Generally, you should use the same matrix size, or finer, for reconstruction as was used for the acquisition (if you have a choice), and you should initially start with the least amount of filtering. Typically, this can be a ramp filter for filtered back projection reconstruction programs (the most widely available type). Keep in mind that you will have the most noisy images with a ramp filter, but if you are looking for resolution, the ramp filter will give you the least amount of blurring of the image. Remember, you are not looking for “pretty” images – you are looking for presence of artifacts or fine structures (the rods or spheres) in the phantom – and these can disappear if you use too low of a cutoff frequency in your filter. Quick Filter Lesson: Filters Can Be Generally Thought Of As Blurring Functions, That Is, They Only Allow Lower Spatial Frequencies To Be Used During The Reconstruction Process. Thus, They Are Often Called “Low-Pass” Filters. Filters Are Typically Designed With At Least One (But Sometimes More) User Specified Parameter Known As The Cutoff Frequency. This Value Is Usually Expressed In Cycles Per Cm, But Can Be In Other Units. A Larger Cycles/Cm Value Means That Less Blurring Is Taking Place. Thus, A Filter With A Cutoff Frequency Of 1.6 Cycles/Cm Does Less Blurring (And Looks Noisier) Than One With A Value Of 0.5 Cycles/Cm. Reconstruct each head individually, and then all heads combined. Perform attenuation compensation with either a measured TCT map or use a value of 0.12 per cm constant value. Note: The True Attenuation Coefficient Values For Water And Lucite  Are Approximately 0.15 Per Cm At Tc-99m Energies. Using A Value Of 0.12 Instead Of The True Value Will Artificially “Compensate” For The Extra Apparent Activity Near The Center Of The Cylinder That Results From Scattered Photons. Note That This Is An Ad Hoc Method, And That It Provides A Close Enough Approximation For Many Applications. It Will Result In A Relatively Flat Profile Across The Transverse Images. Another Point To Remain Aware Of Is That The Reconstruction Software Of Some Scanner Systems Is Configured To Use Units In Terms Of Cycles Per Pixel Instead Of Cycles Per Cm. You Should Determine If Your System Is This Type And Make Adjustments Accordingly. Contact The Vendor Of That System To Obtain Values That Are Appropriate For Your Uses Of The System. It Is Impossible For Us To Provide A Specific Value, Since Converting From Inverse Centimeters To Inverse Pixels Would Depend On The Matrix Size And The Field Of View Of The Camera. For the cold rods sections of the QC phantom, you probably will want to add 10, 20 or more slices together to improve rod visibility and reduce noise. For this to be successful, you must position the phantom in the scanner as perfectly aligned with the axis of rotation as is possible. Otherwise, any tilting will mean rod locations in one slice will be slightly shifted in subsequent slices. When you add them, the rods will not “line up” and you will just have blurred images. For the slices encompassing the spheres, pick the 2 to 4 slices that seem to include the smallest sphere, and then add these images together. In the uniform section of the cylinder (the volume between the spheres and the phantom’s cover plate), add together 10 to 20 or so of these slices. Note: Depending On Your Computer’s Software And The Number Of Counts In The Transverse Images, You Might Have To Scale The Images Down Before Adding Them, So That You Don’t Have Overflowed Values In Some Pixels. Basically, This Just Means You Have To Divide The Images By Some Factor Like 2 Or 5 Or 10 Before Adding Them. Typically, Most Nuclear Medicine Images Today Are Arrays Of 2-Byte Integers (The “Pixels”), Which Means That The Pixel Can Store A Value No Higher Than 32767. (There Is At Least One System On The Market That Has 2 Byte Integers That Have A Value Range From 0 To 65K, But I Did Say “Typically”, Above). If A Pixel Is Told To Be Set To Some Number Greater Than 32767, It “Overflows”. The Value Is Too High For The Integer Format Declared In The Program’s Source Code, And May (Internally In The Particular Bit Pattern Of The Bytes) Appear To Be Negative Or Some Astronomically High Number. It Will Depend On The “Sign” Bit And The Variable’s Declaration In Source Code.