Physics of Radiology Lior Copel, M.D. Assaf Harofeh Medical Center, Zerifin Sackler School of Medicine, Tel-Aviv University

Clinical Case

Jaundice and Fever Subjective: –42 year old female –Malaise and jaundice for 2 months –Fever for 1 week Objective: –Fever – 38.5 –Leukocytes – ; PMN – 78% –Skin and corneal jaundice: Total bilirubin – 11.3 mg/dL Direct bilirubin – 6.7 mg/dL

Diagnosis Pancreatic adenocarcinoma (head) Obstructive jaundice Ascending cholangitis due to obstructive jaundice

Conventional Radiology

The Electromagnetic Spectrum

Photons Electromagnetic radiation is quantized in discrete quantities called photons Photons behave as waves or particles but have no mass Photons energy (E) – Frequency Wavelength C = velocity of light = 300,000 km/sec E = h x f = h x (C / λ)

Photons Photons  X-Rays Photons – high energy - 20 – 200 keV short wavelength m

X-Ray Tube

X-Ray Interaction in Material Pass through (penetrate) Absorbed (transfer energy to the absorbed medium) Scattered (change direction and possibly lose energy)

The Cassette

Radiology Examination Room

X-Ray Advantages Excellent imaging of the chest and skeleton Good evaluation of GIT and GUT Good spatial resolution Can be performed dynamically Availability Relative low radiation dose to patient

X-Ray Disadvantages Poor 3D geometry Poor soft tissue resolution Insensitive to small lesions (esp. lung fields) No evaluation of CNS and PNS

Ultrasound (US)

Natural Ultrasound

Sound Wave

Sound Waves Infrasound - < 20 Hz Audible sound – 15 – 20,000 Hz Diagnostic ultrasound – 1 – 20 MHz (1 MHz = 10 6 Hz) Diagnostic US uses transducers for the production of the waves

Velocity of the US Wave in Different Tissues

Doppler Physics Doppler effect – the change in frequency that results from a moving sample Object Movement: –Toward the detector  higher frequency, lower wavelength –Away from the detector  lower frequency, higher wavelength

Doppler Shift

US - Advantages Excellent soft tissue contrast resolution Dynamic No radiation Safe in pregnancy Available, cheap

US - Disadvantages Operator dependent  not imagined-not seen ! Relatively no anatomic landmarks Cannot penetrate air and bone No imaging of lungs, CNS, PNS and GIT

Computed Tomography (CT)

Computed Tomography Tomography – Greek: –tomos  slice or section –graphein  to write or record CT scan – a diagnostic test that combines X-rays with computer technology A series of X-rays from many different angles are used to create a cross-sectional image of the patient’s body

Computed Tomography Reconstruction “builds” the CT image from the data collected and represents a cross section of the patient

Computed Tomography Four hardware components: –Radiation source –Radiation detector system –Mechanical manipulator –Computer with display

Contrast-Enhanced CT Abnormal tissues (e.g. tumor, inflammation) enhance differently than normal tissues with IV contrast. This enables the abnormal tissue to be identified and characterized Contrast media opacify specific structures (blood vessels, liver, spleen and the urinary tract) enabling abnormal findings to be detected in those structures

Hounsfield Units

The Gantry

CT - Advantages Excellent anatomical data Excellent spatial resolution Good contrast resolution (fair in CNS and MSK imaging) Reconstructions (MPR, MIP, VR, navigation) Fast exam (20sec.- 1min.) Available

CT - Disadvantages Radiation dose Reactions to contrast material Static exam Availability Cost

Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging Concept – 1970 Present – 60 million examinations per year

Hydrogen Nuclei

The Hydrogen Nuclei Have the largest magnetic moment Abundant in the body H protons in each 1 cm 3 of tissue These protons are normally randomly oriented and have no net magnetic moment (magnetization vector)

The Hydrogen Nuclei

Hydrogen Nuclei in Nature

1 Tesla = 10,000 gauss (G) Earth’s magnetic field 50 µT (0.5 gauss) MRI magnet is 20,000 more powerful than earth’s magnetic field The Magnet

Protons in a Magnetic Field

90 Degree RF Pulse

Time Constants Measurements T1T1 –Time to recover longitudinal magnetization –Realigning to B 0 –Range  200 – 2000 msec T2T2 –Time to dephase  loose horizontal magnetization –Range  30 – 500 msec Differences in T1 and T2 provide basis for variations in signal intensities and tissue contrastDifferences in T1 and T2 provide basis for variations in signal intensities and tissue contrast

The time it takes to each tissue to return to its steady state position is the base for MR imaging T1 Relaxation Time

T2 Relaxation Time

Relaxation Time Different tissues return to equilibrium in different time Those difference in relaxation time enables different contrast resolution to different tissues and make it possible to differentiate normal tissue from abnormal one

Contrast Agents – Gadolinium-DTPA Abnormal tissues (e.g. tumor, inflammation) enhance differently than normal tissues with iv contrast. This enables the abnormal tissue to be identified and characterized Contrast media opacify specific structures (blood vessels, liver, spleen and the urinary tract) enabling abnormal findings to be detected in those structures

Planes of Imaging Coronal Sagittal Axial (Horizontal)

MRI Advantages Excellent contrast resolution (esp. CNS, MSK, mediastinum, pelvis and heart) Each tissue / pathology can be evaluated with different sequences No radiation

MRI Advantages Multiplanar imaging Can be performed in pregnancy (not in first trimester) No risk with contrast materials Can be performed dynamically

MRI Disadvantages Problem with cortical bone and calcifications Hazard to patients with metallic objects Problems in trauma and ventilated patients Claustrophobia Long examination Availability Expensive

Be Careful !!!