Medical imaging modalities I

Mónica Abella (monica.abella@uc3m.es)

Medical image

➢ “Map” of some physical or chemical

property in the organism, numbers
not a picture!

Introduction to medical imaging modalities 2

Projection vs. tomography
Projection Tomography
A “shadow” of the body in a From Greek, “tomé”: to cut, “grafos”:
particular direction picture
We have depth information

Sagital Coronal Axial

Introduction to medical imaging modalities 3

Medical image
X-ray Nuclear medicine
➢ “Map” are a map of some physical or
chemical property in the organism,
numbers not a picture!

➢ Medical image acquisition involves the irradiation of the

sample with a certain energy
– Energy able to penetrate the human body and
interact with internal body structures
– Interaction depending on the physical or
chemical property of tissues
– Energy coming from the sample will land on a
sensor material responsive to that type of energy
and this generates an electrical signal

Different acquisition techniques: Modalities

Introduction to medical imaging modalities 4
Image Sensing
➢ Incoming energy lands on a sensor material responsive to that
type of energy and this generates an electrical signal
(generally, proportional to the integral of the incoming
energy)
Examples in medical image
• RF coil
• US transducer
• Photodiode
• Photomultiplier tube

Introduction to medical imaging modalities 5

 Examples in digital imaging systems

➢ Where do you think the sampling is happening?

Flat panel (2000x2000

elements)

Digital camera detector

(4000x3000 elements)
PET detector (8x8 elements)

Introduction to medical imaging modalities 2D US transducers 6

 Examples in digital imaging systems

➢ Where do you think the quantization is happening?

PET detector (8x8 elements)

Flat panel
(2000x2000 Introduction to medical imaging modalities
elements) 7
Main imaging modalities

▪ X-ray imaging ▪ Nuclear Medicine

Same type of energy

▪ MRI ▪ Ultrasound

Introduction to medical imaging modalities 8

X-ray and nuclear medicine
Energy in X-ray and Nuclear medicine
➢ Type of energy: electromagnetic radiation Ionazing energy

– X-ray imaging: X-ray photons

– Nuclear Medicine: Gamma photons
➢ High energy photons: ionizing radiation

Introduction to medical imaging modalities 10

Energy in X-ray and Nuclear medicine
➢ High energy photons (> energy in the inner layers of atoms)

– Able to strip electrons

– Ionization: Formation of
an ionic pair
Creating ions from atoms

➢ Many elements in tissue < 1 keV (O: 0.5 keV, H: 13.6 eV)
Threshold

Introduction to medical imaging modalities 11

Source of energy

➢ X-ray:
X-ray X-ray tube
Current

The electrons moving create cinetic energy

Voltage
➢ Vacuum tube
– The filament in the cathode is heated by a current and electrons are
emitted (thermionic emission)
– High voltage between cathode and anode accelerates the electrons
towards the anode where their energy is converted into x-ray
photons
➢ Most part is converted into heat, only 1% as X-ray photons

Introduction to medical imaging modalities 12

 Source of energy in X-ray imaging

• Current
– Number of photons: signal to noise ratio
– More intensity: better image quality but
more ionizing dose
SNR signal to noise ratio
– Typical values: Errors can come from:
At a point the detector Noise is random and
• Clinics: mA
gets saturated until tht produces different errors
point more signal means Artifacts (always the same
more cuality • Small animal: μA mistake) which means there
is a systematic errors

• Voltage: “hardness” of radiation

– More penetration
– Optimum:
Voltage
better contrast
– Typical values
• Clinics: 80-140 kVp
• Small animal: 30-50 kVp

Introduction to medical imaging modalities 13

Source of energy

➢ X-ray: X-ray tube

➢ Nuclear medicine: Radiotracer

Radioactive
isotope

radiotracer

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 Source in Nuclear Medicine
➢ Radiotracer: 1 chose what we want to study

− Labeled compound Radioisotope

− Incorporated to biochemical pathways

− Detectable from outside
− Trace concentration: does not alter
In order not to alter the body negatively
the system
➢ Radioisotope: radioactive isotope
− Radioactive decay: Spontaneous event for which a nucleus is
transformed into another more stable emitting energy
− Activity: disintegrations per second (Bq/Ci)
• Semi-disintegration period: time to decay to 1/2
Gammografy
SPECT (single proton emision tomografy )
α Glucose to anything thats
consuming energy like tumors
PET (positron emision tomografy) Particles
Unstable β
electros (-) and positrons(+)
nucleus
Electromagnetic radiation: γ photon
Introduction to medical imaging modalities 15
Source of energy in Nuclear Medicine
➢ Radiotracer:
− Labeled compound
− Incorporated to biochemical pathways
− Detectable from outside
− Trace concentration: does not alter
the system Radioisotope
radioisotope

Signaling
component

Determines the biological

Affinity behavior (binding)
component
What biological process
we want to study?
Target
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Source of energy in NM
➢ Decay by gamma emission
– Single Photon Emission Tomography (SPECT)
– Isotopes: 99Tc, 201Tl, 67Ga, 111In, 123I
– Generally, heavy metals: Difficult to “mark”
molecules
– Production: nuclear Reactor or generator:
Less expensive and more available Molibdeno 99 (Mb99) (T½ 2,7d)
→ Tc99 (T½ 6h)

➢ Decay by positron emission (β+) : two gamma photons in

opposite directions
– Positron Emission Tomography (PET)
– Isotopes: 18F(~H), 13N, 15O, 11C
– “Organic” atoms: endogenous
compounds (or analogous)
– Most of them produced in a cyclotron:
expensive and complex
https://www.youtube.com/watch?
v=XO1EoTvejs0
Introduction to medical imaging modalities 17
Source of energy

➢ X-ray: X-ray tube

➢ Nuclear medicine: Radiotracer

Isótopo radiactivo

radiotrazador

Introduction to medical imaging modalities 18

Projective systems
X-ray imaging Nuclear medicine: SPECT/PET

X-rays traverse the sample suffering

Tracer emits gamma photons that
different attenuations and are registered
are registered in the detector
in the detector

Map of attenuation
Map of tracer
properties of
concentration
traversed tissues to
X-rays

Projection image
Introductioninto
The body is “collapsed” to medical imaging
an image modalities
(overlaid information) 19
 Collimation in MN scintigrphy

Hardware
➢ Decay by gamma emission: one photon collimation

– Single Photon Emission Tomography

(SPECT) Detector

➢ Decay by positron emission (β+) : two Electronic collimation

gamma photons in opposite directions
– Positron Emission Tomography
(PET) Detector 1
Detector 2

Direction determined by the detectors

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Collimators in SPECT

Parallel collimator Pinhole collimator Good for small parts

Bigger field of view More spacial resolution

In both cases I know the direction from which the photon came

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Interactions with the tissue

Emitted photons
Measured photons =
Absorption in tissue

➢ Photoelectric effect: Ecin=E0-EBE

– The photon is completely absorbed releasing
an electron (ionization)

➢ Compton scatter:
– The photon looses part of its energy and
changes trajectory

Introduction to medical imaging modalities 22

Origin of the contrast in the image
➢ X-ray imaging: Map of attenuation to X-rays of traversed
tissues
Image without
sample

Emitted photons
Measuredphotons
Measured photons==
Absorption in
Absorption in tissue
tissue

➢ Little difference of attenuation

properties t X-rays between
different soft tissues
– Contrast agents: High attenuation
to X-rays

Introduction to medical imaging modalities 23

Contrast agents in X-ray

➢ X-ray imaging has poor contrast in soft tissue

– Contrast agents: High attenuation to X-rays
➢ Common contrast agents:
– Iodine (blood vessels)
– Barium (digestive tube)
➢ Administration
– Intravenous injection (blood vessels)
– Oral administration (abdomen and pelvis)
– Rectal administration (large intestines and other organs in the pelvis)

Is it a tracer?

Introduction to medical imaging modalities 24

Origin of the contrast in the image
➢ X-ray imaging: Map of attenuation to X-rays of traversed
tissues
Image without
sample

Emitted photons
Measured photons =
Absorption in tissue

➢ Nuclear medicine: Map of tracer concentration

Emitted photons
Emitted photons
Measured photons =
Absorption in tissue

Introduction to medical imaging modalities 25

 Detector
➢ Incoming energy lands on a sensor material responsive to that
type of energy and this generates an electrical signal (generally,
proportional to the integral of the incoming energy)
gamma/X-ray photons ➢ MN: PET/SPECT

Scintillator: converts high energy

photons into light photons

Photodetector: converts light

photons into electric signal ➢ X-ray imaging

Measurement
proportional to incoming
energy

Introduction to medical imaging modalities 26

 Projective systems
X-ray imaging Nuclear medicine: SPECT/PET

X-rays traverse the sample suffering

Tracer emits gamma photons that
different attenuations and are registered
are registered in the detector
in the detector
Measure the
photons detected
to know how many
photons were
retained by the
patient in high
density area Map of attenuation
Map of tracer
properties of
concentration
traversed tissues to
X-rays

Projection image
Introduction
The body is “collapsed” to a
into medical imaging
projection modalities
(overlaid information) 27
Computed tomography (CT)
➢ Godfrey Hounsfield, 1970: Godfrey Hounsfield,
1970: what if I take several radiographies from
different views and combine the information? Map of attenuation to
X-ray

Set of projections

Image
reconstruction
Acquisition

Introduction to medical imaging modalities 28

CT
➢ Clinical ➢ Small animal

Fuente
Detector

Introduction to medical imaging modalities 29

SPECT: Single Photon Emission Tomography
More detectors increase the signal
you recieve
➢ Clinical ➢ Small animal
MILabs

UC3M

Jefferson Lab

Introduction to medical imaging modalities 30

PET: Positron Emission Tomography
➢ Clinical ➢ Small animal

Introduction to medical imaging modalities 31

Medical imaging modalities II
Mónica Abella (monica.abella@uc3m.es)
MRI
Energy in MRI
➢ Type of energy: electromagnetic radiation
– MRI: radiowaves
➢ Low energy: non ionizing

Introduction to medical imaging modalities 34

Spin within a magnetic field

➢ Spin: intrinsic form of angular momentum carried by

+
elementary particles
➢ Many subatomic particles have spin
– Electrons, protons, neutrons
– Almost every element has an isotope with overall nuclear spin ≠ 0
➢ Inside a magnetic field B
– New energy levels (new possible transitions)
– Possibility of emission/absorption of low energy photons

Giromagnetic constant

DE = h g B

Introduction to medical imaging modalities 35

Origin of the contrast in MRI
➢ Hydrogen (water) content results in contrast
between tissues
➢ Many other mechanisms, some based on
relaxation

Introduction to medical imaging modalities 36

The magnet

➢ High magnetic field (0.5-7 Tesla)

➢ Electromagnet made of superconducting cable (resistance close to
0 at 0 kelvin (-273.15o C)
– Resistance ~ 0 at 0 Kelvin (-273.15o C)
– Immersed in liquid helium
• https://www.youtube.com/watch?v=6BBx8BwLhqg
• https://www.youtube.com/watch?v=9SOUJP5dFEg

Introduction to medical imaging modalities 37

 X
Nuclear Magnetic Resonance

RF Antena that emits waves

f= g B0 E = hf=hgB0

B0 f= g B0
RF
Magnetic field

eco

Introduction to medical imaging modalities 38

RF antennas and gradient coils

Antenas to recieve the signals

Introduction to medical imaging modalities 39

 Doing imaging…

RF f= g Bi E = hf=hgB0

Bi=B0+ΔB

f= g Bi
Field gradients K space

RF
Reconstruction
FT echo

Introduction to medical imaging modalities 40

Gradient coils

Introduction to medical imaging modalities 41

Components of MRI scanners

➢ The magnet
➢ The RF subsystem
➢ The gradient coils
➢ Electronics
➢ The reconstruction

Introduction to medical imaging modalities 42

Ultrasound
Energy in ultrasound imaging
➢ Type of energy used: Ultrasound waves
– Mechanical energy
– Ultrasonic waves
• Longitudinal mechanical waves
• Wave motion: elasticity and inertia of the medium

Ultrasound > 20 KHz

Audible sound Surgical: 25-55 KHz
Infrasound < 20 Hz
20 Hz - 20 KHz Diagnostic: 1-10 MHz

➢ Non-ionizing radiation, inexpensive, portable, excellent

temporal resolution

Introduction to medical imaging modalities 44

Interaction with tissue in US
➢ A propagating wave is partially reflected at the interface
between different tissues
– Position of the interface can be extracted from the time delay
– The intensity of the echo is used to determine the brightness of the
image at the reflecting tissue surface
Tissue interface smooth compared to λ and
Z1 = ρ1×c1 Z2 = ρ2×c2 perpendicular to the direction of wave propagation

Reflection
Incident Transmitted Transmitted coefficient

2
𝑍1 − 𝑍2
Reflected 𝐼𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 = 𝐼𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 2
Reflected 𝑍1 + 𝑍2

• At soft tissue/bone interface: reflected signal ~40% of the incident

intensity
– imaging of structures deeper-lying than bone is extremely difficult
• At a soft tissue/gas interface: ~99% of the beam intensity is reflected
– impossible to scan distal structures deeper than lungs or gas-containing bowel
Introduction to medical imaging modalities 45
Mode B (Brightness)
➢ An image is obtained by sending pulses in
different directions
➢ Brightness represents the amplitude of the
eco from each point

Introduction to medical imaging modalities 46

Mode M (Motion)
• Repeated A-mode measurement in time
• Brightness represents the amplitude of the
eco from each point
• High sampling frequency: up to 1000 pulses per
second
– Useful in assessing motion
– Still used extensively in cardiac and fetal cardiac
imaging

Introduction to medical imaging modalities 47

 Source & detector in US
➢ Source-detector: transducer (converts one type
of energy into another)
– Piezoelectric crystal sandwiched between a pair of electrodes
(Pierre Curie,1880)
• Electricity into sound = pulse
• Sound into electricity = echo

Introduction to medical imaging modalities 48

Scanners

Introduction to medical imaging modalities 49

Intraluminal ultrasound

IVUS

Introduction to medical imaging modalities 50

Question

➢ If I can see anatomy with US and CT, what would you consider
to decide which one to use for a patient?
– Differentiate between cyst, fibrosis, or carcinoma

CT

US

Introduction to medical imaging modalities 51

Remember: Contrast agents in X-rays

➢ X-ray imaging has poor contrast in soft tissue

– Contrast agents: High attenuation to X-rays
➢ Common contrast agents: DSA digital subtraction

– Iodine (blood vessels)

– Barium (digestive tube)
➢ Administration
– Intravenous injection (blood vessels)
– Oral administration (abdomen and pelvis)
– Rectal administration (large intestines and other organs in the pelvis)

Is it a tracer?

Introduction to medical imaging modalities 52

Contrast agents in US: Microbubbles
➢ Air-filled microbubbles administered intravenously
➢ High degree of echogenicity: ability of an object to reflect the
ultrasound waves
➢ Blood perfusion in organs, blood flow rate in heart, …

Is it a tracer?

Introduction to medical imaging modalities 53

Clinical applications
Planar X-Ray Applications

➢ Bone radiography:
– Fractures, joint luxation, spine scoliosis
– Lesions, infections, arthritis, abnormal bone growth
(exotosis)
– Bone cancer
➢ Dental

Introduction to medical imaging modalities 55

Planar X-Ray Applications

➢ Thorax
– Lung: pneumonia, lung edema, lung cancer
– Heart diseases (cardiomegaly)

Introduction to medical imaging modalities 56

Planar X-Ray Applications

➢ Abdomen
– Free air (perforation) and ascites (free fluid in the peritoneal cavity)
– Intestinal obstruction
– Renal calculus (kidney stone)
– Gallstone

Introduction to medical imaging modalities 57

Planar X-Ray Applications

Introduction to medical imaging modalities 58

Scintigraphy uses
➢ Bone scintigraphy
– Small functional alterations before they are
visible in a radiography
– Bone metastasis
➢ Lung scintigraphy
– Artery obstruction or thrombus
– Pulmonary embolism (ventilation/perfusion
scan)
➢ Thyroid scintigraphy
– Size increase (tryroditis)
– Nodules
➢ Radioisotope renography/ Renal scintigraphy

Introduction to medical imaging modalities 59

Clinical Uses of CT

➢ Not much for bone

➢ For soft tissue: contrast agents
➢ Fine detail in lung
– Difficult diagnosis: dynamic image (vascularization)
– Emphysema quantification: under research (textures)
– For screening?
➢ Skull (trauma, stroke)
➢ Neck (tumors)
➢ Abdomen: kidney, liver, etc.
➢ Heart: dynamic cardiac CT

Introduction to medical imaging modalities 60

Uses of SPECT

• Brain SPECT
– Perfusion (blood flow): brain functioning
– Alzheimer, dementia, epilepsy
• Cardiac SPECT: Myocardium perfusion
– Rest: death muscle (myocardium infarction)
– Stress (physical or pharma stimulus): it allows detection muscle areas with low
blood flow (coronary ischemia)
– Selection of patients for catheterism

Introduction to medical imaging modalities 61

Uses of PET

• Oncology
– Metastasis and strength of tumors
– Treatment response (surgery, radiotherapy, Chemotherapy)
• CNS
– Brain function
• Cardiology
– Coronary illness
– Evaluation of coronary flow and reserve
– Tejido viable / no viable
– FDG y NH3: Metabolism and flux
• Better image quality than SPECT

Introduction to medical imaging modalities 62

Uses of MRI
➢ Contrast agents:
– Gadolinium (paramagnetic)
➢ Neuroimaging
– Clinics
– Research
➢ Soft tissue damage (muscle)
➢ Cardiology
➢ Angiography

Introduction to medical imaging modalities 63

Uses of US
➢ Head: newborn, retina
➢ Neck: thyroid, salivary glands, lymph nodes
➢ Urogenital tract: kidney, bladder, prostate, testicles, uterus, vagina, and ovaries

Introduction to medical imaging modalities 64

Uses of US

• Musculoskeletal system
• Abdomen: spleen, pancreas, liver
• Thorax: pleura, diaphragm

Introduction to medical imaging modalities 65

Uses of US
• Cardiology: Echocardiography
• Other fields: anesthesiology (near nerves),
vascular studies, breast, nephrology,
endocrinology
• Fetus

Introduction to medical imaging modalities 66

Uses of US

◼ Cardiology

◼ Obstetrics

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