Thoracic Protocol

• Aneurysm
• Dissection
• Aortitis
• Connective tissue disorders (Marphans, Erlers-Danlos…)
• Proximal great vessel disease
• Mural lesions
• Anatomic variations
• Coarctation
• Sequestration
• Pre-cardiothoracic surgery

It is particularly well suited for patients with allergy to iodinated contrast, elevated serum creatinine (Cr >= 2.0 mg/dl) or patients needing to avoid exposure to ionizing radiation.

• Start intravenous line (20 or 22 gauge) and attach SmartSet (TopSpins. Tel: 734-623-6400) If iv caliber is small (i.e. 22 gauge), consider pre-warming gadolinium contrast to body temperature to reduce its viscosity.
• Oxygen, 2-4 liters/min by nasal canula is essential if patient is short of breath.
• Valium (5-10 mg po) or Xanax (1-2 mg po) if patient is claustrophobic may be given.
• EKG Gating: It is critical to get a high quality EKG waveform for gating in order to eliminate cardiac motion artifact especially to see dissection flaps and the aortic root. Otherwise the MRI images will be of lesser quality
• Some tips for improving EKG signal:
• Use fiber-optic EKG gating system
• Abrade skin before placing leads (edema-inflammation improves skin conductivity).
• Make sure there is plenty of conductive gel.
• Use large diameter leads to decrease skin impedance.
• Keep wires straight Do not allow any large loops to form in the wires.
• Place skin contact sites far apart.
• If anterior positioning does not work, try the left side or the back.
• To avoid severe EKG burns: instruct patients to speak-up if the leads get hot and NEVER use EKG leads on large patients (>200 lb) under general anesthesia.
• If EKG gating is not possible, go straight to 3D Gd:MRA and post Gd Axial 2D TOF

Coil: Body coil has the adventage of a large field-of-view that allows imaging the entire aorta (thoracic and abdominal) with homogeneous sensitivity to MR signal. Pediatric patients can sometimes fit into the head coil, which has at least 4 times the SNR of the body coil. The torso array coil has higher SNR then the body coil but less coverage and inhomogeneous signal sensitivity with prominent near-field artifact. If using the torso array coil, consider rotating it 90 degrees for greater S/I coverage.

Patient Positioning: Supine, feet first,

Landmark: Mid-sternum or nipple line à for thoracic aorta only:
                    Xyphoid à for both thoracic and abdominal aorta:

Thoracic aorta anatomy is complex, variable and the blood flow can be fast or slow. As a result, no single protocol will be suitable for all patients. It is necessary to customize the exam based upon the indication, age, cardiovascular status and anatomy.

In general pediatric patients and young adults have very fast flow which allows the aorta to be well seen on spin echo and time-of-flight sequences. In these patients the black blood, T1 weighted spin echo sequences with EKG gating may be sufficient. Use at least a minimum full echo and no gradient moment nulling (no flow compensation) to achieve suffcient "black blood" effect. Older adult patient generally fall into one of two categories. The first category are those with know aortic aneurysm or dissection and the only clinical issue is to determine if the aneurysm or false lumen is increasing in size. To measure aortic diameter, a simple protocol including Coronal T1, Axial T1 and Sagittal oblique T1 spin echo sequences is adequate. The second category are all the other patients where more precise vascular anatomy and lumenal detail involving both aorta and branch vessels must be imaged. For this second category of patients, 3D Gd:MRA is essential.

Sagittal 3D Gd:MRA: If you have a slow scanner and only the only the aorta and proximal great vessels need to be imaged, consider a sagittal 3D Gd:MRA.The sagittal acquisition eliminates the problem of wrap-around artifact from the arms along side the patient and allows coverage of the aorta with fewer slices. With sagittal acquistion you can speed up the scan by using rectangular FOV or you can reduce the FOV to maximize resolution.

Coronal 3D Gd:MRA: But if you need to image the subclavian or renal arteries, a coronal acquistion for the 3D Gd:MRA is preferred. This requires a state-of-the-art scanner with fast gradients to cover the anatomy with sufficient resolution within a breath hold. Start with a Sagittal spin echo locator, then axial T1, then coronal 3D Gd:MRA followed by axial 2D TOF post gadolinium. If the renal arteries are an issue (especially in patients with hypertension) then 3D phase contrast of the renal arteries can be added on at the end to take advantage of the extra SNR from the gadolinium that was administered.

• ADDITIONAL PARAMETERS:
• Gating Screen
• Trigger Type Auto
• # of RR Interval 1
• Trigger Window Auto
• Trigger Delay Minimum
• Inter-Seq. Delay Minimum
• Cardiac Phases Single
• Do not use flow compensation as this will eliminate the black blood effect.
• Since the number of slices is determined by the R-R interval, it may be necessary to reduce the default number of slices to avoid doing a second acquisition. It is also possible to increase the number of slices for more A-P coverage in patients with a relatively slow heart rate.
• If there is thoracoabdominal pathology (typically aneurysm or dissection) then increase to maximum FOV (48 cm on GE Signa) and landmark on the xyphoid.

• ADDITIONAL PARAMETERS:
• Gating Screen:
• Trigger Type: Auto
• # of RR Interval 1
• Trigger Window Auto
• Trigger Delay Minimum
• Iner-seq. Delay Minimum
• Cardiac Phases Single
• Do not use flow compensation as this will eliminate the black blood effect.
• If there is incomplete flow void on the first series (coronal T1) then consider lengthening the echo time (TE) for series 2 and 3 to 20-30 msec to allow more time for intravoxel dephasing. Also consider using a black blood sequence with inversion pulses to null blood signal if available. Soon, SSFSE with a preparatory inversion pulse will be available for black blood MRI.
• Cover from above the aortic arch to the diaphragm. If aortic disease extends below the diaphragm into the abdomen then extend coverage more inferiorly. Alternatively, if the R-R interval is too short to provide enough slices, perform a second axial T1 acquisition for the abdomen to cover abdominal aortic pathology. This can be done without EKG gating for faster acquisition but be sure to include respiratory compensation.

This optional sequence provides the conventional RAO view of the aortic arch in a black blood presentation that is well received by referring physicians. However it is generally unnecessary if the sagittal 3D Gd:MRA is being performed (series 4) as that provides the same view at higher resolution.

• This series can be performed without EKG gating. Use multiple averages (NEX) to average out cardiac motion. This is especially useful in patients with poor EKG gating (arrhythmias, COPD, high skin impedance (fat), old…etc…) However, if the EKG signal is excellent, consider implementing EKG gating with fewer NEX to speed up the acquisition or obtain more coverage.
• To avoid making this scan too long, keep the TR short (typically 300 msec). This will limit the number of slices so it is important to position and orient the volume so that the entire thoracic aorta is imaged in plane with the narrowest possible volume so the slices can still be reasonably thin.
• To position the slices, look at several axial slices superior to the heart (ideally at the level of the right pulmonary artery) to get a sense of the angle of obliquity to fully open up the aortic arch. Prescribe the entire set of about 10 slices. Then check the position on every axial slice to be sure that the entire thoracic aorta is included. Ideally there will be at least one slice that shows an RAO view of the entire thoracic aorta but this is not always possible when the aorta is tortuous. For aneurysmal, unfolded aortas, it may end up being better to do a straight sagittal acquistion.

• ADDITIONAL PARAMETERS
• Vascular Screen:
• Projection Images: 0
• Collapse: on
• User CVs Screen:
• Max. Monitor Period: 40
• Image Acq. Delay: 8
• SPECIAL: off
• Multi Phase Screen:
• Phases per Location: 3
• Delay After Acq. Minimum (550)
• Autoshim: In general autoshim only needs to be performed once at the beginning of the first scan of the examination. Autoshim is not necessary to perform for subsequent series as long as the table position does not change very much. However, one exception occurs in patient who have medal surgical clips, metal orthopedic implants, shrapnel or other metal that may cause a significant change in the field homogeneity even with small table motions. These patients should have the autoshim performed for every gradient echo pulse sequence.
• To determine where to position the 3D Volume use the Coronal T1 images as a guide. First find the aortic arch and draw a line on the screen with a grease pen about 5 cm above the top of the arch. Then mark the right edge of the ascending aorta and the left edge of the descending thoracic aorta. Look in the abdomen for the renal arteries and draw the course of the abdominal aorta down to the bifurcation.
• Scan from the left edge of the descending aorta to the right edge of the ascending aorta. Be careful to include at least 1-2 cm of renal artery bilaterally. Adjust the FOV to scan from about 5 cm above the aortic arch to below the aortic bifurcation.
• Make sure the acquisition time is short enough so that the patient can suspend breathing for the entire scan. To make the scan time shorter consider

• Decreasing NEX to 0.5
• Decrease Phase FOV (only if patient is thin and FOV is large)
• Decrease matrix to 128
• Decrease number of slices and increase slice thickness
• Be careful to cover only the essential anatomy
• Use the "fallback" for optimal right-left alignment
• Check "#of Locs Before Pause" to be sure it is set to 1.
• For the coronal imaging place the patient’s arms over the head or on cushions to get them out from along side the patient where they will wrap around into the imaging volume.
• Test the iv with saline and then fill the SmartSet tubing with Gd contrast (about 7 ml) so that Gd contrast is advanced almost all the way to the iv site.
• Instruct the patient on when to suspend breathing: "This is the most important scan. You will need to hold your breath for 1/2 of the scan, the second half. You can tell when to hold your breath by the change in the sound. Just to be sure there is no confusion, I will squeeze your arm when the sound changes so that you will know exactly when to take in a deep breath and hold it."
• Start scan: Do not begin injecting until the clock begins to count down: about 15 seconds after starting the scan.
• When the clock begins counting down, start injecting at about 1-2 cc/sec ( as fast as you can, for a person of average strength using Magnevist with a 20 gauge iv).
• When the sound changes (bolus detected), signal the patient to Breath Hold by squeezing arm.
• When Gd infusion is complete, flush with immediately 20 cc normal saline.

At the end of the arterial phase scan, have the patient take 3-4 quick breaths and then scan again to catch the portal venous phase. Then allow the patient to breath several times until relaxed and breath hold for one final scan during the equilibrium phase.

Billing

• MRI Chest: 71551
• MRA Chest:71555
• If thoracoabdominal study add MRA Abdomen: 74185
• Computer Reconstructions 76375

ICD9 Codes:

The following ICD-9-CM diagnosis codes are payable for MRA of the chest (71555)only when iodinated contrast is contraindicated: