Yttrium-90 Bremsstrahlung Imaging

Yttrium-90 imaging is probably not the first thing that comes to mind when scanning in Nuclear Medicine as an isotope, namely because it is outside of the realm of our usual Technetium-99m based radiopharmaceuticals. I have written about Y-90 and Theraspheres in the past blogs (parts I and II), but this time instead of discussing the treatment aspect, we will look at the imaging aspect of using Y-90 and its Bremmsstrahlung x-rays that it produces.

Imaging was performed on a patient who was diagnosed with hepatocellular carcinoma.  Early in the treatment planning with computed tomography (CT) and interventional arteriography, a large mass was localized in segment 4A/B in the liver.   Majority of the vascularity was provided by the left hepatic artery.  The middle hepatic artery and gastroduodenal arteries were then coiled embolized to limit perfusion.  This patient was on the third treatment cycle of Y-90 Therasphere.  The growth of the liver lesion was stabilized with the first two treatments but there were some suspicions of new metastases.

Fig. 1  Finding the common hepatic artery during the arteriogram.  Quite honestly, I can not figure how the interventional radiologists navigate through the arteries like that... There are no google maps for this!

Fig. 2  The right hepatic artery.  Notice the blush within the lesion as the contrast was infused.  I find this quite amazing to see in person.  In the end the interventional radiologists decide to use the left hepatic artery to infuse the Tc-99m MAA to determine the lung shunt fraction and in turn was used to infuse the Y-90. 

What we wanted to accomplish was to ensure that the Y-90 Theraspheres that we were infusing had truly localized within the liver segments that we wanted to treat.  In the past we had indirectly measured the activity using a dose rate meter (Bicron - mSv/hr) to examine the exposure rates to various parts of the chest and abdomen after the infusion.  The tricky part of the whole thing was that we had never scanned Bremmsstrahlung x-rays before.

Luckily there is an array of information on the internet, but the problem is trying to decipher all this information.  The Y-90 Bremsstrahlung spectrum looks very different, when low energy all purpose (LEAP), medium energy (ME) and high energy (HE) collimators are fitted onto the gamma camera.

Fig. 3  Top:  Represents the Bremsstrahlung spectra on a gamma camera without collimation.  Second:  Represents the spectra with a low energy all purpose (LEAP) collimator.  Third:  The spectra with a medium energy (ME) collimator.  Bottom:  The spectra with a high energy (HE) collimator.  Note 1:  If anyone who is reading this - can someone confirm what the lower peak is?  (It's a scatter peak of some sort - it may be a dumb question, but I need to ask it for my knowledge).  Note 2:  Diagram was taken from:  Planar Gamma Camera Imaging and Quantitation of Yttrium-90 Bremsstrahlung

Fig. 4  There is a characteristic x-ray photopeak between 75 - 79 keV with collimation.  This is the range where we decide to use our window for imaging.  However, note on the far right in Fig. 3 TOP, and in Fig. 4 another interesting peak occurs on around 180 - 210 keV.  This is probably the result of septal penetration due to some high energy Bremsstrahlung x-rays.

Furthermore, where do we centre the energy window and how big should the window be?  There is a characteristic x-ray peak, as indicated above, around 75 keV.  But what is interesting is that some protocols use a 79 keV peak with a 26% window, while others use a 90 keV peak and a window width of 15%. So what is the optimal imaging parameter in regards to the energy peak and energy window?  I am not really sure, and I think it really has to do with the testing that we need to do on our camera system (Seimens Symbia T-6) to figure this out in terms of sensitivity, resolution and target to background ratios.  However we did not have the luxury of time to test these parameters.  When we first started the trial, it was never our intention to image the Y-90 Therasphere patients, and this patient was a "one off".

However there is some agreement with respect to the collimation.  From what I have read between ME and HE collimators, generally speaking, most have used the ME's.  In our case, the administered dose was approximately 2.03 GBq and imaging was performed 3 days later, we opted for the ME's since there were not going to be any significant differences between sensitivity and resolution between ME and HE collimators for equivalent energy windows (ie. whether we were using a 26% or 15% window).  

So these are our results:

Fig. 5 Coronal fused section from the SPECT/CT (Symbia T-6) with localization in the segment 4 of the liver

Fig. 6  Sagittal fused section from the SPECT/CT (Symbia T-6)

Fig. 7  Transaxial fused section from the SPECT/CT (Symbia T-6)

The MIP after reconstruction looks like a "big blob".  It didn't look too bad with regards to the planar images which we also obtained.  We had captured them to quickly examine the target lesion to background liver fraction.

Fig. 8  Anterior image.  We imaged for 600 secs and obtained approximately 700K to 1700K total counts (posterior/anterior images respectively)

Fig. 9  A quick target lesion to liver background ratio was calculated to determine the amount that remained in the liver 3 days after the infusion

In the end we decided to use the MD Anderson's protocol.  It was a shot:

NM Planar & SPECT Y-90 Bremsstrahlung Imaging:

– 79keV/26% window, MELP collimation, 128x128 matrix, 4.8 mm2 pixels, 128 

views/360°, 28 s/view, non-circular step-shoot

So we did all this, BUT some might say... what about imaging the patient on the PET/CT unit? Well I wish we could, as you know, Y-90 is a beta emitter and this is certainly within the realm of possibilities, and with it, a huge array of literature on this as well.  We thought about it, but we couldn't get imaging time on the PET/CT unit... so we opted for regular gamma camera imaging.  QED.

Something Doesn't Look Right!

Fig. 1  This is an anterior TBBS image, but something doesn't seem quite right.

Fig.2  The posterior TBBS image.  Expand the image.

When we first looked at these images we were unsure what the problem was, but the images did not look right.  The total body bones scan was taken on a Philips ADAC Skylight system, but generally the images do not look this way.  This is what I mean.... the images look hazy and not indicative of a normal quality bone scan. Some of you may have figured it out by now, but for those who are still guessing, it was caused by a student.

The patient had come to the Nuclear Medicine department to determine if osteomyelitis had developed in the right shoulder.  They have had several corrective surgeries to their joints already:  bilateral shoulder replacement, bilateral hip replacement and a left knee replacement.  Let's put it this way, when she came into the department the patient was held together by duct tape... joking of course but the patient was in pain.

However knowing her condition, we had to repeat the images because something was amiss.  Check out the images below, which were also performed on the Philips ADAC system.

Fig. 3  Corrected TBBS images.  Compare to Fig. 1 and Fig. 2.  Expand the image.

So the problem is this... when the student had placed the patient on the bed, they had unknowingly changed the collimators to high energy collimators instead of the high resolution collimators (VXGP).  While setting up the patient in the computer for the acquisition, the computer had prompted the student that the wrong collimators were installed.  Majority of our acquisition protocols are customized to the organ system that we are scanning (like most modern departments), with a predetermined energy spectrum and energy window, collimation, length of time for the scan etc.. In this case the student did not bother to read the prompt which notified that the wrong collimators were installed for this particular acquisition protocol.  They ignored it and proceeded with the scan.  Thus the 140 keV gamma rays hitting the crystal were limited by the thicker collimation of the high energy collimators, since they were designed for high energy isotopes to minimize the amount of cross talk.  Collimation is one of the fundamental principles of Nuclear Medicine Instrumentation.

Fig. 4  An example of a parallel hole collimator.  With high energy collimators the "holes" are smaller due to the thicker septa and the longer bore, thus limiting the amount of detection or the sensitivity of the collimator.  The reason for this is to limit the amount of cross talk when imaging with high energy isotopes like iodine.  If these modifications were not present, it would increase the amount of scatter and thus decrease the resolution of the image.

Another possible way of getting images to look like those in Fig. 1 and Fig. 2 is by having the wrong energy or energy window settings.  Although a lot of the camera systems all now have presets, but it's always a good habit to check your parameters and never ignore your error messages.

In the end this patient had their total body bone scan, along with their In-111 WBC with sulphur colloid two days later and found no osteomyelitis in the right shoulder.

How Does Attenuation Correction Work?

I think it's one of those questions where you know the answer, but really don't know the answer, because you know the reason why we do attenuation correction (AC), but really don't know how it is applied on our patients.  Thus the question, "how does attenuation correction work?"  

To start, the principles of AC applies to both SPECT/CT and PET/CT alike, even though we are using different isotopes and energy levels for these systems.  

The CT unit is the main equipment piece that helps us to make this work.  The reason why is, it is the x-ray's that shoot through the body, which gets collected by the CT detector array on the other side that provides the AC data.... "but we know that already!".  The problem lies in the fact that the x-rays and the energy of the isotopes are different.  Let's use Tc-99m as an example.  The problem is this, how do you use a 70 keV x-ray to correct for the nuclear medicine SPECT scan if the energy level for acquisition is at 140 keV?  These energies interact and attenuate differently as it traverses through the body!

Fig. 1  At the very top is the CT x-ray tube (rectangle), with x-rays shooting outwards in a fan beam array through the body (ellipse), being received by the detectors arrange in an "U shape" at the bottom.

When the x-rays are produced from the x-ray tube, there are a spectrum of low to high energies generally, where low energy x-rays get absorbed by the tissues whereas the high energy x-rays pass right through the body, if it is used unfiltered. This is something that we do not want because it makes it harder to figure out the AC maps. Thus the x-rays are filtered (wedge or bow tie) to produce an average of 70 keV's to help simplify the process.  The process isn't perfect, but it "hardens" the beam to eliminate the "ends" of the x-ray spectrum.

Fig. 2  The the x-rays are filtered to help simplify the process of AC.

Once we collect the x-ray data from the detectors, the information is reprocessed. The most important piece of information is the linear attenuation coefficient (u), which is calculated by using this formula, aka Beer's Law:

Formula taken from CT1 course, CAMRT

The little (u) can now be used to calculate the CT numbers to differentiate the different densities in the body based on the Hounsfield unit scale on our display monitors.

So the question now is, "How do we put all this information together to understand AC?!?!"

We've acquired our CT and what you see are whole bunch of CT numbers converted to the Hounsfield's scale, based on the (u) values of the CT x-rays.  The next step is to convert this data and create a correction factor for the corresponding voxels in the SPECT scan.  To do this, we need to calculate the new (u) value that is scaled to 140 keV.  Basically, what would be the new (u) value if we converted 70 keV x-ray to 140 keV (the Tc-99m energy that we want to scale it to)?

There's two parts to this scaling:

1.  Formulas to calculate and scale the 70 keV to 140 keV based on the original CT number acquired for your CT scan

2.  A bilinear graph (model) used to convert CT numbers to (u) values for specific radionuclides like Tc-99m, In-111, Ga-67, F-18 etc.

The formulas:

Fig.3  Top:  Formula for (u) for CT values less than 0 (0 being water).  Bottom:  Formula for (u) for CT values greater than 0.  These are bit hard to see, but look them up in this article:  SPECT/CT Physical Principles and Attenuation Correction

The bilinear graph, works hand in hand with the formulas, because it just gives a graphical overview of how the CT numbers are treated when the CT values are below or above CT=0.  The reason for the bilinear graph is to account for the different densities in the body (ie. water and air mix OR water, soft tissue and bone mix).

Fig. 4  Bilinear graph depicting the corresponding (u) values in relation to the original CT numbers based on the CT scan.

Fortunately most of these scaling values have been pre-calculated and installed in the computer in a stored look up table for the reconstruction algorithm... it's been all worked out. To finish it all off, the CT image has to be "dumbed down" to fit the overlying SPECT image, since CT images have higher resolution than nuclear medicine images thus it needs to be reformatted so that it matches the same matrix, slice width and slice position as the SPECT data.  The new scaled AC map is then incorporated into the original radionuclide SPECT projection image, and reconstructed.  The AC map is used to correct the emission counts from the uncorrected SPECT data to provide the final corrected SPECT data.

Burnt Out

Top:  Anterior feet (detector 1); Bottom:  Posterior feet  (detector 2)

What's wrong with this picture?  

First off, this is an In111-WBC scan to localize infection in the ankle.  The top image is the anterior view (detector 1) and the bottom image is the posterior view (detector 2).  What is interesting is that there is a "bite" taken out of detector 2.  As you know, this is probably not something that you would want in your image, but rather a technical artifact in which a photomultiplier tube has burnt out during the image acquisition.

There are 2 things to discuss here:

1.  White Blood Cell (WBC) scans:  

The decision to use Tc-99m WBC versus In-111 WBC is based on the a decision tree approved by our physician.  It really revolves around whether the patient is diabetic and or has a prosthetic or implant post surgery.  With WBC scans, it's primarily used to determine infection such as osteomyelitis, which for the most part represents 99% of our cases.

a.  Not diabetic or no implant - Tc-99m WBC

b.  Diabetic with (?) foot infection and/or has prosthetic implant - In111-WBC 

All WBC cases have a bone scan first followed by either the Tc-99m or In-111 WBC scan afterwards.  The one main thing to note is that if the In-111 route is taken, they are given Tc-99m sulphur colloid first before the In-111 WBC injection on the same day to determine if there is any bone marrow involvement with the site in question.

Having said this, I have yet to see a positive scan to date.  The current case above was to determine post surgical infection of the right ankle.  The scans for both the sulphur colloid and the In-111 WBC were unremarkable, but there were post surgical changes on the right distal fibula and ankle (image not shown) on the bone scan.

What about gallium?  At our site, gallium is not the isotope of choice unless discitis is indicated.  In-111 WBC's are not as effective in relation to Ga-67 in conjunction with a bone scan, to determine discitis/vertebral osteomyelitis.  Accuracy for WBC's are generally low for this type of imaging (Henkin et al. Part IV p. 1129)

2.  Photomultiplier tubes (PMT)

Basic premise with PMT's are to convert light into an electrical current.  There are a multiple of reasons why PMT's burn out.  Daily quality control and monthly preventative maintenance programs are important to detect these irregularities.  PMT's have been around for many years, but with new solid state cameras like the CZT (Cadmium Zinc Telleride) equipped systems, PMT's may be a thing of the past.  Who knows?