Cardiovascular imaging

Question 1

What is used for anatomical imaging?

Answer 1

X-rays

CT scans

Question 2

X-Ray

Answer 2

Chest X-ray is the most common radiological examination:

The image is produced when radiation passes through the body to expose sensitive film on the other side.
The ability of radiation to penetrate structures depends on their energy and density of the tissues and bones
Bones are dense and will absorb most of the radation, whilst air-filled lungs will let radiation pass thorugh

Question 3

What is seen in X-Rays

Answer 3

In terms of the heart we can see:
Size of heart – important clinical feature
Cardiac Thoracic Ratio ; CTR should be no more than 1:2 (heart should be no more than half of the whole chest diameter)
Size and outline of aorta
Evidence of stents, clips, wires (cannot see valves directly)

Question 4

How is an X-ray carried out?

Answer 4

This is achieved using posterior to anterior projection such that the anterior part of the chest has to be closest to the film
Generally little information attained from X-rays hence next step is cross-sectional imaging

Question 5

Computer Tomography (CT) Scan

Answer 5

The cross-sectional anapmty gives a much better evaluation of the structures and therefore often carried out if there are concerns seen in chest X-ray

1.General:
Looking at the main strucutes of the heart

Assess size

Thickness of myocardium

2. Calcium scoring:
   Check for calcification within heart of coronary arteries
3. CTCA (CT coronary angiography):
   Check for aneurysm sof heart or aorta itself

Question 6

What is calcium scoring in CT?

Answer 6

Calcium scoring/ CT Coronary angiography:

More precise evaluation of cardiac heart disease e.g coronary artery stenosis
Allows for evaluation of size/ calibre of the coronary arteries

Question 7

How do general CT scanners work?

Answer 7

CT scans are also produced with X-rays, however the source of the X-ray is rotated around the patient such that x-rays pass through the body and are detected by an array of sensors on the other side. Information from the sensors is computer processed and then displayed as an image on a video screen showing organs of interest at that one selected slice of the body – each scan is a single slice

Question 8

Problems with first generation of CT scanners

Answer 8

1st generation of scanners lacked the temporal and spatial resolution to accurately image the coronary arteries – most severe cardiac problems arise from coronary artery disease; Invasive coronary angiography was the only way to directly visualise the coronary vessels

Question 9

What assessed the calcification within the coronary vessels (shows up as white in CT scan) and how?

Answer 9

CALCIUM SCORING:
Calcification of coronary arteries is a sign of atherosclerotic disease
Amount and site of calcification can be quantified with CT using visual interpretation and a computer algorithm – given a calcium score

Calcium Score & presence CAD:

0: No evidence CAD
1-10: Minimal evidence CAD
11-100: Mild evidence CAD
101-400: Moderate evidence CAD
>400: Sever evidence CAD

Question 10

Advantages of calcium scoring:

Answer 10

Quick (10-20 seconds acquisition)
Non-invasive (no contrast/drugs required for patient); no side-effects
Low dose <2mSv (equivalent to 10 chest x-rays)
No preparation required
Useful prognostic indicator in low risk group – 70% of patients would have no calcium therefore reassured such that coronary heart disease can be excluded

Question 11

CTCA (CT)

Answer 11

CTCA:
Development of 64 slice with multi-row detector CT scanners allowed high resolution and faster imaging for accurate coronary artery visualisation

Dose 7-10mSv so a bit higher
Generally Non-invasive – only i.v. iodinated contrast (we are looking at lumen of vessel rather than calcium, hence kidneys need to work in order to excrete contrast) - patient is monitored only for a short period before discharge
This new technology allows for 3D Anatomical and multiplanar Reconstruction of coronary arteries

Question 12

Advantages of CTCA:

Answer 12

More accurate than any other non-invasive imaging modality

Studies show excellent diagnostic accuracy in detection of significant stenoses: Sensitivity 94% Specificity 97%
Negative predicative value 95%

-if it is reported as “normal” the patient will not have any significant CAD!

Question 13

Limitations of CTCA:

Answer 13

Slow Heart Rate required (<80/min) to prevent blurry images; medication can be given to slow heart rate

Difficulty assessing narrowing with severe calcification; difficult to decipher complete occlusion with narrow passage

No functional information because these are generally single slices

Cannot Intervene if there is a stenosis, you have only gained the information but you do not have any arterial access at the time

Question 14

How are closed vessels imaged?

Answer 14

The imaging of the vessel is obtained by Digital Subtraction Angiography DSA

Coronary angiogram is invasive, however it allows for direct intervention angioplasty stenting by placing a balloon or stent into the closed vessel:

Direct Arterial Access with advance of Catheter from Brachial arteries or femoral artery; catheter is passed all the way to origin of coronary vessel and contrast is injected

Question 15

How doe digital subtraction angiography (DSA) work?

Answer 15

Iodinated contrast used

Overlying structures make it difficult to see vessels directly; a mask is taken before contrast is administered which is a negative template showing all bones and stuff

A second image is taken and and computer subtracts the template from the original leaving only the vessels of interest such that we can get a good evaluation of the vessel

Question 16

What is used for functional imaging?

Answer 16

Ultrasound
MRI
Isotope Scan

Question 17

What is ultrasound?

Answer 17

(Echocardiogram)
Uses sound waves to create an image
Sound wave is omitted and is reflected back to detector; depending on time it takes for wave to reach the target and back defines the image produced

Beam can be put through area of interest (e.g mitral valve) and remain constant; The “M” mode using a stationary transducer but a moving recording scanning the ultrasound pattern across the screen so it is possible to study the dynamics of moving structures such as the ventricular wall or the valve leaflets.
The combination of Doppler and “M” mode demonstrates the Haemodynamics of Blood flowing within the Heart – pulsatility index flowrate

Question 18

What does US show?

Answer 18

Shows:
Movement of the valves
Movement of walls of heart
Direction of blood flow; the Doppler effect for moving blood source because when sound source is moving towards you the waves are compressed such that there is a higher frequency, pitch is lower when red blood cells are moving away from you

Question 19

How is an US conducted?

Answer 19

Beam can be put through area of interest (e.g mitral valve) and remain constant; The “M” mode using a stationary transducer but a moving recording scanning the ultrasound pattern across the screen so it is possible to study the dynamics of moving structures such as the ventricular wall or the valve leaflets.

The combination of Doppler and “M” mode demonstrates the Haemodynamics of Blood flowing within the Heart – pulsatility index flowrate

Question 20

How is a cardiac MRI conducted?

Answer 20

MRI uses a large magnet that surrounds the patient by a magnetic field up to 8,000 times stronger than that of the earth.

MRI produces images by decoding the frequency signals emitted from the body’s hydrogen atoms
The scanner subjects the nuclei of the body’s hydrogen atoms to a radio signal, temporarily knocking select ones out of alignment.
When the signal stops, the nuclei return to the aligned position, releasing their own faint radio frequencies from which the scanner and computer produce detailed images of the human anatomy.

Question 21

Cardiac MRI vs CT

Answer 21

Similar images to CT scan however advantages as:
No radiation /ionising xrays involved
Therefore we can do functional studies

Like CT, MRI produces images, which are the visual equivalent of a slice of anatomy. MRI, however, is also capable of producing those images in an infinite number of projections through the body (any plane that you wish rather than just axial like in CT)

No radiation involved so can keep scanning same area; functional studies are obtained by stacking images acquired from separate time points of the cardiac cycle in a cine-loop.

Question 22

Cardiac Isotope Scan conduction:

Answer 22

Radioactive substances are injected into the body; these are then emitted and detected by a camera which assigns the direction from which these gamma rays have been emitted
Gamma Camera detects the source of the radiation to build a picture

Question 23

When is technetium isotope scan (MI scan) conducted?

Answer 23

done 12 hours after a suspected heart attack.
The scanning is performed 3 hours after the isotope is injected and the image acquisition takes about one hour.
The technetium accumulates in heart tissue that has been damaged, leaving “hot spots” that can be detected by the scintillation camera.

Question 24

Nuclear heart scan- why does it use technetium?

Answer 24

The Nuclear heart scan uses technetium Tc-99m as it has a short physical half life of 6 hours, thus the amount of radioactive exposure is limited and provides better image quality than previously used radioactive agents such as thallium because it has a shorter half life and can thus be given in larger doses.

Question 25

How often is the study repeated and is Cardiac Isotope Scan more accuurate?

Answer 25

The study is repeated after several weeks to determine if any further damage has occurred
Clinical studies demonstrate that technetium heart scans are just as or more accurate in detecting heart attacks as electrocardiogram findings

Question 26

Describe the process of a myocardial perfusion scan?

Answer 26

These scans are used to study blood flow to the heart and can indicate conditions that could lead to a heart attack e.g atherosclerotic disease
MPS is a non-invasive nuclear imaging technique that uses i.v radioactive imaging agents to assess the coronary blood flow to the LV myocardium.
Radioactive tracer injected into vein travels through coronary arteries and settles into LV myocardium.
A narrowed/diseased coronary artery will not allow as much tracer through it as a normal one resulting in reduced amount of tracer to settle in the affected area of myocardium.

Question 27

Myocardial perfusion stress test

Answer 27

Normally exercise causes coronary atery dilation such that more blood/isotope goes to the muscle

In CHD, vessel is affected/calcified such that it does not dilate; stenosis remains the same

The area that is healthy will dilate more such that the difference between the perfused and less perfused area will be exaggerated

Comparison of rest and stress images differentiates between reversible ischemia and established myocardial infarct – do this on treadmill or use pharmacologic agents that increase HR e.g adenosine, dubotamine: