| Patient preparation | Sag Locator | Coronal 3D Gd | Axial 2D TOF |
|---|---|---|---|
IMAGING PARAMETERS | |||
| Plane | Sagittal | Coronal | Axial |
| Mode | 2D | 3D | 2D |
| Pulse Seq | Spin Echo | Vosc, TOF, SPGR | Gradiant Echo |
| Imaging Options | RC, NPW | Fast, ZIP2 | RC, FC, NPW, Seq |
SCAN TIMING | |||
| # of Echoes | 1 | 1 | 1 |
| TE | Min Full | Minimum | Minimum |
| TR | 275 | -- | 18 |
| Flip Angle | -- | 45 | 60 |
| Bandwidth | -- | 31.25 | 15.63 |
| SAT | None | None | None |
ACQUISITION TIMING | |||
| Freq | 256 | 256 | 256 |
| Phase | 160 | 160 | 192 |
| NEX | 2 | 0.5 | 4 |
| Phase FOV | -- | 1 | -- |
| Locs Before Pause | -- | -- | -- |
| Freq DIR | S/I | S/I | R/L |
| Auto Center Freq | Peak | Water | Water |
| Auto Shim | On | On | On |
| Contrast | -- | 42 ml | 42 ml |
SCANNING RANGE | |||
| FOV | 44 (40-48) | 40(36-48) | 32(26-44) |
| Slice Thickness | 10 | 4.0(3-5) | 8(6-12) |
| Spacing | Interleave | -- | 0 |
| Start - End | L95-R95 | See Graphic | See Graphic |
| # Slices | 20 | 40(30-50) | -- |
| Scan Time | 3:25 | 0:28 | 0:14/slice |
Common Indications:
Coil: Body coil has a large field-of-view and uniform sensitivity to the MR signal. However, the torso array may give higher SNR especially in small patients. Rotate 90 degrees for greater S/I coverage and less R/L coverage to help reduce wraparound artifact of the shoulders so that a smaller field of view can be prescribed.
Patient Positioning: Supine, feet first
Landmark: Mid sternum at about the nipple line.
General Strategy:
There are two approaches to pulmonary 3D Gd:MRA. One is to image both lungs simultaneously in the coronal plane with a single injection of gadolinium contrast agent. The other is to acquire separate sagittal acquisitions for each lung with two separate injections, one for each lung. The sagittal acquisitions have the advantage of being able to use a smaller field-of-view with higher resolution without having to worry about wrap-around artifact. However, the main pulmonary artery is not well seen because it is at the edge of each volume. Furthermore, the requirement of two separate injections means that the dose for each injection must be lowered to keep the total dose from being excessive. With a smaller dose, bolus timing becomes much more critical. The coronal acquisition shows the main pulmonary artery well and requires only one injection. But it has lower resolution because of the large field-of-view required to avoid wrap-around artifact from the shoulders in the coronal plane. Here we will describe the coronal approach because it is preferred by our referring physicians who want a good image of the main pulmonary artery.
Any sagittal locator is fine although we prefer not to make the patient breath hold for this scan because then they become tired out and have a more difficult time breath holding for the subsequent coronal 3D Gd MRA.
Scan from posterior to the spinal canal and anterior to the ascending aorta. It is acceptable to miss part of the anterior chest because most of the major pulmonary vessels are posterior. Adjust the number of slices and slice thickness to be sure to cover posteriorly within a short enough duration that the patient is able to suspend breathing for the entire scan.
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It is absolutely critical that this acquisition be completed with no respiratory motion. Prescribe it as a multi-phase with at least three phases so that in the event the first phase is not perfectly free of motion perhaps a second or third phase will come out acceptably. If the patient is on a ventilator, ask for assistance from respiratory therapy to suspend respiration during the scan. Heavy sedation may help the ventilator patients tolerate temporary cessation of the respirator.
Bolus timing for the pulmonary arterial phase is critical. Typical optimal timing is to begin injecting that 2 cc’s per second and then to begin scanning 10 seconds later. Be sure to follow the contrast injection immediately with saline flush at the same rate. Use at least 20 cc of saline flush to completely purge the iv tubing and also to help flush contrast through the arm veins.
A large contrast dose is essential in order to obtain enough signal noise to evaluate the small pulmonary arteries and also to compensate for bolus timing errors that are generally inevitable. A large dose of at least 0.3 mmol/kg (40-60 ml) helps to show pulmonary enhancement to find perfusion defects that accompany occlusive emboli.
This sequence is a good back-up in the event that the patient was not able to suspend breathing for the 3D Gd:MRA. Respiratory motion artifact is minimized by using respiratory compensation and 4 NEX with 256 phase encoding steps for maximum signal averaging. Scan from 2 cm. above the main pulmonary artery down to just above the dome of the diaphragm.
Billing:
ICD9 Codes:
Sample Normal Dictation:
The patient was imaged with a sagittal locator sequence, coronal 3D MRA with dynamic gadolinium-enhancement and 2D time-of-flight post gadolinium. The 3D image data was reformated on a computer workstation to obtain views optimized for each pulmonary artery down to the segmental level
There is normal enhancement of the lung parenchyma with no evidence of pulmonary infarction and no masses identified. The pulmonary arteries are normal in calibre and no emboli are identified down to the segmental level. Smaller, subsegmental pulmonary arteries are not reliably evaluated by MRA.
Impression:
Normal Pulmonary MRA, no emboli.
Accuracy of 3D Gd:MRA for Pulmonary Embolism
| Investigator | Year | Number of Patients | Sensitivity | Specificity |
| Meaney | 1997 | 30 | 100% | 95% |
| Gupta | 1996 | 36 | 85% | 96% |