Presentation on theme: "Advanced Biomedical Imaging"— Presentation transcript:
1 Advanced Biomedical Imaging Lecture 7Factors affect CT image quality&Advanced CT machinesDr. Azza HelalA. Prof. of Medical Physics Faculty of Medicine Alexandria University
2 Points to be coveredImage qualityNoise, contrast, spatial resolution.Radiation dose from different CT scansArtifactsAdvances 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 A pixel is 2D element of the image A voxel is 3 D element of the image Slice thickness is the 3rd dimension of the voxel.Kanal
5 Noise (Grainy image)variation in no of x ray photons absorbed / voxel.Causes: Any cause decreases no of photonsZoom enlargement and narrow window.The decrease in slice thickness and scan timeObese/ Patient thickness and high attenuation materials as bone and prosthesis in slice.
6 It limits image quality (decreases CR & SR). Noise is reduced byAny cause increases photons no absorbed / voxel.S/N ratiokvMA 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 widthSmaller slice thickness, pixel size & FOV (small voxels)More projections & lower PitchReconstruction & decreased Patient motion.
8 Contrast resolution Patient size (thin patient) . low KV (U α Z3/E3) 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 & WindowingContrast Resolution is improved byCauses increase no of photons (dose increases) as:More mAs,increasing FOV, pixel size, slice thick.Patient size (thin patient) low KV (U α Z3/E3)
9 To ½ noise, dose increases by 4 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 2To ½ noise, dose increases by 4Typical effective dose is in range of 5-10msv.
10 Factors affect patient dose 2. mAs Doubling mAs double dose, U α Z3/E32. Radiation dose140KV120KV80KVParameterPoorIntermediateBestContrastLeastAverageMostNoisePenetrationlowestintermediatehighestPatient doseFactors affect patient dose1. KVp Low Kv increase dose & contrast2. mAs Doubling mAs double dose,3. Pitch Dose α mAs/pitch.4. Slice thickness thin slice increases dose5. Scan time Increase time increase dose
11 How to reduce the dose in CT scan? Decrease mA or currentIncrease KvIncrease slice widthDecrease the time, 1/2 the time, 1/2 the dose.Double the pitch 1/2 the dose.Reducing patient dose AFFECT image qualityIncrease noiseDecrease contrast & SRThick sliceSo decrease patient dose As far as not affect image qualityand hence diagnosis
12 Typical effective dose (msv) (millirem)Typical effective dose (msv)Examination100.1Chest X-ray1501.5Head CT3003Screening mammography5305.3Abdomen CT5805.8Chest CT360–8803.6–8.8CT colonography (virtual colonoscopy)9909.9Chest, abdomen & pelvis670–13006.7-13Cardiac CT angiogram150015Barium enema200020Neonatal abdominal CTEffective dose to the patient from different CT scans
14 3.ArtifactsAppearance 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/ avoidArtifacts
15 a. Metallic artifactX-ray beams pass through metal implants are highly attenuated and detector detects no transmission, streaks.
16 Graining on image caused by a low SNR; thin slice Blurring , image ghosting, failure to correct u of moving object during reconstruction ;streaks.Graining on image caused by a low SNR; thin sliceb. Motion artifactc. Noise artifact
17 d. Partial volume effect (PVA)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 pixelDR Azza Helal
18 d. Partial volume effect Floor of ant cranial fossa, as frontal bone is irregular (bone and brain) DDx hge (bright)
19 one or many "rings" appears within an image. due to a detector fault. e. Ring artifact
20 (high energy), low u, low ct no along path of x ray beam, f. Beam hardeningAttenuation 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 tissueThe beam becomes hard(high energy), low u, low ct noalong path of x ray beam,black signal,post fossa black, e.g Petrous bonescupped appearanceThis is easily corrected by filtration
21 4. Advances in CTa. Helical CT (6th generation)pitch value determines how fast the body is movedBody moves continuous through x-ray beam. creates a continuous data which sliced during reconstruction.1. Fast scan 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
22 b. Multiple slice / detector arrays (MDCT) (MSCT)
23 MSCT can be used for:Fast imaging for larger tissue volumeFewer motion artifactEfficient use of x-ray beam & dose reduction.Reconstruction in different slice widthsPossibility of isotropic Imaging (better MPRsand 3D images with reduced image artifacts).Thinner slices for better z-axis resolution
24 c. PET/CTA 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 Thus functional imaging obtained by PET can be correlated with anatomic imaging obtained by CT. PET CT images show metastases to left supraclavicular lymph node a and to liver b
26 QuestionsMention the main factors that affect quality of CT image?Enumerate the factors that improve CT image resolution?What are the factors that help in reducing patient dose during CT imaging?Mention the factors that reduce noise in CT image?