DYNAMIC MR CONTRAST IMAGING AND DELAYED SCAN FOR MYOCARDIAL PATHOLOGY
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Double inversion recovery (DIR) is a type of “black blood” technique used for visualizing the walls of the cardiac chambers and blood vessels (including the coronary arteries).
As the name implies, the sequence involves an application of two RF-inversion pulses in close succession after an EKG trigger. The first inverting pulse is spatially non-selective, while the second pulse is spatially selective.
Dark blood double IR sequence. After the two inversion pulses, tissue within a slice is unaffected, but the magnetization of blood outside the slice is inverted. As this blood flows into the slice for imaging, it produces no signal since TI is chosen to be null its signal. The image acquisition module is typically 2D fast (turbo) spin echo.
The term non-selective means that the first 180 degrees – pulse inverts all spins within the entire active volume of the transmit coil. The second 180 degrees –pulse is spatially selective, meaning that its effects are restricted to the single slice being imaged. The second RF-pulse thus restores longitudinal magnetization for both blood and myocardium within the imaging slice. For spins outside the slice, however, the longitudinal magnetization remains inverted.
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Bright Blood Imaging
Bright blood imaging describes the high signal intensity of fast-flowing blood and is typically used to evaluate cardiac function. The main pulse sequences used for bright blood imaging include GRE and the more recently introduced, but related, the technique termed “steady-state free precession” or “SSFP”. GRE sequences (i.e., spoiled gradient recall[SPGR], turbo FLASH, turbo field echo, fast-field echo [FFE]) are produced by emitting an excitation radio-frequency pulse that is at least two directions, which create an echo signal that can be detected. SSFP sequences (i.e., fast imaging employing steady-state acquisition [FIESTA], fast imaging with steady-state precession [FISP], balanced FFE) are similar but incorporate a short TR with gradient refocusing that is less vulnerable to T2* effects compared with standard GRE. SSF
Dark Blood Imaging
Dark Blood Imaging refers to the low-signal-intensity appearance of fast-flowing blood and is mainly used to delineate anatomic structures. Traditionally, spin-echo (SE) sequences have been used for dark blood imaging. SE has been supplanted by the newer FSE and turbo spin-echo (TSE) techniques in cardiac imaging. Although these techniques have lower signal-to-noise ratio than SE, they enable rapid imaging, which minimizes the effects of respiratory and cardiac motion. Basic FSE and TSE sequences consist of radio-frequency pulses with flip angles (A) of 90 degrees and 180 degrees followed by an acquisition of 1 or 2 signals. FSE and TSE sequences can be T1- or T2-weighted acquired over a series of single or double R-R intervals.
Steady State Free Precession (SSFP) is a family of sequences which are currently the backbone of cine cardiac MR imaging. Members of the SSFP family include Balanced Fast Field Echo(FFE), imaging that produces bright blood images with excellent contrast between myocardium and blood within the heart (blood pool). It is a relatively fast acquisition, however, image quality is very dependent on the homogeneity of the magnetic field.
The high temporal resolution and excellent contrast of SSFP make it well adapted for evaluation of wall motion and volumetric measurement, which require clear delineation between myocardium and blood pool.
DOUBLE INVERSION RECOVERY
- Double inversion recovery sequences are designed specifically to null the signal from flowing blood.
- There are 2 prepulses. A nonselective 180 degrees RF(radio frequency) pulse inverts all protons. This is followed by a slice-selective 180 degrees pulse that reverts all protons in the imaging slice back to the original alignment.
- There is no effect on stationary protons in the imaging slice. However, the flowing blood in the imaging slice will have experienced only the non-selective pulse (the blood that experienced both pulses will no longer be in the slice at the time of imaging).
- Double inversion recovery sequences begin imaging when the magnetization vectors of the flowing blood crosses the null point- the inversion time.
STEADY-STATE VERSUS SPOILED GRADIENT ECHO IMAGING
- In steady-state GRE sequences, spoiling is not performed, and residual transverse magnetization is retained. The retained residual transverse magnetization increases the signal-to-noise ratio (SNR) of steady-state sequences relative to spoiled sequences.
- The image contrast will depend on the T2-to-T1 ratio. As stated previously, this is undesirable for many applications. In steady-state sequences, only fluid and fat will have a high signal (fluid and fat have comparable T1 and t2 times, while in most other tissues, T2 time is much shorter than T1 time).
- However, in bright blood cardiac MRI, hyper intense blood relative to other tissues is exactly what is needed; thus, steady-state GRE sequences are optimal for cine cardiac imaging (cMRI).
LATE ENHANCEMENT SEQUENCE (LOOK-LOCKER TECHNIQUE)
- The shots should be acquired in mid diastole to keep the cardiac motion as small as possible.
- Inversion delay time to cancel the normal myocardium is patient dependent and cannot be calculated before handle. It also depends on the time after the contrast injection. A longer time after injection results in a slower TI relaxation and left contrast in the myocardium.
- Longer inversion time should be used (200-300 ms).
- Use a Look-Locker sequence which is CINE scan that utilizes a single inversion pulse applied once every heart beat immediately after the R-peak. TI relaxation to made visible over the individual cardiac phases.
- Start with a TI of 200 ms, and then increase that TI using small steps of 20 or 30 ms.
- Change the inversion delay in real time during an interactive scan. Make sure that the scan is running in continuous mode. Inversion time depends on the steady state that is reached over multiple heart beats.
- Lock-Locker technique to quickly find the zero-crossing point of the myocardium within a breath hold.
Case – 1
63 YEARS OLD MALE WITH COMPLETE LAD ACCLUSION.
- Known case of anterior wall of MI
- Cath angiography – complete occlusion of LAD
- For myocardial viability
- Contrast enhanced cardiac viability was done
- Dynamic contrast study followed with delayed scan up to 20 minutes
Case – 2
CASE OF 50 YEARS OLD PATIENT
- Arrested for acute myocardial infarction.
- Post resuscitation developed right ventricular outflow tract obstruction.
- Echocardiography shows right ventricular outflow tract obstruction.
- Mixed echogenic lesion at the right free wall.
Causes of RV free wall hematoma
- Intramyocardial hematoma is a rare disease, usually associated with
- Myocardial infarction,
- Chest trauma,
- Coronary artery bypass operation,
- Complication of percutaneous coronary intervention (PCI)
- Can occur spontaneously.
Case of ARVD
RIGHT VENTRICLE SHOWS TI HYPERINTENSE LESIONS – FATTY INFILTRATION OF RV WALL
- Hypertrophic cardiomyopathy (HCM) is a relatively common form of genetic heart disease and the most frequent cause of sudden cardiac death in the young.
- The clinical phenotype is characterized by otherwise unexplained left ventricular (LV) hypertrophy and a myriad of patterns of wall thickening.
- Although the distribution of LV hypertrophy has shown little relation to clinical outcome, the magnitude of LV wall thickness conveys prognostic significance, showing a direct relationship to the risk of sudden death.
- 2D echocardiography is the standard imaging modality for clinical identification of the LV hypertrophy.
Cardiac MRI (CMR)has the capability of acquiring tomographic images of the hypertrophied LV chamber, with tissue contrast and border definition that are often superior to that achievable with echocardiography
- HCM is a heterogeneous cardiac disease in which clinical diagnosis is predicated on the demonstration of otherwise unexplained LV hypertrophy, which appears in a myriad of diverse patterns,
- Although the distribution of hypertrophy is often diffuse, it is also frequently segmental and confined to relatively small regions of the LV chamber.
- Although the anterior portion of the ventricular septum is the area of the wall most commonly involved in the hypertrophic process, localized wall thickening may preferentially involve posterior septum, apex, anterolateral, or even posterior free wall.