1. BMME 560 & BME 590I Medical Imaging: X-ray, CT, and Nuclear Methods X-ray Imaging Part 3

2. Today Resolution vs. Noise Assignment 3 Applications –Conventional radiography –Angiography –Fluoroscopy –Mammography

3. Key Point There is an essential and inescapable tradeoff between noise and resolution in every imaging system. Noise variance Resolution (FWHM)

4. Noise and Resolution Factors limiting resolution –Detector design –Focal spot –Scatter –Magnification Factors contributing to noise –Number of photons collected per pixel per unit time –Background

5. Noise and Resolution Detector design –Improve resolution with smaller pixels, but detect fewer photons per pixel. –Improve QE with thicker scintillator layer, but degrade resolution due to light scattering. Focal spot –Decrease focal spot size by making components smaller, but decrease flux available due to heating considerations.

6. Noise, Contrast, and Resolution Scatter –Apply anti-scatter grid to improve contrast, but detect fewer photons and fewer primary photons. Magnification –Shorten source-to-detector distance to increase magnification, but increase nonuniformity due to angular effects.

7. Other tradeoffs Temporal resolution versus spatial resolution Magnification versus field of view Contrast versus dynamic range

8. Exam Question Choose one of the following techniques of X-ray imaging. Explain one significant application, a disease state, or purpose for which it is used. Relate the source of X-ray contrast in the disease state or application. Explain any special procedures or aspects of instrument design. Discuss the advantages and limitations of X-ray for this disease state or application. –Angiography –Mammography –Fluoroscopy –Interventional radiology –Dental radiography

9. Applications Conventional Radiography Angiography: imaging of blood vessels Fluoroscopy: real-time imaging, interventional radiology Mammography: breast-cancer screening and diagnosis

10. Conventional Radiography Typical X-ray applications –Chest –ExtremitiesExtremities –Dental Refer to Yale Cardiothoracic Database –Rib fractureRib fracture –PneumoniaPneumonia –Lung metastasesLung metastases

11. Angiography Visualization of blood vessels through use of injected contrast Without contrast agent, there is nothing to see. Only the vessel lumen is visualized. Refer to Yale Database –Pulmonary angiographyPulmonary angiography –Coronary angiographyCoronary angiography Other –NeuroangiographyNeuroangiography

12. Fluoroscopy Real-time X-ray imaging Different requirements: –Low dose –Fast detection –Small FOV For interventional radiology, also needinterventional radiology –Patient access –Interventionist shielding

13. Fluoroscopy –Barium swallowBarium swallow Yale –Coronary stenosisCoronary stenosis –Dynamic angiography and ventriculographyDynamic angiography and ventriculography

14. Mammography Screening and detection of breast cancer One of few X-ray procedures done as a large- scale screening procedure (dental) –Most of the patients imaged will be healthy! –Cost is important; so is dose. –Minimize false negatives Patients with suspicious findings will go on to other procedures (biopsy, CT, etc)

15. Mammography The breast is almost all soft tissue and fat –No need to see bone –Lower energy is used in order to improve contrast between soft tissues 20-30 kVp –Molybdenum sources are used – characteristic peaks at 17.5 keV and 19.6 keV –Maximizing dynamic range of contrast in the design is essential (scatter rejection) –Compression gives uniform thickness and pulls in chest wall.

16. Mammography in screening –Compare two breasts in symmetry –If a difference “Normal variation” Benign Suspicious –Asymmetric density –Masses and architectural distortions –Calcifications: regular vs. irregular - “broken glass”

17. Mammography Mammography case studies –Normals –Asymmetric density –Masses –Calcifications