Liquid or foam is a choice made at your visit, so this one can wait.
This page shows what three sources say about the two forms, where their framing differs, and which questions to bring. It does not tell you which form to pick.
Both forms exist
Cleveland Clinic lists the treatment among nonsurgical options: "Your provider injects a foam or liquid solution into your spider vein or varicose vein." So the form is part of the plan, not a separate treatment. For wider background on the vein problem, see MedlinePlus on venous insufficiency.
Two framings of when foam is used
RadiologyInfo.org ties foam to vein size: "For larger veins, a foam is injected to close and seal the vein." The NHS ties foam to the alternatives: "You'll usually be offered foam sclerotherapy if endothermal ablation is not suitable for you."
These are different reasons, and they may not conflict, but neither source explains the other. One looks at the vein. The other looks at what else has been ruled out. Ask your clinic which reason applies to your legs, and ask it to explain the term endothermal ablation.
Questions to bring
- Which form do you recommend for my veins, and why?
- If foam, is it because of vein size or because another option does not suit me?
- How should I prepare and what happens at home afterward? Kaiser Permanente's at-home page can help you shape that last question.
What you can decide alone
Nothing about liquid versus foam is yours to settle without an exam. Your job is to arrive with questions. If you are near Des Plaines, ask a local clinic what it offers and how it decides.
For a cosmetic overview of the whole procedure, the American Society of Plastic Surgeons spider vein page is a quick read. If one leg suddenly swells or hurts, set the question aside and call a clinician the same day.