MR Advance Techniques Cardiac Imaging Class IV
Heart The heart is a muscular organ responsible for pumping blood through the blood vessels by repeated, rhythmic contractions. Layers of the heart Endocardium Myocardium Epicardium
Heart The heart is usually felt to be on the left side because the left heart (left ventricle) is stronger (it pumps to all body parts). The base of the heart is at the level of the atriums. The apex is the blunt point situated in an inferior (pointing down and left) direction
Heart Chambers Heart chambers Atria Right atrium Left atrium Ventricles Right ventricle Left ventricle
Heart eptum The Heart is a dividing wall between the right and left sides of the heart. Interatrial septum Interventricular septum Atrial eptum
Heart Valves Heart valves Atroventricular (AV) Tricuspid valve (RT) Mitral valve or bicuspid (LT) emiluvar Aortic valve Pulmonary valve
Cardiac Phases YTOLE
Cardiac Phases DIATOLE
Blood Circulation in the Heart Deoxygenated blood enters the right atrium via the superior and inferior vena cava. The right atria contracts and forces blood through the tricuspid valve and into the right ventricle. The right ventricle contracts and forces the deoxygenated blood through the pulmonary valve and into the pulmonary arteries and to the lungs.
Blood Circulation in the Heart The now oxygenated blood returns via the pulmonary veins, entering the left atria. The left atria contracts forcing the blood through the mitral valve and into the left ventricle. The left ventricle contracts and forces the blood through the aortic valve and into the aorta which sends it on it s way to the rest of the body.
Blood Circulation in the Heart During pulmonary circulation (circulation between the heart and lungs) oxygenated blood is carried by the pulmonary veins and deoxygenated blood is carried by the pulmonary arteries. To prevent confusion review the definition below: The true definition of arteries and veins: Arteries carry blood away from the heart. Veins carry blood toward the heart.
Blood Circulation Blue= O2 poor blood Red=O2 rich blood
Conventional MRI Vascular Techniques Cardiac imaging follows the same principle as vascular imaging: Black blood imaging Bright blood imaging
Black Blood in the Heart
Black Blood Imaging & IR Inversion pulses can produce black blood imaging in GRE pulse sequences. pecially on the heart where blood flow goes in different directions and pre-sat bands does not work properly. TI
Black Blood Imaging & IR Inversion pulses to produce black blood in GRE sequences can be known as driven equilibrium. These pulse sequences begins with a NON slice selected 180º pulse and then another slice selected 180º pulse. A TI equivalent to the null point of flowing spins entering the slice will be applied.
Double IR This technique is also known as Double Inversion Recovery or Double IR or driven equilibrium. Driven Equilibrium
Double IR 180 Non lice 180 90 elected lice elected At TI of Blood (650 ms)
Longitudinal Magnetization TI of Blood Double IR 180 Non lice elected 180 lice elected 90 lice elected TI of Blood (650 ms)
Longitudinal Magnetization TI of Blood TI of Fat Triple IR 180 Non lice elected 180 lice elected 180 lice elected 90 lice elected TI of Blood (650 ms) TI of Fat (150 ms)
Double IR Vs. Triple IR
TI TI 500 TI 650
Gaiting Gaiting is a very general term used to describe a technique of reducing phase mismapping from periodic motion cause by respiration, cardiac motion and pulsatile flow.
Types of Gating Cardiac gating Respiratory gaiting
Cardiac Gaiting There are several forms of cardiac gating: Electrocardiogram (ECG, EKG) Peripheral gating Pseudo gating
Cardiac Gaiting Application Cardiac gaiting can be used: Reduce cardiac motion Reduce pulsatile flow Acquired cine images of the heart, blood vessels and CF.
ECG waves: A P wave that represents atrial systole (atrial depolarization) A R complex that represents ventricular systole (ventricular depolarization) A T wave that represents ventricular diastole (ventricular repolarization)
Heart Rate Heart rate is the speed of the heartbeat measured by the number of contractions of the heart per minute (bpm). The heart rate can vary according to the body's physical needs. Changes in the heart rate are known as cardiac arrhythmias. 60 seconds Normal heart rates are between 60-100 bpm.
ECG The increase in the number of heartbeats per minute (bpm) is know as tachycardia. Tachycardia is a fast heart rate, defined as above 100 bpm at rest 60 seconds
ECG The decrease in the number of heartbeats per minute bpm is know as bradycardia. Bradycardia is a slow heart rate, defined as below 60 bpm at rest 60 seconds
Cardiac Gaiting Cardiac gating monitors cardiac motion by coordinating the excitation pulse with R wave of the cardiac cycle. This achieved by using an electrical signal generated by the cardiac motion to trigger each excitation pulse.
Cardiac Gaiting The peak of R wave is used to trigger each pulse sequence, because electrically, it has the greatest amplitude. This is called the R to R interval and is controlled by the patient s heart rate. Cardiac Cycle R to R Interval
R to R Interval To calculate the R to R interval we can use the following formula: R to R = 60 000ms / heart beat There are 60 000 milliseconds in 1 minute If the heart beat is 80 beats per minute: R to R = 60 000ms / 80 R to R = 750ms
R to R Interval If the patient has a rapid heart rate, the RR interval decreases. If the heart rate is 120 bpm R to R = 60 000ms / 120 R to R = 500ms R 500 ms 500 ms R R P T P T P T
R to R Interval If the patient has a slow heart rate, the RR interval increases. If the heart rate is 60 bpm R to R = 60 000ms / 60 R to R = 1000ms R 1000 ms 1000 ms R R P T P T P T
ECG gating Electrocardiogram gating uses electrodes and lead wires that are attached to the patient chest to produce an ECG. This is use to determine the timing of the application of each excitation pulse.
No Cardiac Gaiting
Cardiac Gaiting
R R R R P T P T P T P T R 500 ms 500 ms 500 ms R R R P T P T P T P T
If the rate changes at all, data is obtained at different times during the cardiac cycle, and the images contain a great deal of artifact. 500 ms 500 ms 500 ms 700 ms 800 ms 800 ms P R T P R T P R T P R T P R T P R T P R T The safeguards are waiting periods before and after each R wave. They are named: Trigger window Trigger delay
ECG Triggering The trigger window: which is the period before each R wave, usually expressed as a percentage of the RR interval, where the system stops scanning and waits for the next R wave, it is about the 10 to 20% of the RR interval. R R P T P T Trigger window
ECG Triggering Trigger delay is the waiting period after each R waive. There is always a slight hardware delay between the system detecting the R wave and transmitting the RF to excite the first slice (few ms). R R P T P T Trigger delay
ECG Triggering The available imaging time is the actual time available to acquire the slices. It is defined as the effective TR minus the trigger window and the trigger delay. R R P T P T Available imaging time Trigger delay Trigger window
ECG Triggering Available imaging time = R to R interval (trigger window + trigger delay) If the R to R interval is 1000 ms, trigger window 10% and trigger delay 100 ms, the time available to acquire the data is: 1000 ms 100 ms 100 ms = 800 ms
Available Imaging Time The available imaging time is purely the time allowed to collect data, and governs the number of slices that can be obtained. R R R R P T P T P T P T Available Imaging Time
Effective TR The effective TR is the time between the excitation of slice 1 in the first R to R interval, to its excitation in the second R to R interval. Effective TR P R T P R T P R T P R T
Heart Rate & TR The TR, depends entirely on the time interval between each R waves (cardiac cycle). If the patient has a rapid heart rate, the RR interval decreases, making shorter the effective TR. horter TR will: Decrease scan time Decrease maximum number of slices per TR Increase T1 Effects on the image TR 500 TR 1000
Heart Rate & TR If HR is slow (bradycardia) the effective TR will be longer. Longer TR will: Increase scan time Increases maximum number of slices per TR Decrease T1 Effects on the image TR TR 2000 1000
Peripheral Gating Peripheral gating works exactly the same way as ECG gaiting. This method uses a light sensor (pulse oximeter) attached to the patient finger to detect pulsation of blood through the capillaries. It is estimated that the R wave of the ECG occurs approximately 250 ms before blood reach the fingers capillaries.
Peripheral Gating It is estimated that the R wave of the ECG occurs approximately 250 ms before blood reach the fingers capillaries. Not a very accurate method because factors such as age, weight, health can alter this estimated time.
Peripheral Gaiting Very useful for procedures that don t required exact timing such as PC-Angiography and areas with slower flow such as CF. R 250 ms P T 250 ms R R R P T P T P T
Pseudo Gating This method calculates the R to R interval and set the Repetition Time (TR) based on the RR. If hart rate changes motion will result on the image. TR 1000 ms TR 1000 ms R 1000 ms R R P T P T P T
Multiphase Cardiac Imaging In this technique a spin echo pulse sequence is used with slices acquired at precise phases of the cardiac cycle.
Cine If 18 phases are collected each slice must demonstrate 18 different positions of the heart in one cardiac cycle. This is referred to the number of phases per cardiac cycle.
Cine Cardiac cine acquisition are acquired with gradient echo sequences with retrospective gaiting technique Retrospective gaiting uses a method of collecting data continuously throughout the cardiac cycle. Data from each slice location can be acquired at different phases during the cardiac cycle.
The Uses of Cine Cine is useful for dynamic imaging of the vessels and CF. For example evaluate aortic dissection and cardiac function. In the brain, it may be useful to demonstrate dynamically the flow of CF in patient with hydrocephalus.
PC-MRA ystole Diastole ubtraction - =
PAMM is a technique used in MRI to detect infarcted areas. PAMM = patial Modulation of Magnetization. PAMM technique is like a grid that moves with the heart muscles. It is used in association with a multi-slice, multi-phase acquisition and acquires data along the short axis of the left ventricle. PAMM
PAMM In normal hearts, the stripes move along with the cardiac muscle. However in cases of infarction, the infarcted area does not contract along with the normal muscle and can, therefore, be easily identified in relation to the stripes.
Myocardial Perfusion Myocardial perfusion is used to evaluate the coronary arteries. At rest coronary arteries might supply enough blood to the myocardium, but during stress they might not.
Different to a stress test (Nuclear Medicine) were the heart is stressed by physical activity, in MRI the heart is stressed with the application of medication (Adenosine) then it is scanned to evaluate the level of perfusion. Myocardial Perfusion
Myocardial Perfusion This technique uses a T1 weighted images, to observe the enhancement of the tissues. The slices are repeated several times during the bolus injection allowing the evaluation of the level of perfusion.
Myocardial Perfusion
Respiratory Compensation When imaging the chest and abdomen, respiratory motion along the phase axis produces phase mismapping.
Respiratory Compensation Breathing Motion compensation techniques: Breath hold technique Respiratory gaiting Multi-average imaging
Breath Hold The best way of reducing breathing motion is: Use gradient echo pulse sequences to be able to scan faster Ask the patient to hold his breath during image acquisition (breath hold).
Motion Compensation Breath hold technique: helps to minimize motion form breathing. Tips: Explain the patient before start examination Always follow same instructions Aloud time for the patient to recover
Respiratory Gaiting Respiratory gating or respiratory compensation is achieved by monitoring the patient breathing cycle.
Respiratory Gaiting This is accomplished by placing a breading detection device on the patient. This breading detection device (belt or couching) is connected to the scanner and it will advise the scanner about breathing cycle.
Respiratory Gaiting A more sophisticated option is the use of a detection voxel on top of the liver to detect the liver motion during breathing activity.
Respiratory Compensation The image acquisition will always be at the same point during the respiration cycle. This technique is very effective but scan time is significantly increase.
This technique is very effective but scan time is significantly increase. No Respiratory respiratory gaiting (4(20 min) s)
T1 and Respiratory Gating The breathing cycle is slower then the cardiac cycle, this will result in longer effective TR s. Longer TR will significantly reduce the T1 effects on T1 weighted images, resulting in PD weighted images. Example: Respiratory rate: 20 breath p/m Effective TR = 60,0000 ms / 20 Effective TR = 3000 ms
T1 and Respiratory Gating 3000 ms 3000 ms 3000 ms
Multi Average Acquisition Increasing the number of excitations may also help, as this increases the number of times the signal is averaged. Motion is averaged out of the image as it is more random in nature than the signal itself.
NA & Motion Acquisition 1 Acquisition 2 Acquisition 3 Average of the 3 Acquisition
NEX & Motion ince moving tissues change position during different acquisitions the motion tend to disappear when several acquisitions are average out. Acquisition 12 Average of the 3 Acquisition = 79
Navigation ystem The navigation system is a combination of cardiac and respiratory gaiting at the same time to obtain a image free of respiratory and cardiac image. This application will increase imaging time.