decrease contrast & SR  Thick slice So decrease patient dose As far as not affect image quality and hence diagnosis

12 (millirem) Typical effective dose (msv) Examination 100.1Chest X-ray Head CT 3003Screening mammography Abdomen CT Chest CT 360–8803.6–8.8CT colonography (virtual colonoscopy) Chest, abdomen & pelvis 670– Cardiac CT angiogram Barium enema Neonatal abdominal CT Effective dose to the patient from different CT scans

13 Organ Dose  Head CT  Thyroid mGy  Eye lens - 40 mGy  Chest CT  Breast - 21 mGy  Abdomen CT  Uterus – 8 mGy = 8mSv  Gonads - 8 mGy  Pelvis CT  Uterus – 26 mGy  Gonads - 23 mGy Dose to pregnant 1mGy (1mSv) worker. 5mGy (5mSv ), 500 mRem)

14 Artifacts Appearance of signal in an image location not representative of actual properties of object it is necessary to understand why artifacts occur, appearance, cause, and how to prevent/ avoid 3.Artifacts

15 a. Metallic artifact X-ray beams pass through metal implants are highly attenuated and detector detects no transmission, streaks.

16  Blurring, image ghosting, failure to correct u of moving object during reconstruction ;streaks. Graining on image caused by a low SNR; thin slice b. Motion artifact c. Noise artifact

17 a high contrast object occupies part of voxel (bone). scanner is unable to differentiate between a small amount of high-density material (e.g. bone) and a larger amount of other tissue densities (brain). The processor average out the two structures, it raises CT No of pixel & appears higher than it is. It is avoided by thinner slice & smaller pixel d. Partial volume effect (PVA)

18 Floor of ant cranial fossa, as frontal bone is irregular (bone and brain) DDx hge (bright) d. Partial volume effect

19 one or many "rings" appears within an image. due to a detector fault. e. Ring artifact

20 f. Beam hardening Attenuation of bone is greater than that of soft tissue, bone causes more beam hardening (low energy x ray are attenuated ) than an equivalent thickness of soft tissue The beam becomes hard (high energy), low u, low ct no along path of x ray beam, black signal, post fossa black, e.g Petrous bones cupped appearance This is easily corrected by filtration

21 1. Fast scan 2. low dose (rad. is less concentrated) But; reduced contrast & image details in Z axis, More noise & more heating of the tube so need low Ma & PVA occurs a. Helical CT (6th generation) Body moves continuous through x-ray beam. creates a continuous data which sliced during reconstruction. 4. Advances in CT pitch value determines how fast the body is moved

22 b. Multiple slice / detector arrays (MDCT) (MSCT)

23 MSCT can be used for: Fast imaging for larger tissue volume Fewer motion artifact Efficient use of x-ray beam & dose reduction. Reconstruction in different slice widths Possibility of isotropic Imaging (better MPRs and 3D images with reduced image artifacts). Thinner slices for better z-axis resolution

24 c. PET/CT A medical imaging device which combines in a single gantry system both PET and CT components. Images acquired from both devices can be taken sequentially, in the same session from the patient and combined into a single image.

25 PET CT images show metastases to left supraclavicular lymph node a and to liver b Thus functional imaging obtained by PET can be correlated with anatomic imaging obtained by CT. ab

26 Questions 1.Mention the main factors that affect quality of CT image? 2.Enumerate the factors that improve CT image resolution? 3.What are the factors that help in reducing patient dose during CT imaging? 4.Mention the factors that reduce noise in CT image?


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