Physics of RadiologyPhysics of Radiology

Lior Copel, M.D.Lior Copel, M.D.Assaf Harofeh Medical Center, ZerifinAssaf Harofeh Medical Center, Zerifin

Sackler School of Medicine, Sackler School of Medicine,

Tel-Aviv UniversityTel-Aviv University

Clinical CaseClinical Case

Jaundice and FeverJaundice and Fever

• Subjective:– 42 year old female– Malaise and jaundice for 2 months– Fever for 1 week

• Objective:– Fever – 38.5– Leukocytes – 16700 ; PMN – 78%– Skin and corneal jaundice: Total bilirubin – 11.3 mg/dL Direct bilirubin – 6.7 mg/dL

DiagnosisDiagnosis

• Pancreatic adenocarcinoma (head)

• Obstructive jaundice

• Ascending cholangitis due to obstructive jaundice

Conventional RadiologyConventional Radiology

The Electromagnetic SpectrumThe Electromagnetic Spectrum

PhotonsPhotons

• 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)

PhotonsPhotons

• Photons X-Rays

• Photons – high energy - 20 – 200 keV

short wavelength - 10-10 m

X-Ray TubeX-Ray Tube

X-Ray Interaction in MaterialX-Ray Interaction in Material

• Pass through (penetrate)

• Absorbed (transfer energy to the absorbed medium)

• Scattered (change direction and possibly lose energy)

The Cassette The Cassette

Radiology Examination Room Radiology Examination Room

X-Ray Advantages 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 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)Ultrasound (US)

Natural UltrasoundNatural Ultrasound

Sound WaveSound Wave

Sound WavesSound Waves

• Infrasound - < 20 Hz

• Audible sound – 15 – 20,000 Hz

• Diagnostic ultrasound – 1 – 20 MHz

(1 MHz = 106 Hz)

• Diagnostic US uses transducers for the production of the waves

Velocity of the US Wave in Different TissuesVelocity of the US Wave in Different Tissues

Doppler PhysicsDoppler 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 ShiftDoppler Shift

US - AdvantagesUS - Advantages

• Excellent soft tissue contrast resolution

• Dynamic

• No radiation

• Safe in pregnancy

• Available, cheap

US - DisadvantagesUS - 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 (CT)

Computed TomographyComputed 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 TomographyComputed Tomography

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

Computed TomographyComputed Tomography

• Four hardware components:

– Radiation source

– Radiation detector system

– Mechanical manipulator

– Computer with display

Contrast-Enhanced CTContrast-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 UnitsHounsfield Units

The GantryThe Gantry

PROPOSALS FOR THE IONISING RADIATION(MEDICAL EXPOSURE) REGULATIONS 1999

REPIACINGThe Ionising Radiation (Protection of Persons Undergoing Medical

Examination or Treatment) Regulations 1988

CONSULTATIVE DOCUMENT

Comments to be sent to:Ms P M Brown

Room 525 Wellington House133-155 Waterloo Road

London SEI 8UGNo later than 31 May 1999

DEPARTMENT OF HEALTH MARCH 1999

TTyyppiiccaall eeffffeeccttiivvee ddoosseess ffrroomm ddiiaaggnnoossttiicc MMeeddiiccaall eexxppoossuurreess iinn tthhee 11999900ss

Diagnostic procedure

Typical Effective Dose (mSv)

Equiv. No. of chest x-rays

Approx. equiv. Period of natural background radiation

X-ray examinations: Limbs and joints (except hip)

<0.01 <0.5 <1.5 days

Chest (single PA film) 0.02 1 3 days Skull 0.07 3.5 11 days Thoracic spine 0.7 35 4 months Lumbar spine 1.3 65 7 months Hip 0.3 15 7 weeks Pelvis 0.7 35 4 months Abdomen 1.0 50 6 months IVU 2.5 125 14 months Barium swallow 1.5 75 8 months Barium meal 3 150 16 months Barium follow through 3 150 16 months Barium encma 7 350 3.2 years CT head 2.3 115 1 year CT chest 8 400 3.6 years CT abdomen or pelvis 10 500 4.5 years

CT - AdvantagesCT - 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 - DisadvantagesCT - Disadvantages

• Radiation dose

• Reactions to contrast material

• Static exam

• Availability

• Cost

Magnetic Resonance Imaging (MRI)Magnetic Resonance Imaging (MRI)

Magnetic Resonance ImagingMagnetic Resonance Imaging

• Concept – 1970• Present – 60 million

examinations per year

Hydrogen NucleiHydrogen Nuclei

The Hydrogen NucleiThe Hydrogen Nuclei

• Have the largest magnetic moment

• Abundant in the body

• 1023 H protons in each 1 cm3 of tissue

• These protons are normally randomly oriented and have no net magnetic moment (magnetization vector)

The Hydrogen NucleiThe Hydrogen Nuclei

Hydrogen Nuclei in NatureHydrogen 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 MagnetThe Magnet

Protons in a Magnetic FieldProtons in a Magnetic Field

Protons in a Magnetic FieldProtons in a Magnetic Field

90 Degree RF Pulse90 Degree RF Pulse

90 Degree RF Pulse90 Degree RF Pulse

Time Constants MeasurementsTime Constants Measurements

• T1T1 – Time to recover longitudinal magnetization – Realigning to B0 – Range 200 – 2000 msec

• T2T2 – Time to dephase loose horizontal magnetization – Range 30 – 500 msec

• Differences in T1 and T2 provide basis for Differences in T1 and T2 provide basis for variations in signal intensities and tissue contrastvariations 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 TimeT1 Relaxation Time

T2 Relaxation TimeT2 Relaxation Time

Relaxation TimeRelaxation 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-DTPAContrast 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 ImagingPlanes of Imaging

• Coronal

• Sagittal

• Axial (Horizontal)

MRI Advantages MRI Advantages

• Excellent contrast resolution (esp. CNS, MSK, mediastinum, pelvis and heart)

• Each tissue / pathology can be evaluated with different sequences

• No radiation

MRI AdvantagesMRI Advantages

• Multiplanar imaging

• Can be performed in pregnancy (not in first trimester)

• No risk with contrast materials

• Can be performed dynamically

MRI DisadvantagesMRI 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 !!!Be Careful !!!