Friday, September 25, 2009

Pulse Sequences for Cardiac MRI

Bright Blood Technique

Dark Blood Technique
A non-selective 180º inversion pulse excites all the tissues and blood within the entire heart when the R-wave trigger is detected at the beginning of the cardiac cycle. Immediately following, a slice-selective re-inversion pulse excites only the tissues and blood within the image slice. The net result is that everything within the slice is flipped back to normal because it experienced both the inversion and re-inversion pulses, whereas everything outside the slice remains inverted. Next, the systolic contraction forces the blood within the slice to be replaced by blood from outside the slice. After enough time delay has occurred to allow the blood to be fully replaced within the slice, the image data is collected during mid to late diastole of the cardiac cycle.

Double Inversion Recovery

The red line represents the signal of the blood that flows into the image slice, whereas the green line represents the signal of the myocardium within the image slice. Note that the myocardium signal is initially inverted and then immediately re-inverted, so it remains positive during the cardiac cycle. But the blood signal is initially inverted and then gradually recovers toward positive during the cardiac cycle, passing through zero at some time during diastole. The time delay required for the blood signal to pass through zero is known as the blood null time. If the data acquisition is carefully timed to occur exactly when the blood signal is passing through the null point, the blood appears dark in the image.

Triple Inversion Recovery

The third IR pulse and the fat signal are shown in yellow. Although the delay time for blood to cross its null point is relatively long, the delay time for fat to cross its null point is much shorter. If the sequence timing is carefully controlled such that both the blood and fat cross their null points at the same time that data is collected, both blood and fat will be dark in the image.

Short RR Interval
Reduce the data collection segment by reducing the number of lines per segment, or reduce the echo spacing by increasing the bandwidth and/or using faster RF pulses and gradient pulses. If the pulse sequence is single shot - Haste or TrueFisp, reduce the data collection segment by reducing the Phase FOV or Phase matrix, use the minimum possible TE & iPAT.

Long RR Interval

TI Scouting for Delayed Enhancement
An IR pulse and cine data acquistion are repeated every second heart beat. Since T1 recovery is occuring during this period, as shown by the yellow curve, each cardiac phase has a different TI relative to the initial IR pulse.

Inversion Recovery - TurboFLASH for Delayed Enhancement

T1-weighting is maximized by using a 180 degree inversion pulse to flip the signal of all tissues to the negative axis. The signals of tissues with different T1 recovery rates are allowed to separate out over time, as shown by the red and yellow curves. The total recovery period spans through 2 cardiac cycles to allow enough time for all tissues to fully recover back to their positive axis. During this recovery period, the necrotic tissue has higher signal than viable tissue due to its faster T1 recovery rate. Data is collected ideally when there is maximal separation between necrotic and viable tissues. Maximal contrast between necrotic and viable tissues is achieved if the TI is adjusted so that the viable tissue is crossing the null point when the data is collected.

Phase Sensitive Inversion Recovery for Delayed Enhancement

Saturation Recovery - TurboFLASH for Myocardial Perfusion

Sunday, September 20, 2009

Cardiac MRI - Imaging Planes for Basic Cardiac Views

Pseudo 2 & 4-Chamber Views

Short Axis View

4-Chamber View

Left 2-Chamber View

3-Chamber View

Left Ventricular Outflow Tract (LVOT) View

Right 2-Chamber View

Right Ventricular Inflow View

Right Ventricular Outflow Tract (RVOT) View

Right Ventricular Inflow / Outflow View

Off-Resonance Artefacts

TrueFisp Off-Resonance Banding Artefacts

Saturday, September 19, 2009

Cardiac Anatomy


Cardiac Axis

Electromechanical Activity

Cardiac MRI Visualization

Sunday, September 13, 2009

MR Urography (CINE Display)

Normal cine MR urography shows intermittent distension of the ureters with urine. Only the distal ureters are not visible, owing to the distended urinary bladder. Note minimal interference from intestinal fluid due to administration of negative enteric contrast material prior to the study.

Cine MR urography with uteropelvic junction obstruction of the right kidney. Note that no urine is visualized in the ureter below the uteropelvic junction owing to the high-grade obstruction.

Coronal 3D gradient-echo images from excretory MR urography performed at 1.5 T with a 3-mm through-plane resolution.

Magnified coronal fat-suppressed 3D gradient-echo images through the right kidney from full-field-of-view excretory MR urography performed at 3.0 T with a 2-mm through-plane resolution.

Cine MR urography of patient with surgically proved necrotic clear cell carcinoma of the right kidney and partially obstructing blood clot in the right proximal ureter. Note standing column of urine in the ureter above the filling defect in the proximal ureter and intermittent distension of the ureter below this level, indicating partial obstruction. Partial obstruction was also confirmed on contrast-enhanced images (not shown).

Cine MR urography shows persistent right-sided hydronephrosis and standing column of urine in the right ureter to the level of the ileal anastomosis. Note peristalsis in ileal loop.

Keywords: MR Urography, MRU

Saturday, September 12, 2009

Quality Assurance (Clinical MRS)

Source: Kreis R. Issues of spectral quality in clinical 1H-magnetic resonance spectroscopy and a gallery of artifacts. NMR Biomed. 2004;17:361-381.

Friday, September 11, 2009

Spectral Quality & Artefacts

Effect of spurious echoes. Insufficient amplitude of gradient crusher pulses in combination with local B0 inhomogeneities can lead to the refocusing of unwanted echoes (e.g. 2 pulse echo in a PRESS sequence). (A) The FID from a PRESS acquisition (TE 20 ms, TR 3 s) localizing developing white matter in a female preterm neonate (34 weeks gestational age). The encircled part of the FID originates from an unwanted echo. (B) The typical appearance of spurious echoes, often called ghosts, in the spectrum. Because extended phase cycling was used (phase rotation) in data acquisition, the origin of the spurious signal could be identified in a separate trace after Fourier transformation along the phase rotation dimension (C). The particular phase evolution proved the spurious signal to arise from a two-pulse echo of the initial 90 and last 180 pulse. In the current case, elimination of the ghosting artifact can easily be accomplished by zeroing the latter half of the FID. The resulting spectrum is plotted in (D).

Effect of head movements. All peaks are doubled in a spectrum from a neonate because the baby had moved its head between two distinct positions during the scan (E). The repeat examination shows single peaks with perfect shim and lineshape (F), when the baby was soundly asleep. (41 weeks gestational age, ROI in thalamus, PRESS TE 20 ms, TR 2 s, 128 acquisitions

Effect of eddy currents. All lineshapes are distorted in spectrum (G) due to eddy currents in a short-TE PRESS spectrum (TE 20 ms) of occipital GM in a 14-year-old boy. (H) The same spectrum after restoration of the lineshape using the phase information from a water reference scan.

Effect of gross head movement. An ROI was placed in the putamen on a localizer image (A). The resulting spectrum is shown in (B). However, unknown to the operator, the examined subject had turned his head a little towards his left, which was picked up from the repeated localizer scan after the MRS acquisition (C). As ROIs are prescribed in magnet coordinates, the ROI targeted at the putamen ended up in insular GM, such that the spectrum in (B) was in reality acquired from insula, not putamen. This explains the narrow lines, which are atypical for basal ganglia. A spectrum from the putamen was then acquired (D) and shows that a completely wrong diagnosis would have resulted when the spectrum from insular GM was taken as originating from putamen. Verification of proper ROI placement is crucial. (Scan parameters: 26-year-old man; MRI: fast spin echo sequence, echo train length 16, TR 3 s, TE 100 ms; MRS: 2.2 cm3 ROI, PRESS, TE 20 ms, TR 3 s, 128 acquisitions).

Signal bleed from outside the targeted ROI. Signal from outside the selected ROI can give dominating signal contributions, if the transition zone of the slice selective pulses falls into regions with large lipid content. This is illustrated for PRESS spectra obtained from a 40-year-old woman. The original ROI dimensions of 10 x 15 x 27 mm, used for spectra (A) and (B) were reduced to 10 x 15 x 22 mm for spectrum (C). This diminished the transition zone of the longest dimension of the voxel pointing towards the lipid-containing areas and the lipid contribution vanished. Just moving the ROI away from the skull, (E)–(F), did not completely eliminate the lipid contamination in the spectrum, (A)–(B). (Scan parameters: TE 20 ms, TR 3 s, 1953 Hz spectral width, 1024 points zero-filled to 2048 points, outer volume suppression pulses disabled).

Conspicuity of artifacts in MRI and MRS. If a patient leaves the magnet half-way through a scan, even a layman will refrain from interpreting the resulting image (A). If this happens in a MRS scan, even the expert will not be able to recognize this fact from the resulting spectrum (B), since only signal-to-noise and absolute concentrations will be affected. Spectra (B) (half of the acquired FIDs contain noise only) and (C) (normal acquisition) were scaled to the largest peak, resulting in an apparent signal-to-noise difference, while quantitative analysis would yield a 50% deficit for all metabolites. Unless double-checking mechanisms are put in place and plausibility arguments are used, the resulting diagnosis will be completely wrong. (Scan parameters: 38-year-old healthy woman; MRI, fast spin echo with TE 102 ms, TR 3 s, 256 x 256, 4 mm slice thickness; MRS, PRESS with TE 20 ms, TR 3 s, 6.7 cm3 ROI in periventricular GM, 128 acquisitions).

Source: Kreis R. Issues of spectral quality in clinical 1H-magnetic resonance spectroscopy and a gallery of artifacts. NMR Biomed. 2004;17:361-381.

Friday, September 4, 2009

Lipid Contamination (MRS)

Top left: T1W image with spectral FOV (white grid), a PRESS-selected region (blue box) and a region with lipid-contaminated voxels (purple box).  Top right: Same image with the extendd spectral FOV through the unaliasing algorithm (white grid) and the correctly located lipid region (red box).  Also shown are spectra of the purple box before (a) and after (b) lipid unaliasing, and the lipid spectra unaliased into the red box that is an FOV away (c).

Application of B1- and T1-insensitive VSS sequence to 3D MRSI of the brain in volunteers. a: MR image showing the positions of the PRESS box (white box) and VSS band (black line). b: The inclusion of subcutaneous lipids at the left side of the PRESS box caused spectral distortions throughout the entire spectral array (white grid). c: The VSS band was placed to reduce the lipid contamination. The black line indicated the edge of the VSS band. All voxels within the left two columns showed virtually complete suppression of lipid signals.

Spatial response function (SRF) or point spread function (PSF) in chemical shift imaging (CSI).  The PSF, which depends on the sampling and post processing scheme, shows how much signal is contributed to a pixel in CSI from outside this pixel.  The diagram above illustrates how the low-resolution (24 x 24) SI data exacerbates the problem of signal bleeding.  Substantial signals are observed way outside of the selected voxel.