Lecture 1 - Imaging Modalities Slides

Introduction to RadiologicalIntroduction to Radiological Modalities

Department of Radiology,p gy,Vancouver General Hospital

Dr. Savvas NicolaouDirector of ER/Trauma Radiology

[email protected]

Objectives

• Understand the basic principles of the following modalities:• Ultrasound• Radiography• Computed Tomography• Magnetic Resonance• Positron Emission Tomography• Nuclear Medicine• Nuclear Medicine

• Understand the relative radiation risk of the different imaging modalities

• Be able to recognize the five major densities found on an x-raye ab e to ecog e t e e ajo de s t es ou d o a ay

Objectives

• Be able to define the following terms:

• Attenuation, radiodense, radiolucent, Hounsfield Unit

B bl t d fi th f ll i i l i fil• Be able to define the following views on x-ray plain film

• X-ray

- LateralLateral- Anteroposterior (AP)- Posteroanterior (PA)

• Be able to recognize and define the anatomical planes on cross sectional

imaging:

• Sagittal• Sagittal

• Coronal

• Transverse/AxialTransverse/Axial

What is Radiology?

What is Radiology?

• Area of clinical medicine involving anatomical imaging to help identify

pathologiespathologies

• Requires in-depth knowledge of anatomy, physiology and pathology

• Medical imaging began with the discovery of X-rays but continues to include

additional modalities and improve on old ones as technology develops

Why is understanding radiographic imaging and anatomy important to me?

A f t h i i t f ill h t d d i t t•As future physicians, most of you will have to order and interpret

radiographic images

•Imaging is how you will be viewing anatomy in the future it isImaging is how you will be viewing anatomy in the future, it is

important you can correlate what you see on an image with basic

anatomical structures

• As future physicians most of you will have to order and interpret

Why is understanding radiographic imaging and anatomy important to me?

• As future physicians, most of you will have to order and interpret

radiographic images

• Most of you will be ordering images on real patients and need to understand

the benefits and risks of what is being ordered

25 yr old female VS 70 yr old male

with persistent diarrheawith persistent diarrhea……

Choice of imaging may be different!

Radiology in the curriculum

Provided throughout your four years of medical school

PRIN 10 k h f d t di d liti d l ti•PRIN – 10 workshops- focus on understanding modalities and correlating

anatomy with radiographic images

•FMED (next semester and next year)– Clinical radiology emphasizing theFMED (next semester and next year) Clinical radiology emphasizing the

development of a system to interpret radiographs and specific pathologies

•3rd year – survival skills for clerkship, opportunities for electives, and relevant

imaging for emergency medicine

•4th year – Refresher course for LMCC

Overview of imaging modalities

• X-ray systems

• Radiography (plain films, fluoroscopy, angiography)

C t d T h (CT)• Computed Tomography (CT)

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

• Ultrasound• Ultrasound

• Nuclear Medicine (NM) / Positron Emission Tomography (PET)Nuclear Medicine (NM) / Positron Emission Tomography (PET)

Similar Theme of imaging (for all modalities)

Signal (x-radiation or

Tissue changes signal (absorbs

Changed signal provides radiation or

sound) is projected at or through tissue

signal (absorbs, scatters, reflects, selectively takes up source etc)

information which is used to

construct an imageimage

K th th i i d d th d litiKeep these themes in mind and the modalities will be easier to understand!

Radiography

X-rays are high energy electromagnetic radiation

capable of penetratingcapable of penetrating human tissues

Discovered in the 17th

century, their ability to help y y pvisualize anatomical

structures gave birth to diagnostic radiology in 1895 Early Xray Machine 1899

Radiography

Most common examination in

radiologygy

X-rays are passed through

patient

X-rays detected on other side by

film or digital detectordetector

Images often referred to asreferred to as

‘x-rays’ or ‘plain film’ X-ray Unit

Radiography

As x-rays pass Tissues with a Tissues with a lowery pthrough the body,

they are attenuated (absorbed and scattered) by

Tissues with a higher density will attenuate more x-rays and appear

lighter (radiodense)

Tissues with a lower density will

attenuate less x-rays and appear

darker (radiolucent)) yinteraction with body tissues

lighter (radiodense) on a plain film

darker (radiolucent) on a plain film

Radiography

5 major densities are found on radiographs, and are demonstrated on this imageg

Air - fat - water - bone - metal Soft tissue

Lower Higher fat

Note that THICK structures attenuate more radiation than THIN structures of the same composition

bonecomposition

air

metal

Radiography

Liver

L2 spinous

A t i t tIliopsoas

L2 spinousprocess

Anatomic structures are visible when they are outlined in whole

muscle

B lor part by tissues of different x-ray density

Bowel gas

Naming Radiographic Views

Most views are named by how x-rays pass through the patient

Posteroanterior Anteroposterior

Naming Radiographic Views

Most views are named by how x-rays pass through the patientMost views are named by how x-rays pass through the patient

Lateral Oblique

Posteroanterior (PA) view

• Regularly used for chest x-rays

• Decreases cardiac shadow

compared to anteroposterior (AP)

view

• Reason for use in chest x-raysReason for use in chest x rays

Posteroanterior (PA) vs Anteroposterior (AP)

APPA AP

Anteroposterior (AP) view

• Regularly used for non-chest x-Regularly used for non chest xrays along midline

Lateral view

• The name is self-explanatory, a view from th l t l idthe lateral side

Oblique View

Not parallel to either AP or lateral views

(may appear lateral but try comparing to previous slide)

Importance of using different views

Fluoroscopy

Continuous low dose radiography used to monitor part of the body in real-time

Often used in conjunction with contrast, can be helpful to evaluate motion such as gastrointestinal peristalsisgastrointestinal peristalsis

Used less commonly inurban settingsurban settings

NB: Contrast is an exogenous agent used to alter the attenuation of aalter the attenuation of a structure

Barium Swallow

Contrast Agents

Intracavitary contrast eg barium f th GI t t (t k ll GI is normally full of air

or fecal matter, but with barium appears radiodense

for the GI tract (taken orally or introduced per rectum)

radiodense

I t t t i liFilling defect

b l i LIntravenous agents to visualize arterial system (angiography)

-embolus in L pulmonary artery

Catheter

Normally cannot visualize blood vessels without contrast

Computed Tomography (CT)

Computed Tomography

• Similar to radiography in that it uses ionizing radiation from a source to

produce an image

• Cross-sectional image based on the same principles of attenuation asCross sectional image based on the same principles of attenuation as

an x-ray

• 3D information based on tissue density


X ray SourceSource and detector rotates 360º around• Source and detector rotates 360º around

patient (modern scanners less than 0.5s)

• Less time averaging

Detector• Computer uses number of profiles to construct

3D information in 2D slices

Object

CT (and MRI) views

Because the data is multi-planarBecause the data is multi planar, images can be reconstructed in different planes

• Coronal view• Sagittal view• Transverse/Axial view

CT/MRI Axial views

Imagine feet are coming out of the screen

Anterior

Posterior

CT Views

CT A i lCT Axial

CTCT Coronal

CT Sagittal


A Hounsfield unit is

assigned for every volume

element (voxel) •This unit is relative to the

attenuation of water

•The Hounsfield unit can help

identify certain tissues - Eg Air -

1000; fat -100; water 0; blood 30

b 500 HU; bone 500+ HU

NB the upper and lower values may change slightly depending on the scanner, but the

l l l ti t t (0)values are always relative to water (0)

Hounsfield Unit (HU)

ColonGall bladder-1000 HU6 HU

Liver

50 HUFat -120

50 HU

Air -1000; fat -100; water 0; blood 30 ; bone 500+ HU

CT windows

• We can differentiate only ~16 shades of grey

• CT data (Hounsfield units) cover a much larger range of values (-CT data (Hounsfield units) cover a much larger range of values (

1000 to +500)

• To interpret images, we limit the number of Hounsfield units

displayed

• The computer is able to convert the ranges used to shades of grey

we can see

CT windows

Window width:

•The range of HU of all tissues of interest

Ti i thi ill b di l d i i h d f•Tissues in this range will be displayed in various shades of grey

•Tissues with HU outside the range are displayed as black or white

Window level:

•The central HU of all the numbers in the window width•The central HU of all the numbers in the window width

CT Windows

Wide Wide Narrow Narrow Hounsfield UnitsHounsfield Units

+400+400

300300

WindowWindow WindowWindow

+300+300

+200+200

+100+100Window +100+100

00

--100100

Window Level

100100

--200200

--300300

--400400

CT windows

Same patient – use of windowing allows inspection of areas with completely different density

Bone Window W:2500 L:480 – Frontal bone skull fracture

Soft Tissue Window W:80 L:40 – Epidural and parenchymal hemorrhage

Dynamic imaging, can follow anatomy

Transverse/Axial Coronal

CT Strengths and weaknesses

Strengths Weaknesses

• Excellent bone detail

• Good soft tissue

contrast

• Availability

• Cost (though less

expensive than MRI)contrast

• CT angiography (use of

contrast in blood vessels)

expensive than MRI)

• Radiation

• Weight limitation 160 kg

• 3D image rendering (350 lb)

• Contrast complications

(allergies, nephrotoxicity)

• Artefacts with metal

Magnetic Resonance Imaging (MRI)

Physics is complicated..………….

Simplified:

• Protons in a strong magnetic field

are bombarded with low energy

(non ionizing) radiowaves

• In different tissues

(environments) they absorb and ( ) y

release the energy at different,

detectable, and characteristic Looks like a CT but much different

rates


The released energy can be detected over time and images can be `weighted` to

enhance different characteristics of the soft tissue

• T1 weighted – usually best anatomical detail, gadolinium can

be used as contrast

• T2 weighted – usually most sensitive for pathological lesions g y p g

(Water is White in T2 - World War 2)

MRI Images

Which plane is this? p

Sagittal

T1 weighted T2 Weighted

MRI vs CT for Soft tissue

Which plane is this?

Axial

CTCT T2 Weighted MRIT2 Weighted MRI

showing same mass in R axilla

CTCT T2 Weighted MRIT2 Weighted MRI


Strengths Weaknesses

• excellent soft tissue

contrast

• availability/cost (very

expensive)

• no radiation

• MR angiography

• gadolinium IV contrast is not

• procedure time (immobile,

elderly, sick patients, children)

• Movement can severely• gadolinium IV contrast is not

nephrotoxic and may be used

in setting of renal failure

• Movement can severely

affect image quality

• poor bone detail

• metal and other artifacts

• risk of complications related to

magnetic metal implants

(pacemakers, aneurysm clips

etc )etc,)

• rare serious contrast reactions

Ultrasound (US)

• High frequency sound waves• High frequency sound waves

• Sound reflected by body

structures is converted to grey g y

image

• Doppler ultrasound uses the

principal that flow changes the

sound frequency, can detect

bl d flblood flow

• First line imaging for heart

Ultrasound (US)

• Ultrasound imaging relies on sound echoes from tissue interfaces in the body

• Strength of reflections depends upon the difference in acoustic properties of• Strength of reflections depends upon the difference in acoustic properties of the interface tissues

• Bone and air reflect virtually all the sound so US cannot be used near

ultrasound

bone or the lungs very well.

ultrasoundtransducer

sound pulsesound pulse reflected pulses

Ultrasound Imaging

• Determination of cystic (fluid filled) vs. solid structures - cysts do not

reflect sound and are anechoic

• Evaluation of

• bile ducts

• gall bladdergall bladder

• Renal/ovarian/breast cysts

• hydronephrosis Etc.

• Evaluation of stones

• gall stones

l l li• renal calculi

• Evaluation of abdominal and pelvic organs

Ultrasound Terminology

T i l D i tiTerminology Description

Anechoic • black with no echoes or sound wave reflection • simple cysts, vessels, ascites

Hypo-echoic • dark on the film with some low grade echoes • Organs, nodes, tumors, complex fluid

Isoechoic • the same echogenicity as another structure

Echogenic • bright on the film with significant sound wave reflection • Fat, air, bone, complex fluid

Acoustic shadowing • reflection of sound waves with distal shadowing • Stones, bone, gas

Ultrasound (US)

1 2Anechoic

3Echogenic

AcousticAcoustic Shadow

Hypo-echoic

Longitudinal cross section of the gall bladder. Fluid in bladder shows no reflections (1) whereas calcified stones (2) show strong reflections with decrease of signal beyond (3)

Ultrasound Imaging

Advantages • Disadvantages

• No radiation

• Relatively cheap

• Difficult in obese patients

• View often obscured by air –Relatively cheap

• Modality of choice to

examine the heart

View often obscured by air

lung, bowel, bone

• Operator dependent

• Real-time

• Can be performed at bedside

Most sensitive for fluid• Most sensitive for fluid

Nuclear Medicine

•Branch of radiology that utilizes intravenous radio-pharmaceuticals for imaging

•Radio-pharmaceuticals which may be deposited in certain tissues emit gamma rays

•The rays are detected by a gamma camera

Nuclear Medicine

•Assessment of function

•Most common radioisotope

t h ti 99 b t d ithtechnetium-99m can be tagged with

substances that are selectively taken

up by different organsp y g

•Examples:

•bone scan

•ventilation-perfusion

scanning

•cardiovascular scanning•cardiovascular scanning

•thyroid scanning

Nuclear Medicine - Bone Scan

Multiple metastatic bone lesionsMultiple metastatic bone lesions

show higher retention of radio-

labelled bone seeking agentg g

Hot spot

Nuclear Medicine – Ventilation-perfusion scan

Ventilation-perfusion mismatch – will learn about next semester in pulmonary block – detectable by nuclear medicine

Nuclear Medicine – Positron Emission Topography (PET)

• Positron Emission Tomography g p yPET uses fluorine-18 which is a positron emitter. Positron emission produce photons that can be imaged. CT

• Fluorine-18 can be incorporated into biologically active molecules such as fluoro-18-deoxyglucose –used for cancer staging PET/CTused for cancer staging PET/CT

PET

Nuclear Medicine / PET

Advantages • DisadvantagesAdvantages Disadvantages

• Excellent specificity • ExpensiveExcellent specificity

• Provides physiologic

information

Expensive

• Availability of service (rarely

available)

• Radiation

• Poor spatial resolution

Risks associated with radiation exposure

• It has long been known that radiation can cause a number of

effects through genetic changes:

• Cancer

• Skin damage

• Fetal abnormalities

• At the doses of radiation used in diagnostic Imaging the risk of

these effects are normally very low

• Nevertheless, it is prudent to minimize ANY risk by minimizing

the number of procedures

Radiation Exposure

• The organ most radio-sensitive is the thyroid gland

• Tumors likely to occur after exposure to ionizing radiation include leukemiaTumors likely to occur after exposure to ionizing radiation include leukemia,

thyroid, breast, lung and skin

• A rad is a measure of absorbed dose

R di ti i t d ith h d CT i t th i t d ith h t• Radiation associated with head CT is greater than associated with a chest x-

rays (4000 vs. 12 millirads)

Age and Mortality Risk

• Children are more radiosensitive than adults

• Females are more radiosensitive than males

• Older adults least sensitive ( average age at VGH 55y )

v)ris

k (%

/S 30

25female

mor

talit

y

15

20female

1996 Re-analysis Hiroshima data

Life

time 10

5 male

10 706040 5030200

0Age at time of exposure

Scenario from earlier

25 yr old female VS 70 yr old male

with persistent diarrhea

Point from earlier- More consideration of dose reduction has to be made when imaging a female in reproductive age compared to an older male (especially the abdomen)( p y )

Radiation Doses for common tests

Procedure Equivalent b f

Equivalent P i dnumber of

Chest x-rays

PeriodNatural

Background (~3mSv)

Very Low Dose Knee 0.25 <1 day Bone density Scan 0.5 <1 dayShoulder 0.5 <1 day Low Dose Chest PA 1 2 days Skull series 5 12 days Mammography 20 7 weeks

Xrayg p y

Pelvis AP 30 10 weeks Abdomen AP 35 3 months Thoracic Spine AP 50 4 months Intermediate Dose Lumbar Spine AP 75 6 months CT Head 100 8 months CTCT Head 100 8 monthsIVP 150 1 year Small bowel series 250 1.67 years Higher doses Upper GI Series 300 2 years Spine CT 300 2 years Pelvis CT 300 2 years

Fluoroscopy (real-time Xray)

CTPelvis CT 300 2 yearsAbdomen CT 400 2.7 years Barium enema 400 2.7 years Chest CT (PE) 750 5 years Coronary angiography 800 5.3 years

CTFluoroscopy

CT

For more information on doses see Radiology 2008; 248(1):254-63.

Methods to reduce radiation exposure

• Reduction in unnecessary examinations (e.g. daily ICU films)

• Dose reduction (CT)

• Exposure time reduction (fluoroscopy)

• Use of US and MRI (non ionizing)

Imaging in pregnancy

• No proven risk to fetus of ultrasound

• No proven risk to fetus of MRI, but avoid in first trimester if possible

I t f f i i ti if di ll• Importance of performing examinations if medically necessary

• Importance of re-evaluating "set protocols" e.g. trauma protocols in a

pregnant patientpregnant patient

• Dose reduction

• ShieldingS e d g

• Note shielding unhelpful in nuclear medicine - bladder emptying and

hydration most helpful

• Studies that should be performed only if absolutely necessary with shielding

if possible

Relative Costs of Procedures

Xrays, fluoroscopy Study Relative Cost

CXR 1 y , pyand US are relatively cheap

Abdominal series 2

Barium enema 3

Abdominal US 4

Most nuclear medicine scans are moderately costly

UGI series with SBFT 4

IVP 4

Bone scan 5

Lung VQ scan 6

CT is more costly

moderately costly

MRI is fairly

Lung VQ scan 6

CT abdomen 9

CT chest 10

MRI abdomen 12

N t bl

yexpensive

PET scans are very expensive

MRI of lumbar spine 12

Colonoscopy 16

PET scan 16Not reasonable to order excess tests

very expensive Chest + abdomen + pelvic CT 17

Lecture 1 - Imaging Modalities Slides

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